Physicists at CERN report that they’ve detected evidence of a rare meson particle decay (B⁰→ µ⁺µ⁻) that occurs when the meson’s beauty quark annihilates with a down-type quark. The pair decays into a muon and antimuon, which are particles similar to electrons. Because evidence for this decay helps test theoretical predictions from the Standard Model, physicists have been waiting for it since 1984, when it was first theorized.
The team details the measurements along with those of another slightly less rare decay in a recent paper posted to the arXiv preprint server.
The search for rare meson decays
The Large Hadron Collider often detects the presence of B⁰ and B⁰ₛ mesons, which can decay into various particles. However, some decays are rarer than others. In particular, the decay of either particle into a muon-antimuon pair is incredibly difficult to detect. According to Standard Model predictions, the muon-antimuon decay of B⁰ occurs in only about 1.03 in 10 billion decays, while the muon-antimuon decay of B⁰ₛ occurs about 3.66 times per billion decays. These two-muon decays have relatively precise theoretical predictions, making them useful tests of the Standard Model.
The B⁰ₛ decay has been observed before. The decay was established first in a joint analysis between the updated LHCb and the Compact Muon Solenoid (CMS) and then again at the LHCb in 2017. While some joint evidence was reported for the rarer B⁰ decay, it was not confirmed in subsequent analyses.
The LHCb gets a lucky break
Finally, after over 40 years, evidence of the rare B⁰ decay showed up in proton collisions recorded at the LHCb detector in 2024–2025. The data were combined with results from the earlier LHCb measurements. Analysis showed the first single-experiment evidence for B⁰ → μ⁺μ⁻ at 3.0 standard deviations in new data, a significance that rose to 3.2 when combined with the earlier LHCb data.
The combined data showed a B⁰ decay rate of around 1.14 in 10 billion, with substantial uncertainty. The combined data showed a B⁰ₛ decay rate of about 3.25 per billion decays—now the world’s most precise measurement of this rate.
The team also determined that the effective lifetime for B⁰ₛ decays into muon pairs is 1.60 ± 0.16 picoseconds, with a much smaller systematic uncertainty. The effective lifetime is the average time an unstable particle exists before it decays. The researchers say that all of the results are in line with Standard Model predictions and don’t point to any new physics.
The ideal statistical significance for these measurements is five standard deviations, while the results fall a bit short here at 3.2. Still, the results provide significant evidence and represent an experimental milestone. Future collision data may establish the B⁰ decay more firmly and sharpen its measured frequency.
Source:phys.org