b-quarks and tension in the Standard Model

The big news this week came from an LHCb seminar at CERN. LHCb is an experiment designed specifically to look at the physics of b-quarks. These "bottom" quarks (occasionally also called beauty) are massive, but take a little while to decay. This slight delay relative to other big particles - like top quarks - present physicists with a signature to look for in the otherwise messy data. These "displaced vertices" are used in b-tagging, and greatly facilitate the study of what emerged from those collisions. When the b-quarks finally decay, they decay into familiar particles like electrons and muons, and are historically a great source of precision tests of the Standard Model of Particle Physics.

The tantalizing result published this week is an imbalance just how those b-quarks decay. In some reactions, the standard model predicts no appreciable difference between decays to taus, muons or electrons, (energies not withstanding). Yet LHCb has observed evidence for a difference in rate of these reactions as the b-quarks decay to muons versus electrons.

This difference is wonky, but if the evidence holds up, it requires new physics, beyond the standard model.

Of course, "evidence for" is not the same as a "discovery of". LHCb has tightened the error bars down to "3.1-sigma", which in terms of a normal distribution implies a 96% confidence in the result. Discoveries in particle physics require a 5-sigma deviation from the standard model, which would be closer to 99.97%. Lots of 3-sigma events have turned out to be statistical noise. But given all the new physics that WASN'Tfound at the LHC, we remain hopeful.