Promptly produced particles may interact with hadrons produced at the same time in the pp collision. A very important feature of the LHCb analysis is the ability to compare the promptly produced χ c1 (3872) and ψ(2S) particles with those from decays of B hadrons. This comparison is very interesting because B hadrons live long enough to fly a few mm and decay in the vacuum outside the pp collision region - and therefore the χ c1 (3872) and ψ(2S) produced in B hadron decays never interact with the dense environment at the pp collision point, and so could not experience the suppression effects mentioned above.

The left image shows the fraction f prompt of promptly produced χ c1 (3872) and ψ(2S), as a function of the number of tracks reconstructed in the VErtex LOcator (VELO), which is a measure of the event activity. For both mesons, the value of f prompt decreases as the event activity increases. The right image shows the ratio of the χ c1 (3872) and ψ(2S) production cross sections for prompt and B hadron decay production (denoted "b decays"), again as a function of the number of tracks reconstructed in the VELO. Moving from low to high multiplicity, the data suggest that prompt χ c1 (3872) production is suppressed relative to prompt ψ(2S) production. This would be expected in a scenario where interactions with co-moving hadrons produced in the collision dissociate a large, weakly bound χ c1 (3872) particle more than the relatively compact conventional charmonium ψ(2S) particle. In contrast, the ratio of cross sections for production in B hadron decays does not display any significant dependence on event activity. (The central values of the black points increase gradually, but within uncertainties are consistent with being flat.) These results will be of interest both to theorists studying the possible exotic nature of χ c1 (3872) particle and those investigating dense hadronic matter. Read more in the conference presentation and in the conference note.

Results from LHCb on Open and hidden beauty production as well as Z boson production in pPb and Pbp collisions were also presented at the conference. These allow the study of cold nuclear matter effects, and to how they can be disentangled these from quark-gluon plasma effects (see an introduction to the topic). The images above show the production of different bb quark bound states (ϒ mesons) and Z boson production in pPb collisions as well as production of two charmonium states (J/ψ and ψ(2S)) in ultra-peripheral Pb-Pb collisions. The LHCb Upgrade will provide an excellent opportunity for unique and groundbreaking insights on the intrinsic properties of the heavy ion physics to be made in the LHC Run 3 and 4.

The LHCb Collaboration presented interesting new results at the International Conference on Kaon Physics 2019 in Perugia, Italy, and at the International Conference on B-physics at frontier machines, Beauty 2019 in Ljubljana, Slovenia. Selected topics are listed below.

(1) Hunting for the rarest strange decays at the LHC, K S 0→μμ. Decays of K mesons into pairs of muons played a very important role in the history of particle physics. There are two types of neutral K mesons: the short-lived K S 0 ("K-short"), and the long-lived K L 0 ("K-long"). The results of branching-ratio measurements of the K-long decay into muon pairs in the early 1970s disagreed strongly with the predictions of the particle physics theory of the time, based on the existence of three quarks: u, d and s. The branching ratios were calculated to be of the order of 10-4 while the experimental limits were about 4 orders of magnitude lower. In an attempt to solve this problem Glashow, Iliopoulos and Maiani proposed the existence of an additional quark, called a charm quark – a 1970s version of a new physics model. In the mechanism they proposed (GIM mechanism) a destructive virtual contribution of this new quark greatly reduced the K-long decay rate into muon pairs. The discovery of the J/ψ meson in November 1974 was the first evidence for the existence of charm quark, and at the same time confirmation of the GIM mechanism.

45 years later, LHCb physicists are searching for K-short decays into muon pairs, again on the look-out for new physics. This decay rate is very sensitive to possible contributions from new, yet-to-be discovered particles, such as leptoquarks or super-symmetric particles that are too heavy to be observed directly at the LHC. These could significantly enhance the decay rate, up to existing experimental limits, but could also suppress it, as the charm-quark contribution to the K-long decay did. In the same way, the long-running search for the very rare decay of a B s 0 meson into a muon-antimuon pair was motivated by the search for new physics. According to the Standard Model, the expected decay rate of K S 0 →μμ is about thousand times smaller than that of B s 0 →μμ, making the search particularly challenging. The analysis required a sophisticated online (real-time) selection, and made use of machine-learning tools and parallel processing with graphics processing units (GPUs). In the absence of a significant signal (see image), an upper limit on the branching fraction of 2.1 × 10 -10 is obtained, four times more stringent than the previous best limit , which was also set by LHCb.

(2) Observation of parity violation and search for CP-violation in Λ b 0→pπ-π+π- decays. Charge-parity (CP) violation - a difference in behaviour between matter and antimatter - is a well-established phenomenon in the decays of K and B mesons. Recently, it has also been observed by the LHCb collaboration in the decays of D mesons. However, CP violation has yet to be established in baryonic decays. Similarly, parity violation is well established in weak interactions, but has never been observed in b-hadron decays. (In weak decays of hadrons, parity violation depends on the hadron’s constituents.)

Physicists from LHCb reported the first observation of parity violation in b-baryon decays, with a significance of over 5 standard deviations, using a sample of Λ b 0→pπ-π+π- decays. Searches for CP violation were also performed, giving results that are marginally compatible with the hypothesis of no-CP violation.

The left image above shows the pπ-π+π- invariant mass spectrum with an accumulation at the Λ b 0 mass. The other images show measured asymmetries by grouping data in different ways (different binning schemes), with horizontal black dashed lines at zero representing the hypothesis of P and CP conservation. The open blue points are inconsistent with this null hypothesis and show clear evidence of parity violation, the first observation of the phenomenon in b-baryon decays, while the red full points do not show evidence of CP violation when a statistical test is applied. A second, complementary analysis method is used in which, rather than grouping the data into bins, a single test is used to measure how well the underlying distributions of data points overlap. The result obtained from this method constitutes an observation of parity violation with over 5 standard deviations significance.

Read more in the LHCb conference presentation, in the LHCb CERN seminar and in the paper. This result is an update to a previous paper, which used about four times less signal.

(3) Search for the doubly charmed baryon Ξ cc +. The LHCb collaboration discovered the exceptionally charming particle Ξ cc ++ in 2017. It is a baryon containing two charm quarks and one up quark (ccu), resulting in an overall doubly positive charge. It is the doubly charmed counterpart of the well-known lower-mass Ξ0 baryon, which is composed of two strange quarks and an up quark (ssu). The Ξ cc ++ was then rediscovered in different decay mode and its lifetime has been measured.

Other, similar combinations of quarks are also possible. If the u quark of a Ξ cc ++ baryon is replaced by a d quark, one obtains a singly charged Ξ cc + baryon (ccd), which is expected to have a similar mass but a much shorter lifetime according to a number of theoretical calculations. LHCb physicists reported the results of a search for this particle in the decay chain Ξ cc +→Λ c +K-π+, Λ c +→pK-π+, using the full Run1 and Run2 data sample.

The left image above shows the pK-π+ invariant mass spectrum in which events accumulate around the Λ c + mass. Milions of Λ c + baryons are selected with high purity. The right image shows the Λ c +K-π+ invariant mass spectrum. A search is carried out for a possible "bump" that would indicate evidence for the Ξ cc + baryon in the “RS” (right-sign) distribution; the “WS” (wrong-sign) spectrum is a control channel where no structure is expected to show up. No significant signal is observed and, since LHCb's ability to reconstruct these particles depends strongly on how much the Ξ cc + baryon flies before decaying, a suite of limits are reported under different Ξ cc + lifetime hypotheses. The dashed blue line indicates the mass of the Ξ cc ++ baryon discovered by LHCb, and the dotted red line indicates the mass of the Ξ cc + baryon reported by SELEX experiment at Fermilab.

Read more in the LHCb conference presentation and in the LHCb paper.

The lightest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composed of udd quarks. By replacing one of the d quarks by a heavier strange quark s we obtain a Λ 0 particle composed of uds quarks. Furthermore by replacing in the Λ 0 baryon the s quark by a charm quark c or a beauty quark b we obtain a Λ c + or a Λ b 0 baryon particle. The three quarks udb forming the Λ b 0 are in their lowest quantum mechanical state. Like electrons in atoms quarks can form excited states with different values of angular momentum and quark spin orientation. Earlier, in 2012, LHCb announced the observation of the two excited states of Λ b 0 baryon, Λ b (5912) 0 and Λ b (5920) 0 , discovered in the Λ b 0 π + π - invariant mass spectrum.

Today at the EPS-HEP conference, the LHCb collaboration announced the discovery of two new beauty baryon particles. These particles are interpreted as excited states of the Λ b 0 baryon. The new structures are observed in the Λ b 0 π + π - spectrum using the full LHCb Run 1 and Run 2 data set, corresponding to an integrated luminosity of 9fb -1 .

In the analysis reported today Λ b 0 baryons are formed from Λ c +π- combinations, where the Λ c + baryon is reconstructed using its pK-π+ decay. Λ b 0→J/ψK- decays, with J/ψ→μ+μ-, are also used as a cross-check. The left image below shows the Λ b 0π+π- invariant mass spectrum with Λ b 0 baryons reconstructed in these two cases. A clear peaking structure is observed at a mass of around 6150 MeV in both cases.

Since the mass of the new structure is above the thresholds (minimum invariant mass) of both the Σ b ±π∓ and Σ b *±π∓ combinations, it can potentially decay via these intermediate resonances. To investigate this the Λ b 0π+π- mass spectrum is studied in the Λ b 0π± mass regions populated by the Σ b (*)± resonances. The data with Λ b 0→Λ c +π- are split into three samples: events with a Λ b 0π+ mass around the Σ b ± mass region, around Σ b ∗± mass region and the remaining nonresonant (NR) ones. The Λ b 0π+π- mass spectra in these three samples are shown in the central image above. The spectra in the Σ b and Σ b * regions look different and suggest the presence of two narrow peaks at different masses, denoted in the LHCb result as Λ b (6146)0 and Λ b (6152)0. The right image above shows the corresponding Λ b 0π+ and Λ b 0π- invariant mass spectra in the decays of new particles. As well as nonresonant component, clear contributions from intermediate resonances are visible: Λ b (6152)0→Σ b ±π∓, Λ b (6152)0→Σ b *±π∓ and Λ b (6146)0→Σ b *±π∓. No significant evidence is found for Λ b (6146)0→Σ b ±π∓.

