LHCb Detects First Single-Experiment Evidence for Bd → μ+μ− Decay at 3.2 Sigma

The upgraded LHCb detector, operating at five times prior luminosity since 2024, collected 35 fb⁻¹ of proton-proton collisions. Machine-learning classifiers isolated muon pairs from combinatorial and partially reconstructed backgrounds. A simultaneous fit to invariant-mass and decay-time distributions yielded 3.2 standard deviations above background-only hypothesis, consistent with the Standard Model branching fraction of (1.0 ± 0.1) × 10⁻¹⁰. This marks the first time a single experiment crossed the evidence threshold for this channel.

Quantum loops make the decay sensitive to virtual particles heavier than those directly produced at the LHC. Any measured deviation from the predicted rate would indicate new physics such as supersymmetric partners or leptoquarks. The result aligns with prior Bs → μ+μ− measurements and tightens constraints on scalar operators in effective field theory.

The analysis remains statistics-limited; reaching 5-sigma observation will require the full Run 3 dataset plus Run 4. Cross-checks with Belle II and future CMS data will be essential to control systematic uncertainties in production cross-sections and fragmentation functions.

Future runs targeting 300 fb⁻¹ will also enable precision angular analyses of related modes, sharpening indirect probes of TeV-scale particles inaccessible to direct searches.