Physicists working at CERN’s Large Hadron Collider have captured evidence of an extraordinarily rare particle decay that occurs roughly once in every ten billion events, crossing a threshold that researchers have chased for more than four decades, according to Home.
Capturing Rare Particle Decays at The Large Hadron Collider
The measurement centers on the LHCb experiment, where a neutral B0 meson transforms into a muon and an antimuon. This transformation is predicted by the Standard Model to happen just once in every 10 billion decays, according to WIRED. Because the probability depends on the square of CKM matrix elements, the decay is heavily suppressed through an effect known as CKM suppression, making it exceptionally difficult to isolate from overwhelming background noise in proton-proton collisions.
Work on this scientific pursuit commenced prior to Tim Berners-Lee establishing the World Wide Web at CERN back in 1989. The hunt for this process started back in 1984, when the CLEO experiment set the initial upper limit on its rate. The Large Hadron Collider is a vast machine situated at the European Laboratory for Particle Physics on the Swiss-French border, which smashes atoms together to break them apart and discover what is inside them. More than 40 years later, an international team has successfully tracked the decay of a neutral B0 meson into a muon and an antimuon, written as B0→μ+μ−. The research team posted a preprint of the paper on arXiv on October 30, 2026, and CERN along with the LHCb collaboration formally announced the result on November 3. The paper was submitted to the journal Physical Review X.
The work was signed by the entire LHCb collaboration, with R. Aaij and colleagues listing Marco Santimaria as co-leader of the 19-person international analysis team. Santimaria noted in a press release from CERN that researchers had been chasing this decay for more than 40 years, emphasizing that extreme precision is required to pick the muons out of the overwhelming background. UK researchers contributed to the Ring Imaging Cherenkov system, known as the RICH system, which identifies different types of particles. Nearly 90 British companies took part in the construction, helping train a new generation of scientists and engineers along the way.
To reach the result, the research team combined newly collected data from 2024 and 2025 with older datasets gathered between 2011 and 2018. Ever since the enhanced LHCb detector initiated data collection in 2024, it has achieved a collision recording frequency up to five times greater than its former capacity, providing investigators with an expanded particle pool to analyze. Through enhanced particle classification and machine-learning methodologies, the team successfully filtered out the heavy noise from extraneous processes that would have otherwise obscured the signatures of these rare beauty-particle transformations. Evaluated solely on the newly acquired data, the analysis produced a branching fraction amounting to roughly 1.26 per 10 billion, accompanied by a statistical uncertainty of approximately 0.5 per 10 billion. The Standard Model prediction sits at about 1.03 in 10 billion, and the combined result with older data is 1.14 in 10 billion. The resulting statistical significance reached 3.2 sigma, meeting the threshold required to claim evidence of the phenomenon. In 2015, LHCb and CMS published a joint analysis of Run 1 data in Nature reporting 3.0 sigma evidence, but subsequent updates using data collected up until 2018 did not maintain that significance.
Testing The Limits of Particle Physics
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, of Cambridge University who led the analysis of the data. We’re knocking on the door of this zeptoverse.
Cliff added that seeing clear evidence for this rarest decay for the first time was thrilling, noting that physicists had been looking for this rare type of decay since 1984, which was before he was born. The sub-atomic dance provides researchers with a ringside seat to the behaviour of possible new particles. Lasting merely one and a half trillionths of a second prior to its disintegration into alternative configurations, the unstable particle decays whilst engaging with other microscopic entities across a magnitude of a billion-trillionth of a metre.
The zeptoverse describes a scale of nature involving distances as small as a zeptometre, which measures $10^{-21}$ meters, or one billionth of a trillionth of a meter. Exploring this subatomic domain gives researchers a direct test of the Standard Model and a way to search for undiscovered particles or forces. Highlighting the significance of these findings, Niladri Sahoo, a particle physicist at the University of Birmingham alongside his role as convener of the LHCb Very Rare Decays working group, noted that this prospective discovery—coupled with a much more refined calculation of a previously cataloged sibling decay—offers both an uncompromising evaluation of the Standard Model and a gateway to prospective novel physics.
Alongside the primary measurement, the analysis produced high-precision determinations of the sibling decay involving the heavier B0s meson, which was first observed in 2015. Researchers measured a branching fraction of 3.31 in a billion for the B0s meson against a Standard Model prediction of 3.66 in a billion, calling it the most precise determination to date. They also measured the B0s meson’s effective lifetime in this decay, finding a result of 1.42 picoseconds with a statistical uncertainty of 0.18, which combined with earlier LHCb data totals 1.60 picoseconds. Furthermore, the ratio of these specific decay rates was computed utilizing the formula $mathcal{R}_{mumu}equiv frac{mathcal{B}(B^0tomu^+mu^-)} {mathcal{B}(B_s^0tomu^+mu^-)} =0.035^{+0.014+0.002}_{-0.012-0.001}$, while theoretical calculations from the Standard Model anticipate a value of 0.0280±0.0016, presenting no statistically notable variance.
Researchers Question Future Participation After Funding Withdrawal
While the analysis represents a milestone for research teams supported by the Science and Technology Facilities Council, questions remain over future participation. Expressing enthusiasm for the scientific milestone whilst concurrently criticizing the choice to withdraw financial backing from the UK portion of the CERN venture, University of Manchester particle researcher and television presenter Prof Brian Cox—who operates outside the core analysis team—characterized the findings as profoundly significant.
A UKRI spokesperson stated that they sought independent advice and took forward an exciting portfolio of new infrastructure investments including clean energy, medical imaging, and cultural heritage, though as usual, there were more exciting applications for funding than they were able to support. Despite these budgetary hurdles, the 28 countries involved in the project have agreed to a £150m upgrade to the LHCb detector that would increase the frequency of these decays tenfold, allowing researchers to probe deeper inside the zeptoverse into the early 2030s.
Earlier coverage: CERN Physicists Spot 10-Billion-to-1 Particle Decay After 42-Year Hunt