The masses of the two states measured in this analysis are consistent with theoretical predictions for the 1D doublet of Λ b 0 states. While the newly discovered states are denoted as Λ b 0, their interpretation as other excited beauty baryons, such as neutral Σ b 0 states, can not be excluded.

Recently LHCb physicists reported the first observation of the two new mass peaks in the Λ b 0π+ and Λ b 0π- invariant mass spectrum consistent with the excited Σ b (6097)± charged baryon.

Read more in the LHCb EPS-HEP presentation and in the LHCb paper.

The LHCb collaboration has submitted for publication an updated measurement of the CP-violating phase φ s in B s 0 meson decays. The precision measurement of φ s is one of the most important goals of the LHCb experiment.

In the wonderful world of quantum mechanics a B s 0 meson can decay directly or oscillate into B s 0 meson and then decay. In analogy to the two-slit quantum mechanics experiment, these two modes of decay can interfere. Interference like this is one of the key ingredients for CP violation to occur. CP violation means that particles and their antiparticles have different properties and behave in different ways; it's required to explain why the universe we live in consists mainly of matter rather than antimatter. In this case, the CP violation would manifest itself as a nonzero value of the phase φ s .

Within the Standard Model, the value of φ s can be calculated precisely from other measurements. The predicted value of φ s is small, about -0.037 rad, and New Physics effects could therefore change its value significantly.

In order to obtain this new result, LHCb physicists measured the decay-time-dependent CP asymmetry in B s 0→J/ψK+K- decays using proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of 13TeV in 2015 and 2016 of the LHC Run 2, corresponding to an integrated luminosity of 1.9fb-1. The CP-violating phase φ s was measured using a sample of approximately 117 000 decays with a K+K- invariant mass in the vicinity of the φ(1020) meson. The images below show (a) the J/ψK+K- invariant mass distribution peaking at the B s 0 meson mass, and (b) that of the K+K- combination accumulated around the φ meson mass.

The reported value is φ s = -0.083±0.041±0.006 rad, consistent with expectations based on the Standard Model and with a previous LHCb analysis of this decay using data recorded at centre-of-mass energies 7 and 8 TeV in the LHC Run 1. The combination of the Run 1 and Run 2 results gives φ s = -0.080±0.032 rad. This result is then combined with the recently published measurements of φ s using B s 0→J/ψπ+π- decays obtained with the same dataset, and with previous independent LHCb results. The combined value φ s = -0.041±0.025 rad is consistent with expectations based on the Standard Model, and is the most precise experimental determination.

The phase φ s is not the only interesting parameter that can be measured with B s 0-B s 0 oscillations. Following the rules of quantum mechanics, each of the particles can be expressed as a different combination of two quantum states that have slightly different masses and decay widths (and thus slightly different lifetimes). Their mass difference, Δm s , determinates how fast B s 0 and B s 0 oscillate into each other, see 15 march 2011 news. The difference of their decay widths, ΔΓ s , was measured in this new analysis together with the value of φ s and other parameters such as the difference between the B s 0 and B0 lifetimes.

The left image above shows the various LHCb measurements of φ s used for the combined result, and the combined contour (in blue) in the φ s -ΔΓ s plane. The B s 0→J/ψK+K- (magenta) and B s 0→J/ψπ+π- (red) contours are both marked 4.9 fb-1 and represent combinations of Run 1 and Run 2 data. The Standard Model expectations obtained by the CKMfitter group is indicated by the thin black rectangle (and a similar result is also obtained by the UTFit collaboration). The right image above shows the comparison of the LHCb combined result (in green) with results from other experiments, including the recent ATLAS measurement (in blue), and the world combination by the HFLAV group (in white). The world combination gives φ s = -0.055±0.021 rad.

Read more in the LHCb paper and LHCb CERN seminar.

Today at the Rencontres de Moriond QCD conference, the LHCb collaboration announced the discovery of a new narrow pentaquark particle, P c (4312)+, decaying to a J//ψ and a proton, with a statistical significance of 7.3 standard deviations. In addition, the P c (4450)+ pentaquark structure previously reported by LHCb is also confirmed, but a more complex structure consisting of two narrow overlapping peaks, P c (4440)+ and P c (4457)+, is now emerging, with the two-peak structure having a statistical significance of 5.4 standard deviations compared to a single-peak hypothesis.

In the conventional quark model, strongly interacting particles known as the hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)+ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ b 0→ J/ψpK- decays.

Various interpretations of these structures have been proposed, including tightly bound pentaquark states and loosely bound molecular baryon-meson state. These two possibilities are illustrated in the figure to the left. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The leftmost image illustrates how the quarks could be tightly bound ; the image to the right shows a loosely bound meson-baryon molecule, in which a meson and a baryon are connected by a residual strong force similar to the one that binds protons and neutrons together within nuclei.

The analysis presented today used the combined data set collected by the LHCb collaboration in Run 1 (with pp collision energies of 7 and 8TeV, and corresponding to a total integrated luminosity of 3 fb-1) plus Run 2 (6 fb-1 at 13TeV). From this sample, 2.5x105 Λ b 0→ J/ψpK- decays were selected, nine times more than in the previous Run 1 analysis. The combined data set was analysed in the same way as in the earlier 2015 paper and the parameters of the previously reported P c (4450)+ and P c (4380)+ structures were found to be consistent with the original results. However, analysis of the much larger data sample reveals additional peaking structures in the J/ψp invariant mass spectrum which were not visible in the data sample used before. A narrow peak is observed near 4312MeV with a width comparable to the mass resolution. The structure at 4450MeV is now resolved into two narrow peaks, at 4440 and 4457MeV. The images below show the contribution of these pentaquark states to the J/ψp invariant mass spectra.

The minimal quark content of these states is duucc: four quarks and one antiquark. Since all three states are narrow and lie just below the Σ c +D0 and Σ c +D*0 thresholds (meaning that their mass is slightly smaller than the sum of the masses of a Σ c + and a D0 or a D*0) by amounts that correspond to plausible hadron-hadron binding energies, they provide a possible experimental evidence for the existence of bound states of a baryon and a meson, as seen in the image above. If this interpretation is correct, the decay channels open to the states would be restricted. Being just below threshold, such states would not decay by "falling apart" into a Σ c + baryon and a D0 or a D*0 meson, but could decay instead to a J/ψ meson and a proton. In the baryon-meson configuration shown in the image, it is not easy for the c and c quarks to come close enough together to form a cc bound state (i.e. a J/ψ meson). Therefore, such a baryon-meson configuration is expected to be relatively stable and would be observed as a narrow peak, following the basic rules of quantum mechanics. A description of these states as tightly bound clusters of five quarks is also plausible. A full understanding of the internal structure of the observed states will require more experimental and theoretical study.

Read more in the Moriond presentation, in the LHCb paper, in the CERN news and in the LHCb CERN seminar.

Today at the Rencontres de Moriond EW conference, the LHCb Collaboration presented an updated measurement of the ratio R K , an important test of a principle of the Standard Model of particle physics known as "lepton universality", which states that the Standard Model treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses.

The ratio R K describes how often a B+ meson decays to a charged kaon and either a muon and anti-muon pair (K+μ+μ-) or an electron and anti-electron pair (K+e+e-). These decays are extremely rare, occurring at a rate of only one in two million B+ meson decays. The decays involve the transformation of a beauty quark into a strange quark (b→s), a process that is highly suppressed in the Standard Model and can be affected by the existence of new particles, which could have masses too high to be produced directly at the Large Hadron Collider.

LHCb has studied a number of other such ratios comparing decays with different leptons in beauty particle decays (see R K , R K*0 , R(D*) and R(J/ψ)). These results revealed hints of deviations from lepton universality, none of which was significant enough to constitute evidence of new physics on their own. However, according to theorists who study possible extensions of the Standard Model, taken together these deviations suggest an interesting and coherent pattern. All previous LHCb results used only the Run 1 data sample. During Run 2 (2015-2018), LHCb collected a much larger data sample containing approximately four times larger number of beauty particle decays and these data are now being analysed intensively. Measurements like R K use the technique of blind analysis, in which the physicists analysing the data do not know the result until the analysis method is finalized and frozen, following an extended review within the collaboration. The measurement presented today is the first lepton universality test performed using part of the Run 2 data set (2015-2016) together with the full Run 1 data sample, representing in total an integrated luminosity of 5fb-1.

To minimise the influence of detector and other experimental effects, LHCb physicists used a "double ratio" method: what they measure is R K divided by another ratio, r J/ψ , the true value of which is known to be very close to 1 but which has similar sensitivity to detector effects to R K . This gives an extra layer of protection: the scientists study and correct for all known experimental effects, but if any unknown effects slip through they will cancel in the double ratio between R K and r J/ψ . In more detail, R K is defined as the ratio of probabilities that a B + meson decays to K + μ + μ - or K + e + e - (within a particular invariant mass range; see below), and r J/ψ is defined as the ratio of probabilities that a B + meson decays to J/ψK + with J/ψ →μ + μ - vs J/ψK + with J/ψ →e + e - . The double ratio method greatly reduces systematic uncertainties related to the different experimental treatment of muons and electrons, which largely cancel in the double ratio. In particular, the use of the double ratio method means that the detection efficiency of the decay B + →K + e + e - only needs to be known relative to that of the B + →J/ψ(→e + e - )K + decay, rather than the B + →K + μ + μ - . Moreover, J/ψ meson decays into μ + μ - and e + e - pairs are known to respect lepton universality at the 0.4% level. This means that the measurement of the single ratio, r J/ψ , also constitutes an excellent cross-check, since it does not benefit from the double ratio's cancellation of systematic effects and so is a sensitive and stringent test of the methods used to determine the efficiencies. The value of r J/ψ is found to be 1.014±0.035, consistent with 1. The figures show the measured invariant mass distributions of B + candidates for the four decay modes used in the double-ratio measurement, each with a clear peak around the B + meson mass.

