Nationaal instituut voor subatomaire fysica

LHCb presents evidence for rare B decay


Figure 1: LHCb event display of typical rare B decay candidate. A beam of protons enters the LHCb detector on the left, creating a B particle, which decays into two muons, seen as a pair of purple tracks traversing the whole detector. The right panel shows the zoom around the proton-proton collision point, origin of many particle tracks. The muon tracks originate form the B decay point located 14 mm from the collision.


Nikhef news release 

Amsterdam, 14 November 2012 - Evidence for one of the rarest particle decays ever observed, the long-sought decay of a B0s  meson into two muons, was presented by the LHCb collaboration yesterday at the Hadron Collider Particle Symposium in Kyoto, Japan. Nikhef scientists played an important role in this experimental measurement. The results were eagerly awaited by experimental and theoretical physicists and are a crucial milestone for the LHCb experiment. 

Standard Model prediction
The Standard Model of particle physics predicts that the B0s particle, which is made of a bottom antiquark and a strange quark, should decay into a pair of muons (μμ) about 3 times in every billion (109) decays . This Standard Model prediction for the so-called branching fraction was recently updated by the Nikhef theory and LHCb groups to be (3.54 ± 0.30) x 10-9 . (Read according news item in Dutch here, in English here). 

Latest measurement
From an analysis of data from 2011 and part of that from 2012, LHCb observes an excess of B0s →μμ candidates with respect to the number of events you expect for background alone, that is from other processes also resulting in two muons (see also figure 2 below). The measurement corresponds to a so-called significance  of 3.5σ (standard deviation) and therefore is conventionally classified as evidence for the B0s →μμ decay. (Note: A 4σ result is called an observation, a full 5σ result is a discovery). The value for the measured branching fraction is (3.2+1.5-1.2) x 10-9.  

The result, the culmination of nearly 30 years of searching for this extremely rare decay, has been published on arXiv

Consequences for Supersymmetry
The result, although of limited statistical significance, is in agreement with the Standard Model prediction. This is an important step in investigating possible supersymmetric extensions of the Standard Model (SUSY). The B0s →μμ decay namely proceeds via quantum "loops" within which even particles we do not yet know about may circulate, for example supersymmetric particles. These would have an effect on the decay. A measurement of its branching fraction in agreement with the Standard Model prediction thus squeezes strongly the possible parameters of the supersymmetric extensions of the Standard Model (SUSY), most notably of those predicting that this decay would happen far more often.

Prof. Dr. Marcel Merk, programme leader of the LHCb group at Nikhef and professor at the VU University Amsterdam: "With the evidence of the most rare B-decay the Standard model has passed its most stringent test thus far.”

Prof. Dr. Antonio Pellegrino, deputy programme leader of the LHCb group at Nikhef and professor at the University of Groningen: ”The hunt for new physics remains open.”

The precision of these results will be improved using additional data that will be available by the end of this year thanks to the strong and continuous support from the LHC operations team for the LHCb physics program.

More information

Figure 2 (invariant mass plot) 

If a B disintegrates into a muon pair, the invariant mass of the muon pair equals the B mass. The plot shows the muon-pair invariant mass of events classified as "unlikely to be background" (according to a statistical method called a Boosted Decision Tree). The mass resolution is excellent (25 MeV) and the background almost zero. The evidence for the B0s →μμ decay is provided by the dashed red line (the blue line is the total fit, which includes backgrounds). Although the statistical significance is still in the range where fluctuations might be the real source of the bump, LHCb estimated that the probability that the background could produce such an excess is 5.3 x 10-4, corresponding to a signal significance of 3.5 standard deviations.


References and Links

Article on arXiv, "First evidence for the decay Bs -> mu+ mu-", LHCb collaboration

LHCb public webpage



Prof. Dr. Marcel Merk (Nikhef & VU) - email - phone: 020-5925107

Prof. Dr. Antonio Pellegrino (Nikhef & RUG) - email - phone: 020-5925002

Science Communication Nikhef: Vanessa Mexner - email - phone: 020-5925075