Why is there something rather than nothing? It’s a question so basic and so vast in scope that it seems beyond the realm of quantitative inquiry. And yet surprisingly, it is a question that can be framed and at least partially answered in the study of particle physics.

The standard model (SM) is our current theory of matter and its interactions. Written in the language of relativistic quantum field theory, it has stood up to several decades of tests at high-energy colliders. We know, however, that it cannot be the complete description of the universe, in large part because in several important regards it fails to answer the something-versus-nothing question.

Material existence is connected to three fundamental mysteries. First, why can matter clump to form a rich array of structures without collapsing into black holes? The requisite weakness of gravity arises because the particles that make up everyday matter are incredibly light. In the SM, the mass of all fundamental particles must be less than the electroweak mass, whose value of a few hundred GeV/c2 follows from the physics of the Higgs boson. However, when the effects of gravity are taken into account, the SM electroweak scale receives quantum contributions on the order of the Planck mass, 1018 GeV/c2. That puzzling discrepancy of scale is called the hierarchy problem. In the SM, a particle’s lightness is recovered only thanks to an incredible cancellation among unrelated parameters. There exist more attractive solutions, including supersymmetry and the Higgs boson as a composite particle, that also predict high-energy signatures of new physics at the Large Hadron Collider (LHC).

Second, stuff exists only because at some point in the early universe, there was one extra particle of SM matter for every one billion matter–antimatter pairs. That tiny excess is all that remains today, and it must have been created dynamically in the primordial plasma of the Big Bang. In the SM, that process—baryogenesis—is too weak by many orders of magnitude; evidently, additional particles and interactions must have been present. Theorists have suggested many possibilities, including additional Higgs bosons and modified Higgs couplings that could be detected at the LHC.

Third, strong interlocking evidence from a multitude of astrophysical and cosmological observations suggests that dark matter makes up about 80% of the matter in the universe. We have no definitive knowledge of what it is or how it connects to the SM. The most popular theories predict a tiny interaction of dark matter with ordinary matter. Such a coupling would enable direct detection by nuclear recoils in sensitive detectors and indirect detection in cosmic-ray data that show evidence of dark-matter annihilation into SM particles.

The hierarchy problem, the riddle of matter–antimatter asymmetry, and the nature of dark matter drive much of experimental and theoretical particle physics. Many experimental searches for answers to those mysteries are proceeding vigorously, but so far with null results across the board. That doesn’t mean nothing is to be discovered. On the contrary, the null results may well point us toward the true nature of the universe. There could be hidden sectors—additional particles and forces—with only tiny couplings to the SM. Far from being inconsequential, those new sectors can address all three big mysteries. Their signatures are subtle and easily missed, but luckily, their hidden nature is also the key to their discovery. Invisible long-lived particles (LLPs) can be produced at colliders and decay into energetic SM particles after traveling an appreciable distance. Can we catch those revealing flashes?