The analysis is performed in the range 1.1<q 2 <6.0 GeV 2 , where q 2 is the invariant mass of the μ + μ - and e + e - pair, and benefits from an improved reconstruction compared to the previous LHCb R K measurement . The value of R K is measured to be 0.846 +0.060 -0.054 +0.016 -0.014 , where the first uncertainty is statistical and the second systematic, and is shown at the image as a black point with error bars. This is the most precise measurement to date and is consistent with the SM expectation at the level of 2.5σ (2.5 standard deviations), a value very similar to the 2.6σ obtained in the previous measurement, and show as a grey point. The new measured value of R K supersedes the previous one, is closer to the Standard Model prediction, and has a smaller uncertainty. The result of BaBar Collaboration at low q 2 is shown by the green left point and favors also a value below one. The blue point shows that the result of the R K measurement by the Belle Collaboration is consistent with one in the whole q 2 range up to 22 GeV 2 . Further reduction in the uncertainty on R K can be expected when the data collected by LHCb in 2017 and 2018, which have a statistical power approximately equal to that of the entire 2011-2016 data set used here, are included in a future analysis. In the longer term, there are good prospects for even higher-precision measurements as much larger samples will be collected with the upgraded LHCb detector.

Read more in the LHCb Moriond presentation, in the LHCb paper and in the LHCb CERN seminar.

The LHCb collaboration has just presented at the Rencontres de Moriond EW and in a special CERN Seminar the first observation of CP violation in charm particle decays. Quarks can be split into two sectors: those with the same electrical charge as the up quark (up-type quarks, charge +2/3), and those with the same as the down quark (down-type quarks, charge -1/3). Differences in the properties of matter and antimatter, arising from the so-called phenomenon of CP violation, had been observed in the past using the decays of K and B mesons, i.e. of particles that contain strange or beauty quarks, which are both down-type quarks. By contrast, despite decades of experimental searches, CP violation in the decays of charmed particles, i.e. containing the charm quark, which is an up-type quark, escaped detection so far. The result announced today constitutes the first observation of CP violation in decays of a charmed particle.

CP violation is one of the key ingredients required to explain why today's universe is only composed of matter particles, with essentially no residual presence of antimatter. The phenomenon was first observed in 1964 in the decay of neutral K mesons, and the two physicists who made the discovery, James Cronin and Val Fitch, were awarded the Nobel Prize in physics in 1980. Such a discovery came as a great surprise at the time, as it was firmly believed by the community of particle physicists that the CP symmetry could not be violated. In the early 1970s, building on the foundations laid by Nicola Cabibbo and others some years before, Makoto Kobayashi and Toshihide Maskawa realised that CP violation could be included naturally in the theoretical framework that we know today as the Standard Model of particle physics provided that at least six different quarks existed in nature. Their fundamental idea was confirmed eventually three decades later by the discovery of CP violation in beauty particle decays by the BaBar and Belle collaborations, leading to the award of the 2008 Nobel Prize in physics to Kobayashi and Maskawa. In the Standard Model, the existence and overall size of CP violation are determined by a single parameter, though the way it manifests in a particular decay is influenced by several others. The values of these fundamental parameters can be determined experimentally by measuring many different CP-violating processes. The combined set of these measurements, many of which have been performed by LHCb, agree very well with the Standard Model predictions for all CP-violating effects known so far in particle physics. The Standard Model also predicts a tiny amount of CP violation in charm particle decays, at a level that is difficult to calculate exactly but could be up to 10-3 - 10-4 in decay modes of interest.

b) D 0 and D 0 mesons are produced from so-called semileptonic beauty decays, as for example B + → μ + νD 0 or B - → μ - νD 0 . In this way, the presence of a μ + identifies a D 0 meson, whereas a μ - indicates a D 0 .

a) D 0 and D 0 mesons are produced from D *+(-) meson decays, D *+ → π + D 0 and D *- → π - D 0 . The presence of a π + at this point in the decay chain indicates the presence of a D 0 meson, whereas a π - accompanies a D 0 meson.

LHCb physicists studied the differences in decay rates of neutral D 0 mesons, composed of a charm quark (c) bound by strong interactions with an up antiquark (u), and D 0 mesons, composed of a charm antiquark (c) bound with an up quark (u). The goal of the analysis is to measure the difference between the decay rates of D 0 and D 0 mesons decaying into K + K – pairs or into π + π - pairs. In practice, the measurement consists in counting the numbers of D 0 and D 0 mesons decaying into K + K - or π + π - pairs which are present in the data sample recorded by LHCb in 2011-2018. The LHCb experiment collected an unprecedented amount of such charm decays, allowing physicists from the LHCb collaboration to pinpoint the tiny size of CP violation in the charm-quark sector. However, these decays are identical for D 0 and D 0 mesons: how can you tell if the decaying meson is D 0 or D 0 to understand whether there are more D 0 or D 0 mesons decaying to K + K - (or π + π - ) pairs?

The images show the so-called invariant-mass distributions used to count the number of decays that are present in the data sample. The area of each blue, bell-shaped (Gaussian) peak is proportional to the number of decays of that type recorded by the experiment. The final result, which uses essentially the full data sample collected by LHCb so far, is given by the quantity ΔA CP =(-0.154±0.029)% , whose difference from zero quantifies the amount of CP violation observed. In particular, this turns out to differ from zero by 5.3σ (5.3 standard deviations), thus surpassing the threshold of 5σ adopted by particle physicists to assert without reservation that a discovery is made. This represents the first observation of CP violation in charm particle decays, opening up a new field in particle physics: the study of CP-violating effects in the sector of up-type quarks, and searches for new physics effects using charm CP asymmetry measurements.

Read more in the Moriond EW presentation, in the CERN seminar presentation, in the LHCb paper, in the CERN Press Release in English end French, in The Conversation article.

This week the LHCb collaboration announced the observation of an excited B c + state, confirming measurements by the ATLAS and CMS collaborations. The results were presented at the conference “Rencontres de Physique de la Vallée d'Aoste” held at La Thuile, Italy.

The B c + particle is a very interesting meson composed of two different heavy quarks: an anti-beauty quark (electric charge +1/3) and a charm quark (+2/3) bound together by the strong nuclear force. Just like ordinary atoms, c and b quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses like charmonium and bottomonium systems. The charmonium, cc, and bottomonium, bb, quark bound systems were intensively studied after the discovery of the J/ψ meson on 11 November 1974. (Indeed, LHCb continues to contribute to these studies; see a recent contribution in the news below). The study of the B c + system, cb, is much more difficult. B c + mesons are produced much more rarely, since both a cc and a bb quark pair need to be formed close together. But at the LHC, the large energy and intensity of pp collisions allow sufficient number of B c + mesons to be produced, enabling scientists to study the properties of the cb quark bound system.

In the second step of analysis, the physicists looked for signs of strong decays of excited B c + mesons to the ground state. To do this, they looked for combinations of the B c + ground-state meson plus a pair of opposite sign pions, π + π - . The left image below shows the corresponding invariant mass spectrum, with the black points showing the mass of B c + π + π - combinations: two peaking structures are visible. The same image also includes a red histogram; this represents combinations of a B c + with two same-sign points, e.g. B c + π + π + . Because these same-sign combinations have the wrong electric charge, they cannot contain real excited B c + states and are useful as a measure of the background. The right image below is made from the same data sample but plotted in a different way: the x-axis variable, ΔM, is obtained by subtracting the B c + meson mass from the B c + π + π - invariant mass. The two coloured peaks show the contributions of the two excited states announced at the conference today: the left one (red hatched) is observed with a significance above 5σ while the right one (red cross-hatched) has a local significance above 3σ. The numerical results of the measurements can be found in the LHCb presentation.

B c + mesons are unstable, but they can be reconstructed by looking for their decay products. In the first step of the analysis, LHCb physicists looked for pairings of a J/ψ and a π + meson, one possible B c + decay mode. The image shows the corresponding invariant mass spectrum. The points with error bars are the data, and the red cross-hatched area shows a clear contribution from the decays of the B c + ground state: the lowest-mass state of the cb system. (The ground state is sometimes just called the B c + .)

There is a correspondance between the four cb states shown in the image and the charmonium states η c (1S), η c (2S), J/ψ and ψ(2S) shown in the schema in the recent LHCb news below . The cb state 1S 1 , corresponding to the J/ψ meson in this analogy, has not yet been observed directly.

What are these structures? The four relevant quantum states of the cb system are show in the image to the left. Like many systems in atomic, nuclear, and particle physics, angular momentum plays a key role. The energy of the system is determined by three quantities: the spin arrangement of the c and b quarks, the angular momentum between the quarks, and the radial excitation (size) of the system. Each of the four states shown is marked "S", meaning that there is zero orbital angular momentum between the quarks. There are two main levels: a ground state 1S and an excited state 2S. Within each of those two levels, the quarks can be arranged so that their spins are opposed (↑↓) or aligned (↑↑). Since each quark has spin ½, these correspond to total spins of 0 or 1; like when putting bar magnets side by side, the arrangement with opposed spins is more stable and corresponds to a lower-mass state. The two peaks seen by LHCb are due to the two decays of a 2S excited state to a 1S ground state (2S 0 →1S 0 and 2S 1 →1S 1 ) accompanied by a pair of pions (π + π - ), shown as vertical black arrows in the figure. For the 2S 1 decay chain, there is an extra photon produced (marked γ, from the transition (1S 1 →1S 0 at the bottom of the figure) but it is not reconstructed.

The B c + meson was first observed by the CDF collaboration at the Tevatron collider in 1998. The ATLAS collaboration first reported a single peak in the B c +π+π- invariant mass distribution in 2014, but due to experimental limitations on the mass resolution and signal yield the ATLAS analysis was unable to resolve the two-peak structure. One month ago, in February 2019, the CMS collaboration released a paper in which the two peak structure is resolved. The LHCb and CMS results for the properties of the peaks are consistent.

Properties of the B c meson family can reveal information on heavy quark dynamics and improve the understanding of the strong interaction in the intermediate energy region between charmonium and bottomonium. The two –onium systems have been studied for 40 years. The precise study of B c meson family, the cb system, starts now. The LHCb experiment at the LHC is very well designed to play a leading role in this investigation.

