Nope, Still No LeptoQuarks!

Lepton Flavor Universality maintained in Top Decays.

What is a Lepton?
The most famous, well used and well known elementary particle is probably the electron. It might also be the most sterile. Electronic signals are clean, easily identifiable have been measured extensively.

Electrons have cousins that share their simple aesthetic: the ever-present muon, the ginormous tau and the ghostly neutrino. Together they comprise the family of particles known as leptons. Leptons by definition do not participate in the strong nuclear force.

Lepton Flavor Universality
Part of what makes leptons so clean is their familal discipline. They all seem to interact with the other forces of nature in precisely the same way. For example: they all have the same electric charge. This "flavor universal" behavior extends to include the primordial electroweak force, from which both the electric and weak nuclear forces derive. The leptons are nothing if not organized.

Or so we thought.

Recent Experiments give Conflicting Results
There are plenty of reasons to expect small deviations from this so-called Lepton Flavor Universality. We briefly mentioned one of them back in March. Leptoquarks - which would NOT be directly observable at the LHC - could provide a mechanism to untidy that lepton flavor universality.

Back in March, the LHCb experiment reported results consistent with a slight violation of lepton flavor universality - at a significance of about 3 standard deviations. The gold standard of discovery in particle physics is 5, so it’s entirely possible that LHCb’s result is a statistical fluctuation - akin to flipping a coin “heads” 8 times in a row. Unlucky but not impossible.

The ATLAS experiment - another experiment on the LHC beamline - recently published findings that are more consistent with lepton universality.

The ATLAS result descends from decays of the top quark, which are produced in copious amounts by the LHC. These tops decay into W bosons which in turn decay into taus or muons. The measurement in question is the rate at which W’s decay to either of those two leptons.

The LHCb result from March studied the decay of bottom quarks (whence the b in the experiment name). So in principle they’ve measured distinct physical effects.

The resolution will only come from more data collection - and possibly more precision experiments.