Today,in a seminar at CERN, the LHCb collaboration presented new long-awaited resultson a particular decay of B0 mesons produced in collisions at theLarge Hadron Collider. The Standard Model of particle physics predictsthe probability of the many possible decay modes of B0 mesons, andpossible discrepancies with the data would signal new physics.
Inthis study, the LHCb collaboration looked at the decays of B0 mesonsto an excited kaon and a pair of electrons or muons. The muon is 200 timesheavier than the electron, but in the Standard Model its interactions areotherwise identical to those of the electron, a property known as leptonuniversality. Lepton universality predicts that, up to a small and calculableeffect due to the mass difference, electron and muons should be produced withthe same probability in this specific B0 decay. LHCb finds insteadthat the decays involving muons occur less often.
Whilepotentially exciting, the discrepancy with the Standard Model occurs at thelevel of 2.2 to 2.5 sigma, which is not yet sufficient to draw a firmconclusion. However, the result is intriguing because a recent measurement byLHCb involving a related decay exhibited similar behaviour.
Whileof great interest, these hints are not enough to come to a conclusivestatement. Although of a different nature, there have been many previousmeasurements supporting the symmetry between electrons and muons. More data andmore observations of similar decays are needed in order to clarify whether thesehints are just a statistical fluctuation or the first signs for new particlesthat would extend and complete the Standard Model of particles physics. Themeasurements discussed were obtained using the entire data sample of the firstperiod of exploitation of the Large Hadron Collider (Run 1). If the newmeasurements indeed point to physicsbeyond the Standard Model, the larger data sample collected in Run 2 will besufficient to confirm these effects.
Explorefurther:How universal is (lepton) universality?
Moreinformation: LHCb statement: lhcb-public.web.cern.ch/lhcb-public/Welcome.html#RKstar
LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today