Read more in the LHCb presentation and in the LHCb paper.

This week the LHCb Collaboration announced the discovery of a new particle made of a charm and anti-charm quark. The results, which were presented at the International Workshop "e+e- Collisions From Phi to Psi 2019" held in Novosibirsk, are the first to make use of all the available data recorded by the LHCb experiment from 2011 to 2018, and also include precise measurements of the properties of two other so-called "charmonium" states.

Charmonium states with mass smaller than twice the mass of a charm meson D cannot decay into a pair of charmed particles that each contain a charm c or an anticharm c quark. Instead, the states with higher masses can, and decay typically into charm meson pairs. LHCb physicists studied the decays of charmonium in D 0 D 0 and D + D - meson pairs. The image above shows the corresponding invariant mass spectra. Several peaking structures can be clearly seen and are listed in the figure. The images below show fits to different regions of the mass spectra with models used to describe these structures as charmonium states emerging above the background. The narrow structure marked X(3842) in the image above represents the contribution of a new narrow charmonium state, observed for the first time by LHCb, with overwhelming statistical significance in both decay modes. The left image below shows a fit giving the mass of this state to be 3842.72±0.16±0.12 MeV/c 2 and the natural width 2.79±0.51±0.35 MeV , where the first error is statistical and the second systematic. The observed mass and narrow natural width suggest the interpretation of the new state as the previously unobserved ψ 3 (1D) charmonium state. This represents the first spin-3 charmonium state observed. It is interesting to note that LHCb discovered in July 2014 another spin-3 state in the D s meson system.

The charmonium states are bound systems of a charm quark, c, and an anti-charm quark, c, held together by the strong nuclear force. Just like ordinary atoms, e.g. hydrogen, c and c quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses, all composed of the same fundamental quarks. In recent years there has been a resurgence of interest in charmonium spectroscopy following the discovery of states that do not fit into the conventional charmonium spectrum. It is sometimes difficult to conclude if a new discovered state is a previously unobserved charmonium state or an exotic particle composed e.g. of four quarks, such as a tetraquarks . Knowledge of the spectrum of conventional states is important to help identify exotic states: if all predicted conventional states are accounted for, we can be more confident that the remaining ones are exotic.

The left image shows LHCb contribution to the knowledge of the charmonium spectrum. The newly discovered X(3842) state is marked in red as ψ 3 (1D). The two states, which new parameter measurements were announced at the conference, are marked in green. Other states, in which the most precise measurements comes from LHCb, are shown with blue lines. In particular, LHCb announced in September 2017 a very innovative measurement of the masses of the χ c1 and χ c2 charmonium mesons , performed for the first time by utilising the newly discovered decay χ c1,2 →μ + μ - J/ψ. Two not-yet discovered states h c (2P) and η c2 (1D) are shown with dashed lines. The DD threshold is indicated with dotted line.

The other peaking structures visible in the images above represent prompt hadroproduction, seen for the first time, of the previously observed ψ(3770) and χ c2 (3930) states. The most precise measurements ever of the resonance parameters of these states were made. The low mass structure visible in the D 0 D 0 invariant mass spectrum comes from the χ c1 (3872)→D ∗0 D 0 decays followed by the D ∗0 →D 0 π 0 or D ∗0 →D 0 γ decays, where the γ and the π 0 are not detected.

Read more in the LHCb presentation, in the CERN news in English and in French and also in the LHCb paper.

The LHC Run 2 ended on December 3, see the corresponding news . The current experiment will be largely dismantled and an almost completely new detector constructed during the two-year-long shutdown: a new LHCb will be born in 2021. The major fraction of LHCb's sub-detectors will be replaced or upgraded during this shutdown. Last week about 500 members of the collaboration celebrated the end of the first stage of the experiment with the detector in its original form and the beginning of construction of a new detector. There were memorable sessions reminding members of the collaboration how the ideas of the first dedicated beauty experiment at the hadron collider were born and developed as well as on the history of different sub-detector construction.

The construction of the new upgraded detector will profit from the most recent technological developments. LHCb physicists and engineers have been assisted in their efforts by excellent companies that have collaborated closely with LHCb and their contributions have been crucial in ensuring the successful upgrade of the experiment. The LHCb collaboration selected three outstanding companies, Kuraray , Europractice and Advacam , for LHCb Industry awards and presented these at a special ceremony which took place during celebration.

They were followed by a fantastic party with amazing number of artists giving terrific musical performances, accompanied by video1 , video2 and a slide show presenting images of the old detector and preparations for the installation of a new one. The image shows a musical ensemble playing “A Requiem for LHCb” during the celebration.

The images above show the celebration speech given by the Collaboration Board chairperson Valerie Gibson and the spokesperson Giovanni Passaleva and the presentation of the Industry Awards to the winners.

The Kuraray company delivered 12 000 km of scintillating plastic fibre for the LHCb Scintillating Fibre Tracker (SciFi) detector. The fibre is in principle a product available in Kuraray's catalog, however, LHCb’s technical requirements in terms of attenuation length, light yield and geometrical quality significantly exceeded their standard specifications. The company had to improve production and environmental parameters and invest in measurement and quality control equipment. The stable quality of the product over the full delivery period was excellent, the photo and video shows the successful quality tests being performed at CERN.

Europractice helped to design and produced the CLARO ASIC, a 8-channel amplifier/discriminator for single photon counting with multi-anode photomultiplier tubes. The ASIC , an Application-Specific Integrated Circuit, is an integrated circuit customized for a particular use, rather than intended for general-purpose use. The CLARO is designed to sustain high rate, up to 40 Mhits/s per pixel, with a low power consumption, and it is one of the main building blocks of the LHCb RICH upgrade. Europractice provided access to the software tools needed for ASIC design, and to the silicon foundry for prototyping and production. Nearly 100k chips were produced in total according to the specifications in terms of speed, power consumption and radiation tolerance, and these fulfill the requirements of the LHCb RICH upgrade. The images shows the CLARO ASIC.

Advacam developed a novel thinned ASIC bump bonding technique for the Upgrade of the Vertex Locator of the LHCb detector. The company had a very collaborative approach to the R&D and prompt, large scale delivery of devices in full conformity with tender. LHCb very much appreciated the close collaboration of the company, with weekly meetings and feedback on all aspects of project. The excellent quality control achieved was particularly impressive. The image shows the interconnection bumps of the VeloPix ASIC .

A very busy period starts now. Members of the collaboration will continue to analyse Run1 and Run2 data and, at the same time, finalise construction and then install the new detector. The road towards new discoveries continues.

At the end of every year, the CERN accelerator complex ends its operation for the usual winter shutdown, in which particle accelerators and experiments perform necessary maintence. But today, when the LHC stopped at 4:38 am, marks the beginning of something very different: the end of the LHC's Run 2 operation period. After a two-year-long break known as Long Shutdown 2 (LS2), the collider will restart again in 2021 for the Run 3 operation period. For LHCb, today is the end of data taking with detector in its original form. The current experiment will be largely dismantled and an almost completely new detector constructed during the LS2, and a new LHCb will be born in 2021.

The 2018 data taking period was divided into two parts, and was extremely successful. Proton-proton (pp) collisions started on April 28 th and were followed by the lead-ion-lead-ion (PbPb) collisions starting on November 9 th . The image shows comparison of integrated luminosity recorded by LHCb during different pp data taking periods. This year LHCb recorded 2.19 fb -1 , the best performance ever achieved and slightly higher than the value obtained in 2012, the last year of Run 1. The total luminosity collected in Run 2 is nearly 6 fb -1 , twice the Run 1 sample of 3 fb -1 . Moreover, since the cross-section for b- and b-quark production at 13 TeV proton-proton collisions is about twice that of Run 1 (7 and 8 TeV), the number of beauty particles available for physics analysis is four times higher in the Run 2 data than in Run 1. Excellent perspectives for more and more precise physics results and for the exploration of so-far-inaccessible rare decays are therefore opening for LHCb.

In pp data taking, bunches of protons can cross every 25 ns at the LHC, corresponding to a frequency of 40 MHz (with about 30 MHz of events with collisions). During Run 1 and 2 the event rate was filtered down to 1 MHz with the help of fast electronics, using comparatively simple algorithms to select the most interesting events. Those events were then processed in a dedicated computer farm, located underground close to the detector; this allowed additional, more sophisticated selection criteria to be applied using software (see the 14 October 2015 news for more). For Run 3 and beyond, this will change radically: the fast electronics will be removed and the whole detector will be read out at the full rate of 40 MHz. This will allow the whole selection to be done in software, meaning that it can be much more precise and flexible. Better yet, from Run 3 the pp collision rate at LHCb will be increased by a factor of 5 (for experts: the luminosity will rise to 2x10 33 cm -2 s -1 ). With the higher luminosity and a greatly improved ability to pick out the most interesting events, LHCb can look forward to much larger signal yields.

LHCb, the world's first dedicated b-physics experiment at a hadron collider, is not only producing world-leading results in heavy flavour physics, but it is obtaining important results also in other fields thanks to its excellent detector performances and unique large-rapidity acceptance that make it a general purpose detector in the forward region. LHCb collected the first pPb collision data in 2013 and the first PbPb data in 2015. The 2018 PbPb data taking period was exceptionally successful. The number of collisions per unit time (instantaneous luminosity) was up to 50 times higher than was seen in 2015, thanks to the number of colliding bunches ( 468 ) beeing 20 times higher and the focalization of colliding Pb beams beeing twice better, while the overall number of collected events (integrated luminosity) was 20 times higher. The image shows a clearly visible J/ψ peak reconstructed during data taking from the PbPb collisions and from the Pb beam interactions with the injected Ne gas target . LHCb physicists are now eagerly awaiting the offline data processing to start exploring this large PbPb collision data sample.

