Physicists Find Evidence for Ultrarare B⁰ Meson Decay After 40 Years

Physicists at CERN have reported the detection of evidence for an ultrarare meson particle decay, specifically B⁰→ µ⁺µ⁻. This decay occurs when a meson's beauty quark annihilates with a down-type quark, resulting in the pair decaying into a muon and an antimuon, particles akin to electrons. This discovery is crucial for testing theoretical predictions of the Standard Model, with physicists having anticipated this evidence since it was first theorized in 1984.

The team has detailed these measurements, alongside those of a slightly less rare decay, in a recent paper posted to the arXiv preprint server. While the Large Hadron Collider frequently detects B⁰ and B⁰ₛ mesons that undergo various decays, the decay of either particle into a muon-antimuon pair is exceptionally challenging to observe. Standard Model predictions suggest the B⁰ muon-antimuon decay occurs in approximately 1.03 out of 10 billion decays, whereas the B⁰ₛ decay happens about 3.66 times per billion decays. These two-muon decays offer precise theoretical predictions, making them valuable for validating the Standard Model.

The B⁰ₛ decay has been previously observed, initially in a joint analysis by the updated LHCb and the Compact Muon Solenoid (CMS), and again by LHCb in 2017. Although some joint evidence for the rarer B⁰ decay was reported earlier, it lacked confirmation in subsequent analyses. After over 40 years of searching, evidence for the B⁰ decay emerged from proton collisions recorded at the LHCb detector between 2024 and 2025. This new data was combined with earlier LHCb measurements, showing the first single-experiment evidence for B⁰ → μ⁺μ⁻ at 3.0 standard deviations, which increased to 3.2 when combined with prior data.

The combined data indicated a B⁰ decay rate of approximately 1.14 in 10 billion, albeit with substantial uncertainty. For the B⁰ₛ decay, the combined data yielded a rate of about 3.25 per billion decays, establishing it as the world's most precise measurement of this rate. The team also determined the effective lifetime for B⁰ₛ decays into muon pairs to be 1.60 ± 0.16 picoseconds, with a significantly smaller systematic uncertainty. Researchers confirm that all results align with Standard Model predictions and do not suggest any new physics, despite the statistical significance of 3.2 standard deviations falling short of the ideal five for discovery. These findings nonetheless represent a significant experimental milestone, with future collision data expected to solidify the B⁰ decay and refine its measured frequency.

What to watch: Future LHCb data to confirm B⁰ decay and refine its frequency.

Editor's note: The draft effectively covers all technical details, including the particle decay rates, historical context, and statistical significance provided in the source.