LHCb sees the B0 meson decay to two muons at 3.2 sigma

It is enough for evidence, not for discovery.

According to the LHCb collaboration, this is the first time a single experiment has produced evidence for this decay. For theorists, a prediction chased for four decades now has a measurement attached to it. A channel long seen as a golden channel in the search for new physics is now measurable. Any future gap between the measurement and the prediction would point to particles beyond the Standard Model.

Evidence, not a discovery

Sci.News reported on 3 November that the new data alone give a significance of 3.0 standard deviations. Combined with earlier LHCb data, that rises to 3.2. Both figures sit below the five-sigma threshold that particle physics requires before anyone says discovery, as the University of Liverpool and UKRI noted. INFN put it in plainer terms: about a one in 1,500 chance that background fluctuations produced the signal.

The analysis was submitted to Physical Review X. Cambridge's Cavendish Laboratory led the analysis team. It noted that the results have not yet been peer-reviewed.

Why a rare decay is worth chasing

Muons are the heavier cousins of the electron.

Rarity is the point. According to the University of Manchester, the rarer a decay is, the more sensitive it can be to the influence of new particles. Such particles could alter how often it occurs. Beauty quarks are made in huge numbers at the LHC but live for about one and a half trillionths of a second. Physicists therefore measure how they fall apart rather than the quarks themselves.

The hunt is old. The CLEO experiment at Cornell set the first upper limit on the decay's rate in 1984, according to Sci.News.

The LHC resolves structure down to about a hundred zeptometres, Cambridge says. New particles, if they exist, may sit at a zeptometre, a trillionth of a billionth of a metre. LHCb cannot see that far directly. It looks for the interference such particles would leave in rare decays.

How the measurement was made

The upgraded LHCb detector records collisions at up to five times its previous rate. The larger sample made the analysis possible.

The team combined 2024 and 2025 data with earlier runs from 2011 to 2018, UKRI said. It used improved particle identification and machine-learning techniques. These separate the decay from the far larger background of ordinary collisions. LHCb records hundreds of millions of proton collisions per second, a data flux of 4 terabytes per second, according to INFN.

The Vertex Locator, a silicon pixel detector assembled at the University of Liverpool, sits five millimetres from the LHC beams and forms the first layer of the tracking system. It lets physicists pinpoint where short-lived particles decay. About 20 physicists from the UK, Germany, Italy, Spain and the Netherlands worked on the analysis, Cambridge said.

The numbers against the Standard Model

The new data give a branching fraction of about 1.26 in 10 billion, with a statistical uncertainty of roughly 0.5 in 10 billion. The Standard Model prediction is about 1.03 in 10 billion. Combined with older data, the measured value is 1.14 in 10 billion.

The team also remeasured a sibling process, the B0s meson decaying into two muons, which LHCb first observed in 2015. It found a branching fraction of 3.31 in a billion against a prediction of 3.66 in a billion. The team called it the most precise determination to date. The B0s effective lifetime in that decay came out at 1.42 picoseconds with a statistical uncertainty of 0.18. Combined with earlier data, it is 1.60 picoseconds.

All of it agrees with the Standard Model, within uncertainties.

What more data would settle

More collisions are the obvious next step. INFN says the next LHCb upgrade will increase the data collection rate by a factor of ten over the next decade.

Professor Chris Parkes of the University of Manchester led the collaboration through the construction of the upgrade. He said the team is planning an experiment to exploit the High Luminosity LHC. He also said the UK made the largest national contribution to the upgraded detector. It is a fifteen-year project from a 1,900-member collaboration.

Dr Harry Cliff of Cambridge's Cavendish Laboratory led the analysis. "When we saw the result for the first time, it was a really thrilling moment," he said, according to Sci.News. "To see something that people had been searching for since before I was born was a real privilege."

Marco Santimaria of INFN co-led the team. "Extreme precision is required to pick the muons out of the overwhelming background," he said.

Whether the rate stays where it is, or drifts toward something the Standard Model cannot explain, is now a question of statistics. The answer is not in.