A major fraction of LHCb's sub-detectors will be replaced or upgraded during LS2 in order to cope with these much more demanding data-taking conditions, using the most recent technological developments to design the new detectors. The VErtex LOcator ( VELO ) will be replaced by a new silicon pixel detector that will come as close as 5.1 mm to the proton beams (see more ). The tracking detectors will be replaced by a new high-granularity silicon micro-strip detector, the Upstream Tracker (UT), placed upstream of the magnet, and by the three stations of the Scintillating Fibre Tracker (SiFi), which is placed downstream the magnet, and consists of 2.5m-long scintillating plastic fibre matrices read out by silicon photo-multipliers. The mirrors of the RICH1 detector will have a larger curvature radius, and the current Hybrid Photon Detectors (HPD) will be replaced by multianode photomultipliers in both RICH1 and RICH2 detectors. The scintillating pad detector ( SPD ), the preshower ( PS ) and the first muon chamber ( M1 ) will be removed. The electronics connected directly to the detector (front-end) of all sub-detectors will be modified. And last but not least, the computing power of the LHCb software event selection system (trigger) will be significantly increased as mentioned above, and the entire readout system together with the computer farm will be moved from underground to the surface. The images above show the preparation of a test of the new VELO (left), a bundle of scintillating fibres (right) and the schematic view of the upgrated detector.

Read more in the CERN Press Release in English and French, in the CERN Couriel article and also in the CERN news in English and French.

The LHCb Collaboration presented new interesting results at the Large Hadron Collider Committee (LHCC) open session and at the CKM workshop in Heidelberg. Selected topics are listed below.

The search for and study of these states will cast light on the internal mechanisms governing the dynamics of the strong force that binds quarks inside hadrons. Read more in the LHCb presentation , in the LHCb paper , in the CERN update in English and French and in the CERN Courier article .

In the quark model, the Λ c + and Λ b 0 are each composed of three quarks: udc for Λ c + and udb for Λ b 0 . They are heavier partners of the well-known strange baryon Λ 0 , which is composed of uds quarks and whose discovery pre-dates the Standard Model. The Σ b family of resonances have slightly different combinations of light quarks: uub for Σ b + and ddb for Σ b - . In each of the upper plots, there are two peaks clearly visible (blue) that are identified as the Σ b ± and Σ b *± baryons, which were observed previously by the CDF collaboration. LHCb confirms the CDF results, improving the precision on those particles' properties by a factor of 5. Beyond these two particles, additional excited states are expected at higher masses. The lower images show mass spectra over a wider range and with stricter selection requirements. The peaks in the lower plots (pink) represent the first observation, with significances of 12.7σ and 12.6σ, of two new particles named the Σ b (6097) + and Σ b (6097) - . They are likely to be part of same family of excited states of the Σ b baryons. Theoretically, the family of excited Σ b states whose mass is closer to that of the new observed particles is composed of five Σ b (1P) states, some of which may be difficult to observe experimentally. Since it's possible that the masses of different excited states may be similar, it cannot be excluded that the structures seen are superpositions of more than one state.

(1) Observation of two new particles in the Λ b 0 π ± system. LHCb physicists observed and studied two new Σ b particles—as well as four known ones—in the invariant mass spectrum of the two-body system Λ b 0 π ± , consisting of a neutral Λ b 0 baryon and a charged π meson. These Σ b particles manifest as peaks above the smooth background, as shown in the image; on the x-axis, Q=m(Λ b 0 π ± )-m(Λ b 0 )-m(π ± ). The Λ b 0 baryons are reconstructed via their decay Λ b 0 → Λ c + π - , with the Λ c + baryons in turn decaying to pK - π + .

(2) Evidence for an exotic particle decaying into η c (1S)π-. In the quark model, strongly bound particles (hadrons) are formed from combinations of quarks (q) and antiquarks (q) that have no overall colour charge. Until relatively recently, only two classes of hadron were known experimentally: qq pairs (mesons) and sets of three quarks qqq or qqq (baryons and antibaryons). These are referred to as conventional hadrons; those which do not fit into either of the categories are called exotic hadrons. The LHCb collaboration made important contributions through the study and discovery of exotic particles, like exotic mesons (which could be tetraquarks) composed of two quarks and two antiquarks qqqq, and pentaquarks, composed of four quarks and one antiquark qqqqq. An interesting feature of the exotic particles is that they contain a heavy charm quark-antiquark pair, cc, and therefore charmonium mesons like J/ψ or ψ(2S) (sometimes denoted ψ') were observed in their decays. (For more on those, the Particle Data Group has a review of the charmonium system in pdf format.)

At the LHCC open session, the LHCb collaboration presented the first evidence for an exotic particle decaying into another charmonium (cc) meson, the η c (1S), plus a π - . The η c (1S) was reconstructed through its decay to a proton (p) and an antiproton (p). Instead of looking for any combination of an η c (1S) and a pion, the researchers studied those from the decay of a B meson: B 0 → η c (1S)K + π - . In the absence of exotic resonances, this B 0 decay would proceed predominantly through intermediate kaon resonances, such as K *0 →K + π - , as shown in the left Feynman diagram. The right diagram shows a possible contribution proceeding through an exotic particle denoted the Z c - , with minimal quark content ccdu, which then decays into the two-body system η c (1S)π - .

The images above show the invariant mass spectra of the η c (1S)π- system (from the B0 decays discussed above). The left image shows the results of a best description of the data based on a model using only conventional hadron resonances (particles), like those included in the left Feynman diagram. A discrepancy is visible around 4.1 GeV. The right image shows what happens when contributions from an exotic resonance, the Z c -(4100), are allowed: a much better description of the data is obtained. The significance of this new exotic candidate is more than 3σ when including systematic uncertainties. This result was obtained with the LHC Run 1 data plus part of the Run 2 data (4.7fb-1 in all). LHCb will have recorded approximately twice this amount of integrated luminosity by the end of Run 2, so inclusion of the rest of the Run 2 data in a future update of the analysis will improve the precision and help clarify whether this intriguing hint is indeed a new, exotic particle. Read more in the LHCb presentation, in the LHCb paper and in the CERN Courier article.

Measurements of the branching fractions of the rare charm meson decays D + →K - K + K + , D + →π - π + K + and D s + →π - K + K + were reported. The distribution of D s + →π - K + K + candidates across the Dalitz plot is shown in the image, where the presence of intermediate particles (resonances) in the decays can be identified as highly populated regions or bands. Numerical values can be found in the LHCb presentation and in the LHCb paper , and are the most precise results obtained for these decays up to date.

(3) Measurements of the branching fractions of rare decays of charmed mesons. LHCb physicists presented measurements of the branching fractions of rare decays (for experts: doubly Cabibbo-suppressed) of charmed mesons D s + (with quark content cs)) and D + (cd)). Precise measurements of these branching fractions provide important information for the understanding of the decay dynamics of these particles. Modeling these decay dynamics for charm mesons is a challenging theoretical problem: the charm quark mass is not heavy enough for robust application of theoretical methods that are used successfully in calculations of B meson decays, but not light enough for an exhaustive, brute-force approach either (which has more success for lighter K meson decays).

Today the LHCb collaboration submitted for publication the result of a measurement of the lifetime of the Ω c 0 baryon, i.e. a composite particle composed of css quarks. The Ω c 0 baryon was identified via Ω b -→Ω c 0μ-ν μ X decays, and then by means of the Ω c 0→pK-K-π+ decay.

The decay-time distribution of the Ω c 0 baryons reconstructed in the analysed sample was measured relative to that of D+ meson decays, whose corresponding distribution is shown in the left image above. The lifetime of the D+ meson is known to better than 1% precision. This approach allowed for significant reductions in the systematic uncertainty associated to the measurement. A similar technique was also used, for example, in the Ξ cc ++ lifetime measurement. The right image above shows the measured Ω c 0 decay-time distribution marked as “Data”. The “Fit” distribution is created with the lifetime parameter adjusted to describe (fit) the data in the best possible way.

The measured lifetime value 268±24±10±2 fs is nearly four times larger than, and inconsistent with, the values measured by previous experiments, as visible in the image aside. The Ω c 0 baryon lifetime was measured about 15-20 years ago by the Fermilab photoproduction experiments E687 and FOCUS and the CERN WA89 experiment located at the Σ - beamline of the SPS. The small samples of Ω c 0 observed at the time were produced in collisions with nuclear targets. LHCb physicists analysed about 1000 Ω c 0 baryon decays, which constitute a sample that is at least an order of magnitude larger than those used by previous experiments. The spectrum in the right image above labelled as “τ=69 fs” shows what LHCb physicists should have observed if the Ω c 0 lifetime had the world average (PDG) value. The inconsistency is clearly visible.

It is rare that a new measurement of the properties of a known particle results in such a large difference compared to previous measurements. Since lifetime measurements of hadrons containing heavy quarks are sensitive to the internal structure and dynamics within those hadrons, today’s precision measurement by LHCb will stimulate even more interest in charmed-baryon lifetime measurements as well as renewed theoretical efforts to understand the internal structure of charmed baryons. Moreover, they will help us understand how to incorporate QCD effects into the calculations used to describe the decays of baryons containing heavy (b or c) quarks.

Read more in the LHCb publication, in the LHCb presentation at the 2018 Conference on Large Hadron Collider Physics, LHCP, in Bologna, and also in the CERN Update in English and French, in the CERN Courier article as well as in the Physics Today Research Update.

A year ago the LHCb collaboration announced the observation of an exceptionally charmed particle, the Ξ cc ++ baryon. This particle contains two charm quarks and one up quark, resulting in an overall doubly positive charge. A month ago the collaboration has presented the first measurement of the lifetime of this baryon. Today LHCb physicists submitted for publication a new observation of this particle using a different decay channel.

This measurement provides important information towards an improved understanding of the decays of doubly charmed baryons. Read more in the LHCb paper , and also in the CERN Update and the CERN Courier article .

The left image shows the Ξ c + π + invariant mass distribution. The Ξ cc ++ peak is clearly visible, with a statistical significance of 5.9σ against the background-only hypothesis. The mass peak represents an independent observation of the Ξ cc ++ baryon, since the selected events are entirely different from those used in the previous study. The measured Ξ cc ++ mass is 3620.6±1.5±0.4±0.3 MeV/c 2 , fully consistent with the value of the previous measurement.

