LHCb Finds First Evidence of the Rarest Beauty-Particle Decay Yet Seen at the LHC

Physicists at CERN's Large Hadron Collider have found the first evidence of an extraordinarily rare particle decay that researchers have been hunting for more than four decades: a B0 meson, a particle containing a beauty quark, turning into a pair of muons.

The LHCb collaboration announced the result on Nov. 3. The new data show an excess of events over background at 3.0 standard deviations, rising to 3.2 when combined with earlier LHCb data. That counts as evidence, not a discovery. Particle physicists reserve the word discovery for five standard deviations.

"This process only happens for one in every ten billion beauty particles, so it's incredibly difficult to find," said Harry Cliff of the University of Cambridge's Cavendish Laboratory, who led the analysis. "To see something that people had been searching for since before I was born was a real privilege." The search began in 1984, when the CLEO experiment set the first upper limit on how often the decay could happen.

The measured branching fraction, the share of B0 mesons that decay this way, is about 1.26 in ten billion from the new data alone, with a statistical uncertainty of roughly 0.5 in ten billion. The Standard Model predicts about 1.03 in ten billion. Combined with older data, the figure is 1.14 in ten billion. In other words, the result sits comfortably on the prediction.

The same paper also improves the measurement of the closely related decay of the B0s meson into two muons. Its branching fraction came out at 3.31 parts in ten billion against a Standard Model prediction of 3.66, the most precise determination to date. The collaboration also measured the effective lifetime of the B0s decay at 1.42 picoseconds from the new data and 1.60 picoseconds when combined with earlier data. That quantity can reveal contributions from new physics.

Beauty particles typically live only about 1.5 trillionths of a second before decaying, and the collider can probe distances of around 100 zeptometers, which is why the Cambridge team talks about a window into the "zeptoverse." Rare decays are a favored hunting ground for new particles because undiscovered heavy particles could slightly raise or lower the rate through quantum loops.

The result has not changed the picture so far. It matches the Standard Model, and a decay this rare will need much more data to measure precisely. The paper, "Evidence for the decay B0→μ+μ− and improved measurements of the decay B0s→μ+μ−," is in press at Physical Review X and posted on arXiv as 2611.00230. Researchers from the UK, Germany, Italy, Spain and the Netherlands worked on the analysis led from Cambridge.