The LHCb experiment at CERN has found the first evidence of a particle decay that happens only about once in every 10 billion cases, ending a hunt that began more than 40 years ago. A beauty particle called the B0 meson turns into a pair of muons so rarely that the measured rate came out near 1.26 in 10 billion in the newest data alone. The collaboration reported the result on Nov. 3.
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Evidence is a technical word here. The excess over background reached 3.0 standard deviations in the new dataset, and 3.2 once earlier LHCb data were folded in, which is short of the 5 standard deviations physicists require before calling something a discovery.
A B0 meson decaying into two muons
The decay in question, written B0 to mu-plus mu-minus, sends a meson containing a beauty quark into two muons, the heavier cousins of the electron. According to Sci.News, the Standard Model predicts it at about 1.03 in 10 billion, and the measured value from the new data was about 1.26 in 10 billion with a statistical uncertainty of roughly 0.5. Combined with earlier LHCb runs, the figure moves to about 1.14 in 10 billion, comfortably inside the prediction.
Harry Cliff of the University of Cambridge, who led the analysis team, said the process happens for only one in every ten billion beauty particles, so it is incredibly difficult to find. Marco Santimaria of INFN Laboratori Nazionali di Frascati added that extreme precision is required to pick the muons out of an overwhelming background.
Muons make a convenient signature because they pass through the detector layers cleanly, but they also come from countless other processes. Separating the two-muon signal of a B0 decay from random pairings and from misidentified particles is the central difficulty, and it is why the analysis leaned on machine learning to classify candidate events and on careful modelling of every background source before the final count was made.
Four decades of searching, from CLEO in 1984
The search started in 1984, when the CLEO experiment set the first upper limit on how often the decay could occur. That makes the chase about 42 years by the calendar, and the physicists involved describe it as more than forty. The University of Warwick release quotes Cliff saying that seeing something people had been searching for since before he was born was a real privilege, and quotes Tim Gershon of Warwick, who leads LHCb, calling the result a showcase of the experiment’s ability to probe nature at the fundamental level. Santimaria put the history in plain terms: physicists have been chasing this decay for more than forty years.
What changed was data and technique. The team combined proton collision data recorded in 2024 and 2025 with data from 2011 to 2018, and used machine learning to pull a faint signal out of the noise. The paper, posted to arXiv as 2611.00230, uses about 18 inverse femtobarns of data from the upgraded detector running at 13.6 TeV, and has been submitted to Physical Review X.
The sibling decay and what a Standard Model match means
The same analysis also improved the measurement of a close relative, the B0s meson decaying to two muons. Its branching fraction came out at 3.31 in a billion, against a Standard Model prediction of 3.66 in a billion, which the authors call the most precise determination to date. Its effective lifetime was 1.42 picoseconds in the new data and 1.60 picoseconds combined with earlier results, a quantity that can reveal contributions from new physics.
All the measurements agree with the Standard Model. The paper states that conclusion directly. That is not a letdown for the field: rare decays are sensitive to possible new particles and forces, so a tight match narrows the room those hypothetical particles have to hide. The detector that did the work is described on the LHCb collaboration’s public site, and it sits on the ring of CERN’s Large Hadron Collider.
Guy Wilkinson of the University of Oxford said the announcement opens a new era of discovery for the experiment, as the upgraded detector keeps collecting data. Run 3 of the collider, the period that produced the 2024 and 2025 sample, brought a higher collision rate and a rebuilt detector able to read out events far faster than the machine used in earlier years, which is what makes a signal this faint reachable at all.
The open question is whether the B0 rate stays near the Standard Model value or drifts away from it as the sample grows. At 3.2 standard deviations, a few more years of data decide whether this is a confirmed measurement or a statistical flicker, and LHCb has not yet said when it expects to cross 5 standard deviations. Any sustained excess above the roughly 1 in 10 billion Standard Model expectation would be among the first hints of physics beyond it, which keeps this single, rare decay on the list of measurements the field watches most closely.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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