# LHCb presents evidence of rare B decay

A beam of protons enters the LHCb detector on the left, creating a B0s particle, which decays into two muons (purple tracks crossing the whole detector). (Image: LHCb/CERN)

Today at the Hadron Collider Physics Symposium in Kyoto, Japan, the Large Hadron Collider beauty (LHCb) collaboration presented evidence for one of the rarest particle decays ever observed.

The Standard Model of particle physics predicts that the B0S particle, which is made of a bottom antiquark bound to a strange quark, should decay into a pair of muons (μμ) about 3 times in every billion (109) decays. LHCb's measurement, from an analysis of data from 2011 and part of that from 2012, gives a value of (3.2+1.5-1.2) × 10-9.  LHCb spokesperson Pierluigi Campana told the CERN Bulletin that the value is "in very good agreement with the prediction."

Particle physicists describe the certainty of a result on a scale that goes up to 5 sigma. One sigma could be a random statistical fluctuation in the data, 3 sigma counts as evidence, but only a full 5-sigma result is a discovery. The significance of the LHCb measurement is 3.5 sigma and therefore is classified as the first evidence for the B0s →μμ decay.