LHCb experiment finds evidence of rare particle decay after 40-year search

Evidence of a rare decay

Researchers working with the LHCb experiment at CERN have detected evidence of a particle decay expected to occur only once in approximately 10 billion beauty particle decays. The finding was announced during a seminar at CERN and marks the first time a single experiment has produced sufficient evidence for this extraordinarily rare event.

The decay involves a particle called Bd, which contains a beauty quark and a down antiquark. Scientists found evidence that this particle can decay into a pair of muons (μ+μ-). Muons are similar to electrons but have greater mass. The results agree with predictions from the Standard Model, the leading theory describing fundamental particles and their interactions.

Statistical significance and detector upgrades

In particle physics, scientists use a statistical measure called standard deviations to evaluate how likely an apparent signal is to result from random fluctuations rather than a genuine physical process. A result must reach three standard deviations to qualify as evidence of a new phenomenon. At that level, the probability of random fluctuations producing a signal at least as statistically extreme is approximately 0.3%.

Using data collected during 2024 and 2025, the LHCb researchers identified evidence of Bd → μ+μ- at a significance of 3.0 standard deviations. When they combined those measurements with earlier data gathered between 2011 and 2018, the statistical significance increased to 3.2 standard deviations, exceeding the widely accepted threshold for evidence.

A significant improvement to the LHCb experiment helped researchers overcome the difficulty of detecting such a rare event. Following a major upgrade, the experiment began collecting data in 2024 at five times the rate achieved between 2010 and 2018. The upgraded detector also became better at identifying different kinds of particles. The team combined these improvements with machine learning techniques to distinguish the exceptionally rare signals from the vast number of other particle processes.

"Physicists have been chasing this decay for more than forty years," said Marco Santimaria, who co-led the international analysis team of 19 scientists. "Extreme precision is required to pick the muons out of the overwhelming background."

Why it matters

The decay known as Bd → μ+μ- belongs to a special category that physicists have long considered 'golden channels' for investigating physics beyond the Standard Model. By measuring extremely rare decays with high precision, researchers can look for differences between experimental results and theoretical predictions. Such discrepancies could provide indirect evidence of previously unknown particles.

Although the new findings represent a major achievement, physicists have not yet reached the threshold required to declare a confirmed observation. That milestone requires a significance of five standard deviations. If future measurements achieve that level of certainty, Bd → μ+μ- would become the rarest beauty particle decay ever observed.

The High Luminosity LHC, also known as the HiLumi LHC, is scheduled to begin operating in 2030. Another major upgrade to the LHCb experiment is planned for the middle of the following decade. Together, these developments could eventually increase the experiment's data collection rate by another factor of ten.

Sources

• sciencedaily.com: After 40 years, CERN finds evidence of a one-in-10-billion particle decay | ScienceDaily
• yahoo.com: After 40 years, CERN finds evidence of an extraordinarily rare particle decay
• tech.news.am: CERN obtains first compelling evidence of a particle decay that occurs once in 10 billion cases | NEWS.am TECH - Innovations and science

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