Physicists with the LHCb Collaboration at CERN’s Large Hadron Collider announced today evidence for an extremely rare particle decay that scientists have chased for more than four decades, and their result agrees with the Standard Model of particle physics.
The LHCb magnet. Image credit: CERN.
“What we found in these latest results is an incredibly rare decay of one of these beauty particles, where it turns into two even smaller particles called muons,” said Dr. Harry Cliff, a physicist at the University of Cambridge’s Cavendish Laboratory.
“This process only happens for one in every ten billion beauty particles, so it’s incredibly difficult to find.”
The decay involves the B0 (B0d or Bd) meson, a short-lived particle containing a beauty quark, turning into a pair of muons, the heavier cousins of the electron.
The Standard Model predicts this should happen roughly once in every 10 billion B0 decays.
The search began in 1984, when the CLEO experiment set the first upper limit on its rate.
In a paper submitted to the journal Physical Review X, Dr. Cliff and his colleagues reported an excess of such events over the expected background with a statistical significance of 3.0 standard deviations.
Combined with earlier data from the LHCb experiment, the significance rises to 3.2 standard deviations.
“When we saw the result for the first time, it was a really thrilling moment,” Dr. Cliff said.
“To see something that people had been searching for since before I was born was a real privilege.”
“We’re knocking on the door of the Zeptoverse now. I can’t wait to see what we find as we explore deeper.”
The LHCb team measured the decay’s branching fraction, the share of B0 mesons that decay this way, at about 1.26 in 10 billion in the new data alone, with a statistical uncertainty of roughly 0.5 in 10 billion.
The Standard Model prediction is about 1.03 in 10 billion. The combined result with older data is 1.14 in 10 billion.
“Physicists have been chasing this decay for more than forty years,” said Dr. Marco Santimaria, a physicist at the INFN Laboratori Nazionali di Frascati.
“Extreme precision is required to pick the muons out of the overwhelming background.”
The researchers also measured a sibling process, the decay of the heavier B0s meson into two muons, which is more common but still extraordinarily rare.
They found a branching fraction of 3.31 in a billion, against a Standard Model prediction of 3.66 in a billion, and said it is the most precise determination to date.
They also measured the B0s meson’s effective lifetime in this decay, a quantity that can reveal contributions from new physics.
The new result is 1.42 picoseconds (trillionths of a second) with a statistical uncertainty of 0.18; combined with earlier LHCb data it is 1.60 picoseconds.
“LHCb is a great success story of CERN, and one in which Oxford has been proud to play a leading role, dating back to the inception of the project in 1995,” said Professor Guy Wilkinson, a physicist at the University of Oxford.
“Today’s announcement ushers in a new era of discovery for the experiment, and vividly demonstrates the increased sensitivity we have attained in our quest to probe beyond our current understanding of physics at the most fundamental level.”
“This fantastic result showcases the extraordinary capabilities of LHCb to probe nature at the fundamental level,” said Professor Tim Gershon, a researcher at the University of Warwick.
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R. Aaij et al. (LHCb Collaboration). 2026. Evidence for the decay B0→μ+μ− and improved measurements of the decay B0s→μ+μ−. Physical Review X, in press; arXiv: 2611.00230
