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T: Fachverband Teilchenphysik

T 72: Flavour Physics IV

T 72.6: Talk

Thursday, March 19, 2026, 17:30–17:45, KH 01.011

Branching fraction measurements of the rare decays B_(s)⁰ → π⁺ π⁻ µ⁺ µ⁻ at LHCb — Johannes Albrecht¹, Thomas Blake², Mick Mulder¹, Janina Nicolini³, and •Jan Peter Wagner¹ — ¹TU Dortmund University, Dortmund, Germany — ²University of Warwick, Coventry, United Kingdom — ³CERN, Geneva, Switzerland

Rare flavour-changing neutral-current decays with b → q ℓ⁺ ℓ⁻ quark transitions (q=s,d) provide powerful probes of the Standard Model and are central to the LHCb physics programme. Owing to the smaller CKM matrix elements, b → d ℓ⁺ ℓ⁻ decays are further suppressed relative to b → s ℓ⁺ ℓ⁻ processes, making them particularly challenging to observe. Using Run 1 data, the LHCb collaboration observed the decay B_s⁰ → π⁺ π⁻ µ⁺ µ⁻ with a significance of 7.2 σ, while evidence for the analogous decay B⁰ → π⁺ π⁻ µ⁺ µ⁻ was reported with a significance of 4.8 σ.

This contribution reports the current status of the branching-fraction measurements of B_(s)⁰ → π⁺ π⁻ µ⁺ µ⁻ decays using the full 9 fb⁻¹ data sample recorded by LHCb in Runs 1 and 2 of the LHC. First studies based on Run 3 data are also presented, providing an outlook on the improved sensitivity and future prospects for measurements of rare b → d ℓ⁺ ℓ⁻ and b → s ℓ⁺ ℓ⁻ decays with the upgraded LHCb detector.

Keywords: Rare Decays; LHCb; B physics; branching fraction; FCNC