## Wednesday, May 27, 2020

### $$B$$-mesons at LHCb: $$Z'$$-bosons or leptoquarks?

The LHCb experiment

Charlie Wood wrote an intriguing article for the Quanta Magazine,
Growing Anomalies at the Large Hadron Collider Raise Hopes
which mostly covers this March 2020 LHCb paper about the decay of $$B^0\to K^{*0}\mu^+\mu^-$$ which is CP-averaged. The decay involving the muons is easier to measure.

Because the $$N$$-th generation quarks can't "directly" transform to $$N$$-th generation leptons (the three generations of quarks and leptons are "incommensurable" three generations according to the Standard Model), and because the electron and muon masses are negligible relatively to the quark masses, the decays involving electrons and muons should be almost exactly equally frequent, a part of the "lepton universality" which is the opposite of "flavor violation".

But the LHCb detector – normally considered the third most important detector at the LHC, well below the big ATLAS and CMS detectors (but it has a chance to be the first detector that kills the Standard Model) – has been seeing signs of a violation of the lepton universality i.e. signs of the "flavor violation". Electrons and muons seem to behave differently. This is only possible if Nature deviates from the Standard Model, and if it does, the deviations from the Standard Model predictions must be visible with muons only or electrons only (most likely in both channels).

OK, so this paper measured some deviations and described them as 2.8-3.4 sigma deviations in various detailed measurements (Wood uses an amusing new parameterization of the anomalies, namely that you get 65/100 instead of 50/100 heads or tails; sadly, I have very little intuitive anomaly for "how likely it is", I need to calculate it).

These papers present anomalies that are increasingly strong and, even more obviously, increasingly consistent with each other. The anomalies appear at several places that seem to follow from the same new underlying physics – they are apparently signal-like (although one can't be sure that there is a "real signal" beneath these hints).

One theoretical explanation of such a signal, if real, is a $$Z'$$-boson, a new cousin of the neutral $$Z$$-boson that must couple to electrons and muons differently (and that also comes with a new broken gauge symmetry and a new chance to be promoted to the dark matter particle, too). A new force would exist along with some heavy bosonic particle. Another explanation is a new leptoquark, a particle that carries the same quantum numbers as a bound state of 1 lepton and 1 (anti)quark but that is elementary. A leptoquark may merge with a lepton and produce a quark or vice versa.

Of course, the $$Z'$$-boson would be considered more conservative, more elegant, and more minimalistic but I think it's fair to say that leptoquarks are allowed by the most universal laws of Mother Nature. Papers have claimed that leptoquarks may emerge in the string vacua, too. Although I don't have any excessive emotional attachment to leptoquarks (at least so far I would tend to ask "who ordered them?" if they were really discovered), I do believe that those claims (or at least some of them) are correct: both new massive gauge bosons and leptoquarks may emerge according to the best conceptual framework that we have.

You may also check some recent papers by Joaquim Matias, a theorist who proposes models that could explain this new physics if it were new physics at all – plus to connect this new physics to other problems and interesting topics in high-energy physics.