A tale of two frontiers Section: Choose Top of page ABSTRACT A tale of two frontiers << Solving mysteries Flashes beyond the shrapn... Ongoing searches The lifetime frontier References Physicists built the LHC in part to access new physics at mass scales higher than those characteristic of the SM. However, new physics may be concealed, not by virtue of high mass, but by very small couplings to the SM. We use the term “hidden sectors” to refer to such disconnected non-SM matter and forces, which can arise robustly within the framework of quantum field theories. Like the push for higher energy, the investigation of hidden sectors with tiny couplings to visible particles has important historical precedent. Neutrinos are almost massless, and nowadays they are understood to be related to charged leptons (electrons, muons, and taus) through their shared weak interaction. Wolfgang Pauli postulated their existence in 1930 to restore conservation of momentum to beta decay, but their direct detection via scattering off nuclei wasn’t achieved until 1956, when large neutrino fluxes from nuclear reactors became available. The discovery of the ghost-like neutrino sector was only the beginning, and those elusive particles still aren’t done confusing us. For example, we know that neutrinos have masses and oscillate from one type to another, which is not accounted for in the SM. 1 interactions between the SM and a hidden sector to proceed by means of a heavy and therefore hard-to-access mediator state. The mediator is not part of the SM but interacts with both the SM and hidden sectors. New hidden sectors can be connected to the SM by small but nonzero couplings called portals. Figureillustrates the most important types. Portal couplings are small for several reasons. For example, symmetries can give rise to quantum mechanical selection rules that forcebetween the SM and a hidden sector to proceed by means of a heavy and therefore hard-to-access mediator state. The mediator is not part of the SM butwith both the SM and hidden sectors. Higgs bosons and photons. In rough analogy to an oscillating neutrino, a photon could transform into a hidden photon and interact with hidden states. 1 et al. , Phys. Rev. D 80, 075018 (2009). 1. J. D. Bjorken, 075018 (2009). https://doi.org/10.1103/PhysRevD.80.075018 Higgs boson, with a mass of 125 GeV/c2, is heavy enough to sometimes decay directly into the hidden sector. Such exotic Higgs decays are one of the most promising avenues for producing hidden-sector particles. 2 Phys. Lett. B 661, 263 (2008); et al. , Phys. Rev. D 90, 075004 (2014). 2. M. J. Strassler, K. M. Zurek,, 263 (2008); https://doi.org/10.1016/j.physletb.2008.02.008 D. Curtin, 075004 (2014). https://doi.org/10.1103/PhysRevD.90.075004 It’s possible for some SM states—most importantly the photon and the Higgs boson—to function as a mediator. Although the structure of the theory makes the Higgs-portal and photon-portal couplings smaller than ordinary SM couplings, they are readily made much larger than the couplings of other types of portals. Furthermore, accelerators produce lots ofand photons. In rough analogy to an oscillatinga photon could transform into a hidden photon andwith hidden states.Thewith a mass of 125 GeV/, is heavy enough to sometimes decay directly into the hidden sector. Such exotic Higgs decays are one of the most promising avenues for producing hidden-sector particles. Hidden sectors typically contain massive states that would be stable in isolation but that decay into SM particles in the presence of portal couplings. Precisely because the portal is such a tiny keyhole, the decay can take a relatively long time. That is what makes LLPs and their spectacular decays a hallmark of hidden sectors.

