# LHCb at CERN reports first single-experiment evidence of B0→μ+μ– decay at 3σ in Run 3 data

- Published: 2026-11-05 06:08 UTC
- Section: Science › Physics
- Tags: B0→μ+μ– decay, LHCb Run 3, rare B decay, Bs0 effective lifetime, branching fraction, Standard Model test, new physics search, LHCb Upgrade I
- URL: https://realtech.media/news/rt-bb9e29e9d900bc7f0d788b4e_lhcb_at_cern/

**With Run 1 and 2 data the significance is 3.2σ. The most precise Bs0→μ+μ– branching fraction and effective lifetime to date also agree with the Standard Model.**

![LHCb at CERN reports first single-experiment evidence of B0→μ+μ– decay at 3σ in Run 3 data](https://sgzwpqqelzmajkcubgkl.supabase.co/storage/v1/object/public/rt-media/2026/11/fcc866d0f10b6de4.webp)
*ČT24*

The LHCb collaboration at CERN has submitted for publication a paper reporting the first single-experiment evidence for the B0→μ+μ– decay, with a significance of 3σ, according to lhcb-outreach.web.cern.ch. The sample was recorded with the LHCb Upgrade I detector during LHC Run 3, in 2024 and 2025. After combination with Run 1 and Run 2 data, the significance grows to 3.2σ [1]. The result was presented at a CERN seminar that opened the workshop "Implication of LHCb measurements and future prospects".

The same analysis gives the world's most precise measurements of the Bs0→μ+μ– branching fraction and effective lifetime [1]. The lifetime, from the combined Run 1, 2 and 3 data, is 1.60±0.16±0.01 ps, in agreement with the Standard Model prediction [1]. The Standard Model expectation lies inside the 68.27% confidence region of the two-dimensional B0 and Bs0 branching-fraction profile [1].

## Why the decay is studied

The probability of Bs0 and B0 mesons decaying into two oppositely charged muons is very small in the Standard Model and is calculated precisely. Theories with additional Higgs bosons, Z' vector bosons or leptoquarks allow significant changes to this branching fraction, so a deviation would point to new physics [1]. Some such models leave the branching fractions unchanged but alter the decay-time-dependent observables, including the Bs0 effective lifetime.

The Standard Model rate of B0→μ+μ– is about 30 times smaller than that of Bs0→μ+μ–. B0 mesons are produced 4 times more abundantly than Bs0, so the ratio of yields is about 7 [1]. The central value of the branching fraction makes this the rarest B decay measured so far [1].

## History and next steps

The search for Bs0→μ+μ– lasted more than 30 years. LHCb obtained first evidence at 3.5σ in November 2012, and, together with CMS, the first observation at 6.2σ in May 2015. In February 2017 LHCb announced the first single-experiment observation at 7.8σ. The joint LHCb/CMS analysis also reported evidence for B0→μ+μ–, but later analyses did not confirm it [1].

According to the LHCb material, the results narrow the parameter region in which Standard Model extensions can exist. The Upgrade I detector, completed at the turn of 2022 and 2023, records five times more collisions per second than the original detector of 2010–2018. A further LHCb upgrade in the middle of the next decade could raise the data-collection rate by another order of magnitude [1].

## Sources

1. [lhcb-outreach.web.cern.ch](https://lhcb-outreach.web.cern.ch/2026/11/03/evidence-for-the-decay-b0%E2%86%92%CE%BC%CE%BC-and-improved-measurements-of-bs0%E2%86%92%CE%BC%CE%BC/)
2. [ČT24](https://ct24.ceskatelevize.cz/clanek/veda/cern-zachytil-vzacny-rozpad-castic-klepeme-na-dvere-do-zeptovesmiru-raduji-se-vedci-379268)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-bb9e29e9d900bc7f0d788b4e_lhcb_at_cern/