The decay of a Bs (B-sub-s) particle into two muons is one of the "rarest measureable processes in physics."
How rare? Only a handful of Bs particles per billion will decay into pairs of muons, as The Standard Model of particle physics predicts. That prediction was put to the test by scientists at CERN, who measured a process that was described as one of the "most stringent tests to date" of the Standard Model.
“This is a process that particle physicists have been trying to find for 25 years,” said spokesperson Joe Incandela. “It demonstrates the incredible capability of the LHC and experiments like CMS that are able to detect such a rare process involving a particle with a mass that is roughly 1000 times smaller than the masses of the heaviest particles we are searching for now.”
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