The Ξ cc ++ was first observed via its decay into a Λ c + baryon and three lighter mesons K - , π + and π + , with the Λ c + baryon decaying in turn into a proton p, a K - meson and a π + meson. The Ξ cc ++ has now been "re-discovered" using a different decay, Ξ cc ++ → Ξ c + π + (see graph), in which the Ξ c + baryon decays into a proton, a K - meson and a π + meson.

21 November 2019 update: see Precision measurement of the Ξ cc ++ mass paper.

In a typical proton-proton collision at the LHC, proton constituents, quarks or gluons, interact and move to opposite directions. They are interconnected by a strong interaction coloured field, a so-called "string". (Note an analogy to the electromagnetic field interconnecting electric charges.) The strength of this interaction increases with distance until finally the string breaks (fragments) into many hadrons collimated into two or more directions forming in this way characteristic jets. The remaining proton fragments, made of quarks or gluons which did not interact, continue to move in the direction close to their original trajectory, also carrying colour quantum numbers. Therefore, a coloured string interconnecting the two proton remnants is also formed, which then fragments into a high number of particles. The distribution of these particles is close to flat in the (pseudo)-rapidity η, η=-ln(tan(Θ/2)), being Θ the angle between the particle trajectory and the beam line.

Sometimes LHC protons also scatter at small angle without producing additional particles. This kind of interactions is interpreted as being produced by the exchange of an object named pomeron, which has quantum numbers of the vacuum. This concept was introduced in the Regge theory of strong interactions, a very popular theory which enjoyed a period of interest in the 1960s, and was then largely superseded by QCD.

Pomerons, emitted by each proton, can also interact, producing in turn a small number of hadrons. Since pomerons are colourless, rapidity regions depleted of accompanying particles appear, named “rapidity gaps”. This kind of interaction can also be observed if one of the pomerons is replaced by a photon. In the QCD language, a pomeron is interpreted as a two-gluon exchange in which no overall colour charge is transmitted. The left image shows a set of possible Feynman diagrams producing a J/ψ meson with or without a small number of accompanying particles. This kind of processes is referred as a Central Exclusive Production (CEP). The left image below shows a spectacular example of CEP in which two muons from a Υ(1S)-meson decay are reconstructed by the LHCb detector without any other signal recorded.

The High Rapidity Shower Counters for LHCb (HeRSCheL) detector was built to enhance studies of this interesting physics. It is located in the LHC tunnel at a maximum distance of 114m, on either side of the LHCb interaction point (see right image above). The detector was built in 2014 and installed at the beginning of 2015. It consists of twenty square scintillator modules, each about 30x30 cm2 wide, in which tiny flashes of light are produced when they are traversed by charged particles. The scintillators are placed within centimetres of the LHC beam, just outside the vacuum pipe (see images below and video). They can therefore be used to detect particles produced by collisions at the LHCb interaction point, whose deviation from the beam direction is so small that they are not detected in the main LHCb apparatus but escape down the beam-pipe and only emerge further along the tunnel, near the HeRSCheL detectors. Somewhat counter-intuitively, then, a proton-proton interaction of interest is one that leads to nothing being detected by HeRSCheL! Indeed, these rare occurences correspond to CEP events where no particles are produced, except those from pomeron interactions. Thus absence of signal in HeRSCheL enables to be identified and removed very efficiently the much more abundant proton-proton interactions producing a significant number of hadrons.

The HeRSCheL detector is named after Caroline and William Herschel who, together, made great advances in the field of astronomy during the late 18th and early 19th centuries. Not unlike the work of the Herschel family, the HeRSCheL detector brings together well-known and well-established technologies in a novel application. In so doing, HeRSCheL provides a valuable extension to the LHCb physics programme.

Today the LHCb Collaboration submitted for publication first results of J/ψ and ψ(2S)-meson CEP in pp collisions at 13 TeV. The use of HeRSCheL allowed backgrounds and systematical uncertainties to be reduced significantly. Two muon tracks were clearly identified in the LHCb detectors. Events having additional activity, either in the form of charged or neutral particles observed in the LHCb detector as well as those with significant deposits in the HeRSCheL detector, were removed. The image shows clearly visible enhancements at the mass values of J/ψ and ψ(2S) mesons. The results are in agreement with theoretical calculations of two-meson production mainly through pomeron-photon interactions, the continuum being produced via photon-photon interactions. These results represent an excellent test of QCD, as well as an investigation of the nature of the pomeron, and also a means for constraining the gluon distribution in the proton ( PDF ) at low values of proton momentum fraction.

Read more in the LHCb paper, in the HeRSCheL performance paper and in the CERN Courier article.

Last year the LHCb Collaboration announced the observation of an exceptionally charmed particle, the Ξ cc ++ baryon . This contains two charm quarks and one up quark, resulting in an overall doubly positive charge. The Ξ cc ++ baryon was identified via its decay into a Λ c + baryon and three lighter mesons K - , π + and π + (see graph), with the Λ c + baryon decaying in turn into a proton p, a K - and a π + meson. Following this observation, LHCb is now undertaking precision studies of the properties of this special particle. These studies are made possible by the high production rate of heavy quarks at the LHC and the unique capabilities of the LHCb experiment, which can identify the decay products with excellent efficiency and purity. The precise measurements of the track trajectories by the LHCb Vertex Locator ( VELO ) enable the reconstruction of the proton-proton collision point, marked as “PV”, and of the decay points of the Ξ cc ++ and Λ c + baryons, as seen in the cartoon.

This week, at the 9 th International Workshop on Charm Physics, CHARM 2018 , in Novosibirsk, Russia, the LHCb Collaboration has presented the first measurement of the lifetime of the Ξ cc ++ baryon. The data sample and the event selection are similar to those used in the analysis of the Ξ cc ++ discovery. The experimental technique is based on the measurement of the decay time distribution relative to that of another decay with a similar topology, Λ b 0 → Λ c + π - π + π - . As the lifetime of the Λ b 0 is already known with high precision from previous measurements, once the ratio of efficiencies for reconstructing the Ξ cc ++ and Λ b 0 decays is determined, it is possible to derive the lifetime of the Ξ cc ++ baryon. This approach allows for significant reductions of the systematic uncertainties associated to the measurement. The image shows the background-subtracted decay time distribution of the reconstructed Ξ cc ++ baryons marked as “Data” points. The “Fit” distribution is created with the lifetime parameter adjusted to describe (fit) the data in the best possible way. The lifetime value obtained from the fit is 0.256 +0.024 -0.022 ±0.014 ps.

The measurement of the lifetime is critical to establish the nature of the Ξ cc ++. The result clearly confirms predictions that it would have a relatively long lifetime value, which is a distinctive feature of weak interactions. However, it is very difficult to provide precise theoretical predictions, and thus they span a wide range, between 0.200 and 1.050 ps, depending on the phenomenological model that is used. The value measured by LHCb is within this range, but close to the lower end.

Read more in the LHCb presentation, in the CERN update in English and French, in the LHCb paper, and also in the CERN Courier article.

The activities of the LHCb collaboration have expanded far beyond the original core aims. The excellent performance of the detector has allowed the experiment to make important contributions to a wide range of research sectors becoming in this way a general purpose detector in the forward region. The results presented this week at the Quark Matter 2018 conference in Venice, are a perfect example demonstrating such extended LHCb capabilities. In addition to measurements in the “standard” lead-lead ion (PbPb) and proton-lead ion (pPb) LHC collision conditions, LHCb has been able to perform measurements with data taken in a unique configuration: a fixed-target operational mode. In fact, the LHCb vertex locator (VELO) was built with the possibility of injecting gas at very-low pressure into the interaction region, allowing for recording collisions of the LHC circulating proton beams with target nuclei at rest. This mode has been so far succesfully operated with He, Ne and Ar gases.

The LHCb collaboration presented a wide range of results on charm production in various types of collisions. These include the production of the Λ c baryon in pPb collisions and of D0 and J/ψ mesons in the fixed-target collisions with He and Ar. The production of heavy quarks in nucleus-nucleus interactions is well suited to the study of the transition between ordinary hadronic matter and the hot and dense Quark-Gluon Plasma (QGP). The production of J/ψ mesons in nucleus-nucleus interactions, its possible suppression in the quark-gluon medium and/or later charm-anti-charm quark recombination are all studied in order to shed light into the mechanisms governing such a phase transition. The LHCb pPb and fixed-target results utilising proton interactions with different nuclei at different energies provide precious reference results in conditions in which the formation of the QGP is not expected. The images above show a signal mass peak of Λ c baryons decaying into a proton, a K and a π (above left) as well as of D0 mesons decaying into a K and a π (above right).

In ultra-relativistic heavy nuclei PbPb collisions, two-photon and photonuclear interactions are enhanced in ultra-peripheral collisions (UPC). The collisions are either coherent, where the photon couples coherently to all nucleons, or incoherent, where the photon couples to a single nucleon. In the case of coherent J/ψ production in UPC, the photon-lead interaction can be modelled by the exchange of a colourless propagator, identified as a single object called a Pomeron , that interacts with the photon. The LHCb collaboration reported at the conference the cross-section measurement of coherent J/ψ production in PbPb collisions at 5 TeV and compared this to predictions from different phenomenological models. The image shows that the coherent J/ψ production (blue line) can be clearly separated from the other contributions in the natural logarithm of the J/ψ transverse momentum squared distribution.

Bound states of heavy quark and antiquark, such as the charmonium J/ψ and ψ(2S) as well as the bottomonium Upsilon particles, are very important tools to study properties of the QGP. It is expected that the experimentally observed rate of different bound states should be modified depending on the temperature of the QGP. Therefore the measurement of charmonium and bottomonium suppression can be used as a kind of QGP thermometer. The left image above shows different Upsilon states measured in pPb collisions at √s=8.16 TeV as a clear example that the LHCb detector is very well armed to study this interesting physics sector owing to its excellent particle identification and mass resolution.

