LHCb experiment finds evidence for ultra-rare B_d decay — ScienceDaily
Physicists from the LHCb experiment at CERN have obtained evidence for the exceptionally rare decay of a B_d particle into a pair of muons — μ+μ−. Such a process is expected to occur approximately once in every 10 billion decays of beauty particles. ScienceDaily reported on the results, announced during a seminar at CERN.
The B_d particle, according to the researchers’ description, contains a beauty quark and a down antiquark. During the rare decay, they annihilate, and their energy is converted into a muon and an antimuon. Muons are similar to electrons but have greater mass.
Statistical evidence, not a confirmed observation
An analysis of data collected in 2024 and 2025 yielded a signal statistical significance of 3.0 standard deviations. After combining the results with data from 2011–2018, the figure rose to 3.2 standard deviations. In particle physics, a level of three standard deviations is considered evidence of a new phenomenon; the probability of a random fluctuation at this result is approximately 0.3%.
At the same time, a confirmed observation requires a statistical significance of five standard deviations. Previously, a joint LHCb and CMS analysis in 2015 reported initial evidence for this decay, but subsequent studies using data through 2018 were unable to confirm it.
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Upgraded detector and machine learning
The breakthrough was aided by the LHCb upgrade: since 2024, the experiment has begun collecting data five times faster than in 2010–2018. The detector also improved particle identification. Researchers used machine-learning methods to separate the rare signal from the large number of other collision processes.
The results are consistent with the Standard Model, the leading theory of fundamental particles and interactions. Such rare decays may be sensitive to the influence of as-yet unknown heavy particles that cannot be produced directly at the Large Hadron Collider. The study also provided the most precise measurements of the related B_s → μ+μ− decay, which occurs about three times per billion beauty-particle decays.
The launch of the High Luminosity LHC is planned for 2030, while the next LHCb upgrade is planned for the middle of the next decade. This is expected to increase the data collection rate by another factor of ten.
