LHCb reports first evidence of a decay seen once in 10 billion beauty particles

Evidence, not yet a discovery

The collaboration presented the result at a CERN seminar on 3 November. CERN describes it as the first evidence for the decay from a single experiment.

Data collected in 2024 and 2025 alone produced a signal at 3.0 standard deviations, CERN says. Adding the sample recorded between 2011 and 2018 raised the significance to 3.2 sigma. INFN puts that at about a one-in-1500 chance that the signal is a background fluctuation. The team submitted the analysis to Physical Review X and posted it as arXiv:2611.00230. Five sigma is the threshold particle physicists require before claiming a discovery. The collaboration is not claiming one.

The measurement tests the Standard Model at distances far below anything the Large Hadron Collider (LHC) resolves directly. A deviation would point to unknown particles. So far the numbers agree with theory. Confirming the decay needs five sigma. The collaboration says that needs more data.

Why a one-in-ten-billion decay is worth chasing

The Standard Model predicts the Bd to mu mu decay. It happens about once in every 10 billion beauty particles.

The decay turns a beauty particle into a pair of muons. Muons are heavier relatives of the electron. In the new data alone the team measured a branching fraction of about 1.26 in 10 billion, with a statistical uncertainty of roughly 0.5. The Standard Model prediction is about 1.03 in 10 billion, and the combined figure with older data is 1.14. The rarer a decay, the more sensitive it is to particles too heavy to be produced directly in LHC collisions.

The decay probes a billionth of a trillionth of a metre. Physicists have nicknamed that scale the zeptoverse.

Beauty quarks exist for only about one and a half trillionths of a second. Then they decay.

Forty-two years and an upgraded detector

The search began in 1984. That year the CLEO experiment at Cornell set the first upper limit on the decay rate.

A joint LHCb and CMS analysis in 2015 observed Bs to mu mu. It also claimed first evidence of Bd to mu mu. Later updates using data collected up to 2018 did not confirm that evidence. What changed was the detector. The 2024 upgrade lets LHCb collect data at five times the rate achieved between 2010 and 2018. Improved particle identification and machine learning let the team pull the signal out of the background, CERN says. The UK made the largest national contribution to the upgraded detector, UKRI says. Its components were designed and built over more than a decade.

The Vertex Locator is a silicon pixel detector assembled at the University of Liverpool. It sits five millimetres from the LHC beams.

A 19-person international team carried out the analysis. LHCb records hundreds of millions of proton collisions per second, a data flux of 4 terabytes per second. Picking the muons out of the overwhelming background requires extreme precision, said Marco Santimaria, who co-led the team.

The sibling decay gets sharper too

The same analysis measured Bs to mu mu. It is the most precise measurement yet.

The team found a branching fraction of 3.31 per billion, against a Standard Model prediction of 3.66 per billion. Bs to mu mu occurs about three times per billion beauty-particle decays. That makes Bd to mu mu roughly thirty times rarer. The researchers also measured the Bs effective lifetime in this decay at 1.42 picoseconds, with a statistical uncertainty of 0.18. Combined with earlier LHCb data, the lifetime figure is 1.60 picoseconds.

What happens next

The High Luminosity LHC is due to start up in 2030. The 28 countries involved have agreed a 150 million pound upgrade to the detector that would increase the frequency of these decays tenfold, the BBC reported. Last year UK Research and Innovation (UKRI) announced it was withdrawing its share of the money for that upgrade. Tim Gershon, who leads the LHCb international collaboration, told BBC News that the UK's full involvement in the research would be in doubt.

"The UK is at risk of not being involved and making really major discoveries," Gershon said. He spoke to BBC News.

Chris Parkes of the University of Manchester led the collaboration. He led it through construction of the upgrade. He said the team is planning an experiment to exploit the High Luminosity LHC.