LHC Physicists Observe Ultra-Rare Particle Decay

An international team of scientists using the Large Hadron Collider (LHC) at CERN has observed for the first time one of the rarest particle decays ever seen – decay of the B0s particle into two muons.

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 (LHCb / CERN)

Our current knowledge about the most fundamental matter and the forces between them is embedded in the so-called Standard Model of particle physics. The particle masses are a consequence of their interactions with the Higgs field. Exciting the Higgs field in particle collisions at the LHC recently resulted in the discovery of the Higgs boson. However, the Standard Model is not the ultimate theory.

The theory of Supersymmetry is called in to fill some of the gaps of the Standard Model. Since it predicts new phenomena, the theory can be thoroughly tested at the LHC. Physicists predict that the B0s particle, which is made of a bottom antiquark bound to a strange quark, should decay into a pair of muons about 3 times in every billion decays.

The team at the LHC beauty (LHCb) experiment, a gigantic particle detector at one of the collision points on the 27 km LHC collider, has now observed this ultra-rare decay.

“An observation of this very rare decay is a key result that is putting our Supersymmetry theory colleagues in a spin. Results of this quality rely on the dedication and enthusiasm of research post-docs who analyze the data as it pours from the experiment,” said Prof Val Gibson of the University of Cambridge’s Cavendish Laboratory, member of the LHCb team.

The discovery was reported at the Hadron Collider Physics Symposium 2012 in Kyoto, Japan, on November 12, 2012 and published online in a CERN paper.

“The observation is bang on the Standard Model prediction, but comes as very bad news for supporters of Supersymmetry. Indeed, new physics failed to show up where it had the best opportunity.”

“If new physics exists, then it is hiding very well behind the Standard Model,” said Dr Marc-Olivier Bettler of the University of Cambridge, a member of the analysis team.

“Nevertheless, Supersymmetry also benefits from this measurement. This result is important because it tells us what new physics is not.”


Bibliographic information: R. Aaij et al. First evidence for the decay B0s → μ+ μ-. CERN paper, published online 8 November 2012; CERN-PH-EP-2012-335

Albrecht, Johannes. New results on the search for B0s → μ+ μ- from LHCb. Hadron Collider Physics Symposium 2012, Kyoto, Japan, 12 – 16 Nov 2012