Mixed Feelings About a Rare Event

One of the most important missions of the Large Hadron Collider at CERN is to search for phenomena that cannot be explained by the standard model of particle physics. In this context, the latest result from the LHCb experiment, now reported in Physical Review Letters, is a bittersweet victory [1]. The LHCb collaboration has, for the first time, observed evidence for the very rare decay of a neutral meson into a pair of muons. Only about one in every 300 million of the meson’s decays happen this way, and it is no small feat that LHCb has been able to detect the few that do. The rate at which the decay occurs also agrees with the value calculated using the standard model, a theoretical success considering the intricacies involved in the calculations. But many particle physicists were hopeful that the agreement between theory and experiment wouldn’t be quite so good, since a deviation would have been a sign that new physics was at play. So far, no such signs are there, but in the future, the precision of the measurement will improve considerably, potentially allowing smaller deviations from the standard model predictions to be detected.