High-energy collisions involving ions have the best chance to produce gluon condensates, where the gluon wave functions start to overlap producing a collective behaviour. This condensate would be similar to the phenomenon predicted by Bose and Einstein 93 years ago and observed in other boson systems such as ultra-cold atoms. Saturated gluons are expected to be observed only at small angles relative to the beam axes, where the number and the size of the gluons are the largest. LHCb has the unique capability of measuring photons coming from these high density gluon regions. The announcement of initial measurements of these photons caused a lot of excitement at the Quark Matter conference. The image above (right) shows the angular distribution between isolated photons and other particles taken during the 2016 pPb run. This sample is from a region where theorists expect that gluons are saturated. The peak at the angle π indicates the presence of photons from gluons. The blue band is the background from other processes. This is the first indication that gluons can be probed in this region, never achieved by any experiment so far. Members of the theory community expressed interest in studying the upcoming LHCb results and are discussing the mathematical tools that can confirm the discovery of this new form of matter.

Read more in the LHCb presentations [1], [2], [3], [4] and [5]. The LHCb papers and conference contributions will be available shortly.

The 2018 data taking period started officially today. The last 2017 proton-proton collisions took place on 28 November and the LHC machine was shut down during the winter period to allow for planned technical interventions. LHCb used this period to perform maintenance work on many sub-detectors.

The re-commissioning of the accelerator has proceeded very smoothly and first collisions arrived earlier than initially expected. The LHCb detector and its data acquisition system are ready for the last year of Run 2 data taking that will allow the experiment to obtain even more precise and interesting physics results. Follow LHCb data taking by watching live event display as well as live LHC and LHCb status pages . The image displays a typical event recorded today. The two-year Long Shutdown 2 will then start in December 2018, and during this period the LHCb detector will face its first major upgrade, which will allow the experiment to take data at much higher rate.

Four months later proton beams passed again through the LHCb detector. The nominal proton beam has size of a human hair and an energy equivalent to a very fast train. The commissioning of LHC is therefore very complicated and takes about one month. First collisions with very low intensity beams took place on 12 April and the LHC experts were able to declare “Stable Beams” condition on 17 April. Once the “Stable Beams” condition is declared, the LHC operators keep the well focussed colliding beams on their stable orbits, which allows the experiments to switch on their sensitive detectors and start to safely record proton-proton collisions. Each beam consists of packets of protons called bunches. The number of proton bunches was progressively increased and will finally reach the maximum of 2556 bunches of which 2332 will collide inside the LHCb detector. The low intensity collisions are used for detector commissioning and therefore it is not evident at which moment experiments can call the official “start of data taking” used for physics analysis. The LHC operation team and LHC experiments agreed this year that this important milestone will be reached when 1200 proton bunches in each beam will collide. This has happened today.

Today, at the Rencontres de Cuisine, the LHCb collaboration announced the observation of a new particle consistent with the properties of the long-hunted Eggs ball, η gg , the smallest lump of nuclear force, predicted more than 40 years ago by Peter Eggs. Today is thus an eggstra special day in the history of particle physics.

The discovery was possible thanks to the excellent performance of the LHCb detector and the whole CERN accelerator system. The LHCb collaboration produced the η gg by colliding high energy heavy ion beams: a small region of extremely hot nuclear matter was generated, which allowed the coagulation of the η gg . The way to the grand unification between subatomic physics and molecular gastronomy is now therefore cracked wide open.

In 2012 Atlas and CMS discovered the Higgs boson giving mass to other fundamental particles. Another important property of elementary particles is called flavour. The Eggs particle is responsible for the generation of flavour a few minutes after the Big Bang. Without the eggsistence of the Eggs particle all food would now taste the same. By studying the production and the decay properties of this new particle, LHCb physicists confirmed that it is composed of two eggs, that carry the strong sticky force, and hence it is really an egg ball, or more technically an egg-prolate-spheroid. The eggsistance of Eggs balls was predicted more that forty years ago, but it is only today that its reality is unambiguously confirmed eggsperimentally. In this way, the astonishing prediction of Peter Eggs about the strong interaction origin of flavour is confirmed. This idea is not implemented in the Standard Model of particle physics and therefore today’s discovery represents an eggstraordinary manifestation of the eggsistance of New Physics.

Read more in the CERN Update in the English and the French.

The LHCb Collaboration presented new interesting results at the Rencontres de Moriond EW and at the Rencontres de Moriond QCD. A few selected items are listed below.

(1) Search for a dimuon resonance in the Υ mass region . The only known fundamental scalar particle is the Higgs boson with a mass of 125 GeV. However, additional spin-0 particles, called Φ bosons, arise in many extensions of the Standard Model (SM) and are often predicted to be lighter than the Higgs boson. The LHCb detector has good sensitivity to light spin-0 particles produced by gluon-gluon fusion and decaying to a pair of opposite-sign muons, due to its capability of triggering on objects with small transverse momentum and to its high-precision spectrometer. The results of a search for these particles produced in proton-proton collisions at center-of-mass energies of 7 and 8 TeV were presented. No evidence is found for a signal in the mass range 5.5 to 15 GeV and upper limits are placed on the product of the production cross-section and the branching fraction to pairs of muons. The limits are competitive with the most stringent ones over most of the mass region considered, and are the first limits set near the Υ resonances. The image shows the mass spectrum of the muon pair in the whole search region. Mass peaks for five hypothetical Φ-boson mass hypotheses are displayed in green.

(2) First evidence for the decay B s 0 → K *0 μ + μ - . First evidence for the decay B s 0 → K *0 μ + μ - with a significance of 3.4 standard deviations was presented. The left image shows the invariant mass spectrum with the dominant peaking component of the decay B 0 → K *0 μ + μ - , while the zoom at the right is shown to emphasise the B s 0 → K *0 μ + μ - contribution. This decay is predicted to be very rare within the SM, as it occurs only through suppressed loop diagrams. New particles foreseen in extensions of the SM can significantly enhance (or suppress) the rate of this decay. The result presented at the Rencontres de Moriond paves the way to search for new physics using this decay when a larger datasets will be collected by the upgraded LHCb detector during LHC Run-3.

Top pair production was measured at the proton-proton collision energy of 13 TeV, where the production cross-section in the acceptance of the LHCb detector is expected to be ten times larger than it was at the lower Run-1 energies. The higher cross-section allows LHCb physicists to select a high-purity sample, in which the dilepton (μ and e) channel is partially reconstructed by requiring that a muon (μ), an electron (e) and a b-jet are present in the event. The image compares the measured cross-sections within the LHCb acceptance with predictions of theoretical models. Read more in CERN Courier article .

(3) Measurement of forward top pair production . The production of top quarks at hadron colliders represents an important test of the SM. The top quark is the heaviest known fundamental particle and its production and decay properties are sensitive to a number of parameters involved in physics models beyond the SM. The unique forward acceptance of the LHCb detector allows a phase space inaccessible to general purpose detectors such as ATLAS and CMS to be probed. Top-quark production in this region receives a higher contribution from quark-antiquark annihilation than in the central region. In addition, it probes the the proton parton distribution functions ( PDF s) at high values of the fraction of proton momentum carried by quarks or gluons, whose knowledge suffers from large uncertainties. At present precise measurements of top quark production at LHCb can be used to reduce substantially the uncertainty on PDFs in this kinematic region. The large contribution from quark-initiated production also results in a larger expected charge asymmetry in the forward region than in the central region probed by ATLAS and CMS.

(4) Measurement of CP violation in B 0 →D ∓ π ± decays . Decay-time-dependent CP asymmetries in B 0 →D ∓ π ± decays have been measured for the first time at a hadron collider by analysing the decay rate as a function of the decay time of B 0 mesons. The image shows an example of the measured asymmetry as a function of the decay time of the B 0 . The parameters describing the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called Cabibbo-Kobayashi-Maskawa matrix unitarity triangle. The angles of this triangle are denoted α, β and γ, and among these, γ is the least precisely known, see introduction in the news of 5 October 2012 . The measurement of the asymmetries presented for the first time constrain the angle γ to an interval that is consistent with the current world average. The blue line in the image is the result of this analysis, while the red, dashed line shows the expectation in the absence of CP violation. An interesting possibility is to measure precisely the angle γ in processes where new physics contributions are in principle possible and in processes where this is not allowed, looking for differences. This result belongs to the class of measurements in which the contribution of new physics is not expected.

(5) Time-dependent and time-independent CP violation in B (s) 0 →hh . The study of the CP violation in decays of B 0 and of B s 0 mesons into two charged particles (h) that do not contain charm quarks represents a powerful tool to test the CKM picture of the SM, and also to investigate the presence of physics beyond the SM. The results of measurements of the time-dependent CP asymmetries in B 0 →π + π - and B s 0 →K + K - decays as well as of the time-integrated CP asymmetries in B 0 →K + π - and B s 0 →π + K - decays were presented. The results are the most precise from a single experiment and constitute the strongest evidence for time-dependent CP violation in the B s 0 meson decays to date. They also contribute to the determination of the CKM unitarity triangle. The image shows the time-dependent asymmetries for the decays (left) B 0 →π + π - and (right) B s 0 →K + K - .

(6) Measurement of CP violation in the B s 0 →φφ decay . The B s 0 decay into two φ mesons proceeds predominantly via a gluonic loop ( penguin ) diagram and, therefore, provides an excellent tool to search for new heavy particles which could appear within these loops, see the 14 June 2013 news for an introduction. A measurement of the time dependent CP-violating asymmetry in the B s 0 →φφ decay was presented. The results obtained are consistent with the hypothesis of CP conservation. Each φ meson is observed through its decay into a K + K - meson pair. Therefore, the B s 0 meson decay is visible in the invariant mass spectrum of four K mesons, see the red dashed line contribution in the image. At lower mass values, the contribution of the B 0 →φφ decay (green dotted line) is not large enough to be clearly observed yet, and the most stringent limit on its branching fraction was also presented.

Read more details in the LHCb Moriond EW presentations [1], [2], [3], [5], in the Moriond QCD presentations [2], [3] and [4,5,6] as well as in the LHCb papers and conference notes [2], [3].

The 2017 data taking period ended this Sunday. Towards the end of the 2017 run at the centre-of-mass energy of 13 TeV, the LHC provided collisions at a reduced energy of 5 TeV to produce reference data for proton-lead and lead-lead collisions taken earlier in Run 2. Besides the scientific interest of proton-proton (p-p) physics at 5 TeV for the LHCb heavy-ion programme (see [1], [2]), the experiment has been taking at the same time a parallel stream of data from fixed-target collisions with another world first in high energy physics.

