The CMS (Compact Muon Solenoid) and LHCb (Large Hadron Collider beauty) collaborations at CERN’s Large Hadron Collider in Switzerland report the first observation of an extremely rare decay of the B s 0 particle – a heavy composite particle consisting of a bottom antiquark and a strange quark – into two muons. The Standard Model predicts that this rare process happens about 4 times out of a billion decays, but it has never been seen before.

“The discovery came about when two Large Hadron Collider (LHC) experiments recently combined their results and found overwhelming evidence of an extremely rare decay of a particle known as the B s 0 meson,” explained Prof Sheldon Stone of Syracuse University.

“The findings not only provide an indirect way to test new models of new physics, but also shed light on the Standard Model, the theory that best describes the world of particles.”

The findings, published in the journal Nature, are based on data taken at the LHC in 2011 and 2012. These data also contain early hints of a similar, but even more rare decay into two muons of the B0, a cousin of the B s 0.

The B s 0 and B0 are mesons, in other words, non-elementary unstable subatomic particles composed of a quark and an antiquark, bound together by the strong interaction. Such particles are produced only in high-energy collisions – at particle accelerators, or in nature, for example in cosmic-ray interactions.

Physicists at CERN study the properties of particles to search for cracks in the Standard Model, which is known to be incomplete since it does not address issues such as the presence of dark matter or the abundance of matter over antimatter in the Universe. Any deviations from this model could be evidence of new physics at play, such as new particles or forces that could provide answers to these mysteries.

“Many theories that propose to extend the Standard Model also predict an increase in the B s 0 decay rate,” said Dr Joel Butler of Fermilab (Fermi National Accelerator Laboratory), a member of the CMS collaboration. “This new result allows us to discount or severely limit the parameters of most of these theories. Any viable theory must predict a change small enough to be accommodated by the remaining uncertainty.”

“Researchers at the LHC are particularly interested in particles containing bottom quarks because they are easy to detect, abundantly produced and have a relatively long lifespan. We also know that B s 0 mesons oscillate between their matter and their antimatter counterparts, a process first discovered at Fermilab in 2006,” Prof Stone added.

“Studying the properties of B mesons will help us understand the imbalance of matter and antimatter in the Universe.”

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