Solving mysteries Section: Choose Top of page ABSTRACT A tale of two frontiers Solving mysteries << Flashes beyond the shrapn... Ongoing searches The lifetime frontier References Searching for the flashes of LLP decays at the LHC and other colliders will be a major enterprise. It will require dedicated analyses and maybe new detectors, but it is well worth the effort. Here are just a few examples of how new sectors with hidden states address the three big mysteries. 2 electroweak) mass by introducing partners related to the top by a symmetry. In the case of supersymmetry, the top partners take part in the SM’s strong interaction, so they should be produced in large numbers at the LHC. Let’s start with the hierarchy problem. As shown in figure, known solutions cancel the top quark’s large quantum contribution to the Higgs (and thusmass by introducing partners related to the top by a symmetry. In the case of supersymmetry, the top partners take part in the SM’sso they should be produced in large numbers at the 3 Phys. Rev. Lett. 96, 231802 (2006); et al. , J. High Energy Phys. 2015(7), 105 (2015). 3. Z. Chacko, H.-S. Goh, R. Harnik,, 231802 (2006); https://doi.org/10.1103/PhysRevLett.96.231802 N. Craig(7), 105 (2015). https://doi.org/10.1007/JHEP07(2015)105 4 Phys. Lett. B 651, 374 (2007). 4. M. J. Strassler, K. M. Zurek,, 374 (2007). https://doi.org/10.1016/j.physletb.2007.06.055 quantum chromodynamics (QCD), the theory of the SM’s strong interactions. Hidden valleys give rise to low-energy bound states; in analogy to SM protons and pions, they are called hidden hadrons. In theories of neutral naturalness, the top partner does not interact via the SM strong force, but it does interact via those hidden cousins of QCD. A striking signature of that novel interaction involves exotic decays of Higgs bosons to hidden hadrons, which can eventually decay back to SM particles through one of several portals. The result, as shown in the figure We haven’t found any sign of those partners yet, but another solution, called neutral naturalness,relies on hidden valleys,which are a family of theories that are essentially cousins ofthe theory of the SM’sHidden valleys give rise to low-energy bound states; in analogy to SM protons and pions, they are called hiddenIn theories of neutral naturalness, the top partner does notvia the SM strong force, but it doesvia those hidden cousins ofA striking signature of that novelinvolves exotic decays ofto hiddenwhich can eventually decay back to SM particles through one of several portals. The result, as shown in the, looks like a displaced decay. dark matter? Perhaps the best-known candidate is the WIMP (weakly interacting massive particle). As illustrated in figure 3 electroweak coupling and the mass of the dark matter is close to the electroweak mass, one finds roughly the dark-matter abundance measured today. That confluence is called the WIMP miracle. However, the direct coupling of dark matter with the SM also predicts signatures yet to be seen in direct and indirect detection experiments. Dark matter can be produced at colliders but it is invisible and only reveals itself as a momentum imbalance in a collision. What aboutPerhaps the best-known candidate is the WIMP (weaklymassive particle). As illustrated in figure, it freezes out in the early universe when it becomes too diluted to annihilate any further, and its relic abundance is set by its coupling to the SM. If that coupling is the SMcoupling and the mass of theis close to themass, one finds roughly the dark-matter abundance measured today. That confluence is called the WIMP miracle. However, the direct coupling ofwith the SM also predicts signatures yet to be seen in direct and indirectexperiments.can be produced atbut it is invisible and only reveals itself as a momentum imbalance in a collision. figure dark matter is set by the lifetime of a heavy LLP parent that produces dark matter in its decays. 5 et al. , J. High Energy Phys. 2010(3), 80 (2010). 5. L. J. Hall(3), 80 (2010). https://doi.org/10.1007/JHEP03(2010)080 interacting massive particle (FIMP). The LLP can be produced at colliders; typical lifetimes are in the millisecond ballpark. A different possibility shown in theis that the relic abundance ofis set by the lifetime of a heavy LLP parent that producesin its decays.The corresponding dark-matter particle has a much weaker coupling to the SM than a WIMP does and is called a feeblymassive particle (FIMP). The LLP can be produced attypical lifetimes are in the millisecond ballpark. 6 Phys. Rev. D 87, 116013 (2013). 6. Y. Cui, R. Sundrum,, 116013 (2013). https://doi.org/10.1103/PhysRevD.87.116013 4 dark matter and SM matter in our universe are only different by a factor of five, a long-lived WIMP decaying into SM particles could easily generate the required matter asymmetry. The WIMP can be produced at colliders and possibly give rise to LLP signatures. We turn to baryogenesis, which can proceed according to several known mechanisms. In all cases, an out-of-equilibrium process needs to create more matter than antimatter in the plasma of the early universe. A simple way to achieve that imbalance is the out-of-equilibrium decay of a heavy particle. In particular, the scenario of WIMP baryogenesis,as shown in figure, piggybacks off the quantitative success of the WIMP miracle. Since the abundances ofand SM matter in our universe are only different by a factor of five, a long-lived WIMP decaying into SM particles could easily generate the required matter asymmetry. The WIMP can be produced atand possibly give rise to LLP signatures. The above list of theories is certainly not exhaustive; it is meant to illustrate that LLPs and hidden sectors are highly motivated for a variety of fundamental reasons.