There have been typically 1836 bunches of protons circulating in each LHC ring, out of which 1094 collided inside the LHCb detector. LHCb physicists decided to use additional non-colliding bunches to accumulate the largest sample of proton-neon data in a fixed-target configuration. The LHCb experiment has the unique ability of injecting gas, neon in this case, into the interaction region and therefore study processes that would otherwise be inaccessible. This gas-injection system was originally designed to help LHCb measure the brightness of the accelerator's beams, but is now being used for dedicated physics measurements. This kind of operation is called by physicists a “fixed-target” mode in contrast to the standard “collider” mode used at the LHC, as in this case the LHC protons are colliding with stationary neon nuclei.

It has been the first time ever that an experiment has collected data in the collider and fixed-target modes simultaneously. LHCb physicists showed that it is possible to reconstruct both sets of data in parallel, align the detector elements and track particle trajectories correctly. A real challenge has been to develop an online event selection (trigger) system handling efficiently both data taking conditions. The live images (left) obtained by the data acquisition computer programs show reconstructed μ + μ - invariant mass spectra. The J/ψ-meson peaks are clearly visible in the two different operational modes. The two-dimensional plot shows the z coordinate (along the proton beam direction) of the origin of the μ + μ - pair. A strong accumulation around z=0 indicates the p-p collision point. The pink-dashed rectangle highlights the regions were p-p collision events were selected. The two other (red-dashed) rectangles show the region where only p-Ne collisions take place.

LHCb continues to revolutionise data acquisition and analysis techniques. Already two years ago the concepts of “online” and “offline” analysis were unified. The calibration and alignment process takes place now automatically online and stored data are immediately available offline for physics analysis. This time the collider and fixed-target modes of operation have been unified into the same data acquisition framework. In particle physics, a grand-unified theory is one in which at very high energies the electromagnetic, weak and strong interactions unify as a single force. Today LHCb physicists have succeeded to unify very different concepts of data taking and analysis.

A traditional end-of-year shutdown period, so-called Year End Technical Stop (YETS), is starting now. It will be used for maintenance and improvements to the LHC and its detectors. LHCb plans to exploit this period to perform maintenance work on many sub-detectors. It is planned that protons will start to circulate again in the LHC rings at the beginning of April 2018 and that the first p-p collisions for physics will take place in early May, marking the beginning of the last year of Run 2. The two-year Long Shutdown 2 will then start in December 2018, and during this period the LHCb detector will face its first major upgrade, which will allow the experiment to take data at much higher rate.

The 2017 data taking period has been very successful, because of the excellent performances of both the LHC and the LHCb experiment itself. The image shows the growth of integrated luminosity during different years of LHC operation. The 2017 integrated luminosity is higher than that collected in 2016. The overall Run 2 luminosity (2015-2017), 3.7 fb -1 , is already higher than that recorded in Run 1 (3 fb -1 , 2010-2012).

The LHCb detector is used to perform precision measurement across a range of areas of particle physics, notably including searches for new physics manifestations in flavour, studies of heavy-flavour spectroscopy and production of gauge bosons, searches for new exotic particles, and unique measurements with heavy ion and fixed-target collisions in the forward region. Precise calculations from theory are essential for the interpretation of LHCb results, as is evident from numerous news articles on this page. The comparison of LHCb results with precise Standard Model predictions provided by the theory community is the key to looking for discrepancies that would indicate the existence, and the nature, of physics beyond our present knowledge. This series of joint LHCb-theory workshops is aimed at facilitating informal discussions between LHCb experimentalists and theorists, leading to a mutual exchange of information that is valuable for achieving progress.

This week LHCb physicists met with the theory community in the 7 th yearly workshop “Implications of LHCb measurements and future prospects”. The purpose of the meeting is to consider the latest results from LHCb, discuss possible interpretations and identify important channels and observables to test leading theoretical frameworks in the near and long-term future of the experiment.

More than 300 physicists crowded the Main Auditorium through three days featuring various dedicated sessions (see the photo above). The left image shows the steadily increasing number of participants as a function of time since the first workshop was held, clearly demonstrating that this series of workshops is more and more becoming a reference event for both the LHCb physicists and the theorists. Furthermore, intriguing results published by LHCb in recent years have been triggering additional interest and discussions within the physics community.

Yesterday evening the first xenon-xenon ion collisions took place at LHC and the run lasted for seven hours. The left image below shows the corresponding LHC control screen announcing the “Stable Beam” conditions for the data taking. The right image shows a collision event recorded and analysed online by the LHCb data acquisition system.

The images below show the invariant mass distribution of J/ψ mesons decaying to two muons and that of charmed D0 mesons decaying to K and π mesons promptly reconstructed by the LHCb software trigger. Heavy ion collisions are studied at LHCb in order to understand the behaviour of the so-called quark-gluon plasma, a state of matter in which quarks and gluons are moving as free particles, similarly to what happened in the first instants of time after the Big Bang. At the LHC, the properties of the quark-gluon plasma are usually studied in collisions of lead nuclei. Although there are no plans to use xenon-xenon collision for this purpose in the near future, one day of the LHC time was devoted to these collisions in order to study the properties of nuclear matter at high-energy density and high temperature. The comparison of experimental measurements in lead-lead and in xenon-xenon collisions will bring new insights into the properties of the quark-gluon plasma.

Read more about the LHCb’s heavy ion collision studies in this page [1], [2], [3], as well as in the CERN update on the xenon-xenon run in English and French.

This result must be interpreted in the global picture of extensive tests of so-called “lepton universality”. This nomenclature comes from the fact that in the SM all charged leptons, such as taus (τ) or muons (μ), interact with identical strength (or, in physicists’ language, have the same "couplings"). However, mass differences between various leptons play a role which must be accounted for when performing calculations of interaction rates. In particular, the τ is much heavier than the μ lepton, and this leads to a SM prediction for the ratio R(J/ψ) substantially smaller than unity, between 0.25 and 0.28.

Today, at the open session of the Large Hadron Collider Committee, LHCC , the LHCb collaboration presented the result of a new measurement of the ratio of branching fractions, R(J/ψ), between B c + →J/ψτ + ν τ and B c + →J/ψμ + ν μ using τ + → μ + ν μ ν τ decays. The result that was reported is approximately 2 standard deviations from the Standard Model (SM) expectation. Furthermore, an evidence of about 3 standard deviations for the semileptonic decay of charmed-beauty mesons including τ leptons was announced for the first time.

LHCb has also found other intriguing anomalies when performing tests of lepton universality. The theoretically very clean ratios R(K) and R(K*) both show deviations from the SM prediction at the level of about 2.5 standard deviations. All these measurements were performed at LHCb using the entire Run 1 data sample, corresponding to an integrated luminosity of 3fb -1 at centre-of-mass energies of 7 and 8 TeV. Data collected in Run 2 already provide a sample of B-meson decays more than twice as large, and it will be of great importance to see whether future updates of these analyses with increased statistics will confirm the hints of discrepancies. The ability of LHCb to perform measurements using different b-hadron species, notably including the B c meson in this case, but also the B s meson and the Λ b baryon in the forthcoming future, will play a crucial role in order to clarify the global picture of deviations from lepton flavour universality that is emerging.

The left hand side image shows the summary of R(D*) measurements as presented in the 6 June 2017 news. It is interesting to notice that different experiments operating either at pp (LHCb) or e + e - (BaBar, Belle) colliders, using very different experimental techniques, measure values systematically above the SM prediction. The value of R(J/ψ), 0.71±0.17±0.18, reported today and presented in the lower part of the image, is also lying above the SM prediction. (Note the different horizontal scale.)

The R(J/ψ) measurement is very challenging. Due to presence of ν leptons invisible in the detector, both decays, involving either τ or μ leptons, are observed only through the three muons. Two of them are perfectly identified as arising from the decay of a J/ψ meson. The third μ is the key addition to enable semileptonic B c + decays involving τ or μ leptons to be distinguished from the background.

A programme of measurements of lepton flavour universality with τ leptons in the final state is being performed by the LHCb, BaBar and Belle collaborations. The most recent results were reported by LHCb for R(D*), the ratio of branching fractions between B 0 →D *- τ + ν τ and B 0 →D *- μ + ν μ , see the 6 June 2017 news. B 0 and B c + mesons differ only by a quark accompanying the b quark inside a B 0 or B c + meson, as visible in the image above. Any new physics contribution to the decay diagram as for example those sketched in the image would affect both R(D*) and R(J/ψ) ratios in a similar way.

Today, at the open session of the Large Hadron Collider Committee, LHCC, the LHCb collaboration presented the result of a precise mass measurements of χ c1 and χ c2 mesons, performed for the first time by utilising the newly discovered decay χ c1,2 →μ+μ-J/ψ.

χ c1 , χ c2 and J/ψ mesons belong to a family of particles that are commonly referred to as charmonium states. They are bound systems of a charm quark, c, and an anti-charm quark, c, held together by the strong nuclear force. Just like ordinary atoms, e.g. hydrogen, c and c quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses, all composed of the same fundamental quarks. The image on the left represents the complexity of this spectrum along with some of the allowed decay transitions as a function of mass (in the vertical axis) versus spin and other quantum numbers (in the horizontal axis). The decay paths corresponding to the decays under study are highlighted in red. The first charmonium particle, the J/ψ meson, was discovered on the 11th of November 1974. This discovery triggered rapid changes in high-energy physics at the time and these are commonly referred to as the "November Revolution". The charmonium family was then intensively studied by a large number of experiments at different facilities. In particular, the masses of χ c states were measured by studying radiative decays into J/ψ mesons, i.e. χ c1,2 →γJ/ψ, and later by using dedicated experiments employing collisions of protons and antiprotons for their production. As it is experimentally very challenging to measure the energy of a photon precisely in the harsh environment of a hadron collider, high-precision measurements have not previously been made at a collider like the LHC.

The new analysis from LHCb applies an old "trick" in a new situation. This technique was first proposed in 1951 by R.H. Dalitz to study the decay of the π0 meson, which had been discovered one year earlier. Decays of π0 mesons into two real photons were 