One in 10 billion: the decay physicists chased for 40 years

Physicists at CERN's LHCb experiment have found evidence of a particle decay expected just once in 10 billion decays, sharpening a test of the forces that govern matter. ^[1][2]

The collaboration announced on November 3 that it had detected a signal for a neutral beauty particle, called Bd, transforming into two muons. It is the first time a single experiment has reported evidence for this process. ^[1][2]

The result reaches 3.2 standard deviations, above the threshold particle physicists commonly use for evidence but below the five required to claim a discovery. Its measured behaviour agrees with the Standard Model, the theory describing known elementary particles and their interactions. ^[1][2][3]

Such rare decays offer a way to search indirectly for particles too massive for the Large Hadron Collider to produce. Their effects could alter how often the decay happens. By measuring that rate, researchers can narrow the room for new physics even when they find no departure from the existing theory. ^[1][2]

Marco Santimaria, a co-leader of the 19-person international analysis team, said: "Physicists have been chasing this decay for more than forty years". He added: "Extreme precision is required to pick the muons out of the overwhelming background." ^[1]

Two muons, an enormous search

The Bd particle contains a beauty quark and a down antiquark. In the decay under study, those constituents annihilate, turning their energy into a muon and an antimuon. Muons are heavier relatives of electrons. ^[1]

The visible end product is a pair of particles. The difficulty is finding the right pair amid the far larger number of other events recorded by the detector. LHCb used improved particle identification and machine-learning methods to separate the rare signal from that background. ^[1][3]

Its upgraded experiment began collecting data in 2024 at five times the rate achieved between 2010 and 2018. That increase gave researchers more chances to catch a process whose predicted frequency makes even a huge collection of decays a demanding place to look. ^[1]

Data collected in 2024 and 2025 produced a signal at 3.0 standard deviations. Combining those results with earlier data from 2011 to 2018 raised the significance to 3.2. The new data therefore reached the usual evidence threshold on their own, with the older measurements strengthening the combined result. ^[1][2]

Statistical significance measures how strongly a signal stands out against the explanation that background fluctuations produced it. It is not a measure of how important the physics would be. Here, the distinction between evidence and discovery sets the next task: accumulate enough data to make the signal substantially harder to explain away.

Harry Cliff, LHCb's Rare Decays Convener and an analysis co-lead, told the University of Oxford that seeing the result for the first time was "a really thrilling moment". He said it was a privilege to see a process that researchers had been searching for since before he was born. ^[3]

A result with a history of retreat

The search has already had one promising result that failed to hold up. In 2015, a joint analysis by LHCb and CMS, another Large Hadron Collider experiment, reported evidence for the Bd decay. Later analyses using data through 2018 did not confirm it. ^[1][2]

That history makes the new single-experiment evidence a milestone worth marking precisely. LHCb has crossed the evidence threshold with its updated measurements. The collaboration has not announced a discovery. ^[1][2]

The related Bs particle provided an earlier success. LHCb reported evidence for its decay into two muons in 2012, and the joint LHCb-CMS analysis announced its observation in 2015. The Bd process is predicted to happen around 30 times less often than that Bs decay, making it a much harder target. ^[1][2]

The new analysis also delivered what LHCb describes as the most precise measurements yet of the Bs decay into two muons and its effective lifetime, a measure of the timing of those decays. ^[2]

LHCb measured that lifetime at 1.60 picoseconds, with quoted uncertainties of 0.16 and 0.01 picoseconds. A picosecond is one trillionth of a second. The result is consistent with the Standard Model prediction. ^[2]

Measuring both processes gives physicists more than a single rare event to pursue. The collaboration says the findings constrain possible models of new physics, limiting which proposed additions to the Standard Model can fit the measurements. ^[2]

The next test needs more data

LHCb said on November 3 that it had submitted a paper for publication and presented the findings at a CERN seminar. The announcement did not establish a new particle or force. Publication and peer-review status since then, the precise Bd decay rate and the number of candidate events behind the fitted signal are not established here. ^[1][2][3]

The immediate scientific prize is a more precise measurement of the rate. A rare decay can reveal an unseen influence only if researchers can distinguish a real departure from the prediction from uncertainty in the measurement. For now, the agreement with the Standard Model restricts the alternatives. ^[1][2]

More powerful equipment is planned. CERN says the High Luminosity LHC is due to begin operating in 2030, with another LHCb upgrade planned for the middle of the following decade. Together, the upgrades could raise the experiment's data-collection rate by a further factor of 10. ^[1]

Cliff said a future LHCb upgrade would allow researchers to observe more of these decays. After a search lasting more than four decades, the next job is to turn evidence into a measurement precise enough to put the theory under greater strain. ^[1]