Flashes beyond the shrapnel Section: Choose Top of page ABSTRACT A tale of two frontiers Solving mysteries Flashes beyond the shrapn... << Ongoing searches The lifetime frontier References Long-lived particles can be reliably identified only if we know where they are produced. Unlike dark matter or cosmic rays, which occur naturally, LLPs have to be synthesized at colliders, where experimenters can hope to measure the macroscopic distance from the production point to the flash point of the decay. electroweak scale, they might be produced in high-intensity experiments with lower collision energy than at the LHC. Examples of such experiments include the BaBar search at the PEP-II electron–positron collider, 7 et al. (BaBar collaboration), Phys. Rev. Lett. 113, 201801 (2014). 7. J. P. Lees(BaBar collaboration),, 201801 (2014). https://doi.org/10.1103/PhysRevLett.113.201801 8 et al. , Phys. Rev. Lett. 107, 191804 (2011). 8. S. Abrahamyan, 191804 (2011). https://doi.org/10.1103/PhysRevLett.107.191804 9 et al. , Rep. Prog. Phys. 79, 124201 (2016). 9. S. Alekhin, 124201 (2016). https://doi.org/10.1088/0034-4885/79/12/124201 If the LLPs have masses below thescale, they might be produced in high-intensity experiments with lower collision energy than at theExamples of such experiments include the BaBar search at the PEP-II electron–positronthe APEX search at the Thomas Jefferson National Accelerator Facility,and the proposed SHiP (Search for Hidden Particles) experiment at CERN.The abundance of collisions in such investigations allows tiny couplings to be probed by sheer force of numbers. The LHC, however, is unique in that it allows us to probe hidden sectors when the mediator mass or hidden-sector particle masses are at or above the electroweak scale. The high-luminosity LHC upgrade, planned for the mid 2020s, will increase the number of collisions at the facility by a factor of 10 and is projected to yield 108 Higgs bosons over a 10-year period. A tiny Higgs portal can easily force 0.1% of them to decay to hidden-sector states, which would produce plenty of LLPs. Just as the enormous neutrino fluxes at nuclear reactors paved the way to neutrino detection in the 1950s, the combination of high-energy and high-intensity collisions in the current era may give us unparalleled access to new hidden worlds, provided we look in the right places. detectors at the LHC are busy places, with lots of hadronic shrapnel flying around. Luckily, neutral LLP decays are a spectacular signature, and the bursts of energy appearing out of nowhere set them apart from the mundane rubble emanating from the collision point. Looking for LLPs represents something of a paradigm shift from the usual approach, exemplified by the top branch of figure 2 The mainat theare busy places, with lots of hadronic shrapnel flying around. Luckily, neutral LLP decays are a spectacular signature, and the bursts of energy appearing out of nowhere set them apart from the mundane rubble emanating from the collision point. Looking for LLPs represents something of a paradigm shift from the usual approach, exemplified by the top branch of figure, to hunting for new physics. As-yet-undiscovered particles with large coupling to the SM would have large production rates. And that ample production is necessary if the particle signals are to be observed above the SM background noise, because the large coupling also makes the particles decay immediately at the collision point. LLP production typically occurs at much lower rates, but each individual displaced decay is so spectacular that backgrounds are orders of magnitude lower than for particles that couple strongly to the SM. Far fewer observed events are needed to claim discovery. colliders, it must have been in thermal equilibrium with the SM plasma in the early universe. As the universe cooled, the first elements—hydrogen, helium, lithium, and beryllium—came into being. That formation process, Big Bang nucleosynthesis, took place when the universe was about 0.1–1 second old, and it is exquisitely sensitive to ambient conditions. With few exceptions, LLPs decaying during or after Big Bang nucleosynthesis would disrupt the process and yield inconsistencies with measurements of primordial elemental abundances. 10 Phys. Rev. D 74, 103509 (2006). 10. K. Jedamzik,, 103509 (2006). https://doi.org/10.1103/PhysRevD.74.103509 The decay lengths of hidden-sector LLPs could be almost anything from a few hundred microns to astrophysical scales. You might find that a little discouraging: Why should we expect to see an LLP decay in the lab if it could just as well decay near Proxima Centauri? Some detailed theories suggest that LLPs should have relatively short lifetimes, but in any case, there exists a robust ceiling. If the LLP is producible atit must have been in thermal equilibrium with the SM plasma in the early universe. As the universe cooled, the first elements—hydrogen, helium, lithium, and beryllium—came into being. That formation process, Big Bang nucleosynthesis, took place when the universe was about 0.1–1 second old, and it is exquisitely sensitive to ambient conditions. With few exceptions, LLPs decaying during or after Big Bang nucleosynthesis would disrupt the process and yield inconsistencies with measurements of primordial elemental abundances.

