https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464
In: Physics
In the standard model, electrons belong to a family of particles called leptons, which also includes the mu and the tau. The three essentially behave identically – the only difference is their mass.
What was observed here is that b quarks decay into electrons more often than they decay into muons, which is odd because electrons and muons should behave essentially the same. Having such a discrepancy is not inherently a problem with SM, since we know a lot of reasons why particles would prefer to decay to one lepton over another. However, all such examples are relatively easy to explain by combining our understanding of the weak force with basic physics principles. For example, even though the tau largely behaves the same as the other two, it is the most massive, so it can only appear in high energy decays based on energy conservation arguments. A more sophisticated example would be that charged pions decay to muons more than electrons. This is because this decay occurs by the weak force, and the weak force has a preference for certain angular momentum configurations, so we can explain this based on angular momentum arguments. The main point is that all of this “lepton physics” is well understood and has been studied for a long time.
What’s different about this is that they ALSO tried to apply lepton physics to this, but it didn’t work. Hence, they were forced to conclude that the discrepancy is due to the b quark simply having a greater affinity for the electron than the muon. This is surprising because there’s no reason for the b quark to decide among the leptons which one it likes. After all, the b quark mostly follows quark physics, which doesn’t even interact with leptons. Moreover, since the b quark is over 40 times bigger than either particle, the mass difference (that is, the only difference) should be negligible. So, apparently something from quark physics can distinguish leptons, even though as far as we know leptons don’t even appear in quark physics. This is the idea of the contradiction, and it can’t be put to rest quite yet because there’s a lot of quark physics we don’t know.
tl;dr e and μ are similar particles called leptons, which follow lepton physics (easy). b is a large particle that follows quark physics (hard). Lepton and quark physics should be 100% independent. However, we suspect that b prefers e over μ. If correct, the discrepancy can’t be explained by lepton physics, so it lies in quark physics, which mean that leptons are appearing in quark physics when they shouldn’t. This makes quark physics harder, but also more interesting.
(edited for typos, some sentence structure, and the tldr)
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