Particles known as “bottom mesons” are not decaying in the way the Standard Model of particle physics says they should, and it’s causing some excitement

Measurements made by the LHCb experiment at CERN are showing some anomalies which, if confirmed by more data, would signal the breaking point of our most fundamental description of particle physics to date - the Standard Model.

Using proton collisions from the LHC, LHCb has been carefully measuring the production of bottom mesons and how often they decay to kaon and muon particles. It looks like the answer is: not nearly often enough! In fact, this decay occurs at only about three-quarters of the frequency predicted by the Standard Model.

To be sure, statistical fluctuations or systematic uncertainties in the measurement could account for this deficit. However, the chances are small, on the face of it. If the Standard Model is correct, and you made 16000 copies of our LHC and ran them all in the same way, only about 1 of them would measure the data to be in such bad agreement with the Standard Model by chance alone.

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If this isn’t just bad luck, it could be an opportunity. Deep within the bottom mesons, quantum excitations of new particles could be at work, making the bottom mesons decay with the wrong frequency. Recently, many researchers have looked into what these new particles could be like, and it turns out that there are only two types of exotic particle that can explain the low frequency of these decays: “leptoquarks” or “Z primes”.

Leptoquarks are an exotic hybrid between leptons – particles like the electron or muon – and quarks. “Z prime” is just a name for a particle very like the Z boson which carries the weak fundamental force, but more massive.

We won’t know for sure that one of these is responsible (or which one is) unless we discover the leptoquark or Z prime particles directly.

This is like going back to 2011: there were plenty of indirect indications that the Higgs boson existed, but we wanted to produce it and measure its decay, to be sure the theory surrounding it was correct. Given the potential for a historical revolution in particle physics, we got together with our Cambridge colleague, Ben Gripaios, and asked:

What will it take to directly confirm whether these hypotheses are correct?

The obvious way would be to produce the leptoquarks or Z primes in a colliders such as the LHC. However, it is quite likely that they will be too heavy for the LHC to produce directly in sufficient quantities to detect; through Einstein’s famous equation E = mc2 the LHC just wouldn’t have enough energy available to convert into their large mass.

So we had a look at the plans for future higher-energy colliders at CERN. One plan is to roughly double the LHC energy by ripping out the current magnets in the current 27 km tunnel and putting in more powerful magnets. Another alternative is to enlarge the circular tunnel to 100 km, taking it all the way underneath Lake Geneva nearby, thus increasing the maximum energy attainable for the same magnet power (this is because a larger circular tunnel has less curvature and so requires less energy to bend the colliding particles around a circular trajectory). The result is a super-collider that can achieve around seven times the LHC energy.

The conclusion of our study is that these new colliders should have enough energy to produce and directly discover the Z prime scenario. If instead leptoquarks are responsible for the anomalies, our estimate shows that the future circular collider could discover realistic models of leptoquarks, but there is still the possibility that some more specific cases will still evade detection.

Either way, we won’t know for sure until we build such a high energy collider, and our paper provides a very good extra reason to try to discover what’s behind the anomalies. Of course there is always the chance that the anomalous bottom meson decays only disagree with the Standard Model because of the 1 in 16000 fluke we mentioned above. We will find out in about a year, when the LHCb experiment finish their analysis of the latest data. In any case, we believe that there are already good reasons to build higher energy colliders purely for the curiosity of finding out what lurks beyond the Standard Model. If the history of science has taught us anything, it’s that some new surprise awaits when we push hard against the boundaries of experimental knowledge.

Ben Allanach and Tevong You are theoretical physicists at Cambridge University