Ongoing searches Section: Choose Top of page ABSTRACT A tale of two frontiers Solving mysteries Flashes beyond the shrapn... Ongoing searches << The lifetime frontier References LHC are already looking for LLPs, but much more needs to be done to cover the whole range of accessible masses, lifetimes, and production processes. Figure 5 LHC Apparatus) and CMS (Compact Muon Solenoid) detectors at the LHC and the signatures they might observe. Some experiments at theare already looking for LLPs, but much more needs to be done to cover the whole range of accessible masses, lifetimes, and production processes. Figureprovides a basic overview of the ATLAS (A ToroidalApparatus) and CMS (Compact Muon Solenoid)at theand the signatures they might observe. interact electromagnetically or strongly, and those particles can show up in several detector subsystems. 11 et al. (ATLAS collaboration), J. High Energy Phys. 2015(1), 68 (2015); 1029-8479 V. Khachatryan et al. (CMS collaboration), Phys. Rev. D 94, 112004 (2016). 2470-0010 11. G. Aad(ATLAS collaboration),(1), 68 (2015); https://doi.org/10.1007/JHEP11(2015)206 V. Khachatryan(CMS collaboration),, 112004 (2016). https://doi.org/10.1103/PhysRevD.94.112004 detector. Many theories, with and without hidden sectors, predict LLPs thatelectromagnetically or strongly, and those particles can show up in severalsubsystems.For example, due to its high mass, the LLP could deposit a lot of energy in a calorimeter, or it could leave a kinked track if it decays while it traverses the interact with each other via a massless hidden photon that oscillates into the SM sector through the photon portal. Millicharged states are extremely difficult to detect because they are only weakly ionizing when they pass through matter. However, their traces could be observed with a dedicated detector like MilliQan, which would be situated in a tunnel, close to one of the main detectors. 12 et al. , http://arxiv.org/abs/1607.04669. 12. A. Ball It is also possible for stable hidden-sector particles to be electrically millicharged—that is, to have an electronic charge that’s just a tiny fraction of the electron’s. It happens when stateswith each other via a massless hidden photon that oscillates into the SM sector through the photon portal. Millicharged states are extremely difficult tobecause they are only weakly ionizing when they pass through matter. However, their traces could be observed with a dedicatedlike MilliQan, which would be situated in a tunnel, close to one of the main LHC, since the detectors there weren’t primarily designed for such particles. However, if the LLPs decay to charged particles, the tracking systems can reconstruct displaced vertices, 13 et al. (CMS collaboration), Phys. Rev. D 91, 012007 (2015). 2470-0010 13. V. Khachatryan(CMS collaboration),, 012007 (2015). https://doi.org/10.1103/PhysRevD.91.012007 5 hadrons, peculiar configurations of long-lived SM hadron decays, and detector hiccups can conspire to fake a displaced vertex. Such a chain of coincidences can be a problem only because of the enormous rate of SM strong-interaction events, which produce multitudes of hadrons at the LHC. Neutral LLPs are challenging to isolate at thesince thethere weren’t primarily designed for such particles. However, if the LLPs decay to charged particles, the tracking systems can reconstruct displaced vertices,such as is shown in figure. The signals are spectacular, but there is some SM background: Extremely rare events involving production of highly energetic SMpeculiar configurations of long-lived SMdecays, andhiccups can conspire to fake a displaced vertex. Such a chain of coincidences can be a problem only because of the enormous rate of SM strong-interaction events, which produce multitudes ofat the detector size; only a small fraction of those will decay in the detector. The ATLAS detector is able to look for displaced vertices in its largest subdetector, the muon system. If the LHC produces enough LLPs, the ATLAS team can afford for the large majority to escape, and their searches can have sensitivity to decay lengths in the kilometer range. 14 et al. (ATLAS collaboration), Phys. Rev. D 92, 012010 (2015) 2470-0010 ;A. Coccaro et al. , Phys. Rev. D 94, 113003 (2016). 14. G. Aad(ATLAS collaboration),, 012010 (2015) https://doi.org/10.1103/PhysRevD.92.012010 ;A. Coccaro, 113003 (2016). https://doi.org/10.1103/PhysRevD.94.113003 detector, strong-interaction background swamps the signal. How can we probe those especially long-lived particles? The obvious challenges are neutral LLPs with decay lengths much larger than thesize; only a small fraction of those will decay in theThe ATLASis able to look for displaced vertices in its largest subdetector, the muon system. If theproduces enough LLPs, the ATLAS team can afford for the large majority to escape, and their searches can have sensitivity to decay lengths in the kilometer range.But for particles with longer lifetimes and hence lower chances of decaying in the mainstrong-interaction background swamps the signal. How can we probe those especially long-lived particles?