A comprehensive analysis of the $B^0 o K^{*0}μ^+μ^-$ decay

An analysis of the $B^{0}\rightarrow K^{*0}(\to K^+ π^-)μ^{+}μ^{-}$ decay is presented using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 8.4 fb$^{-1}$. The full set of $CP$-averaged and $CP$-asymmetric angular observables is determined in bins of the invariant mass squared of the dimuon system, as well as the branching fraction relative to the $B^{0}\rightarrow J/ψ(\toμ^{+}μ^{-})K^+π^-$ decay. For the first time, the full set of observables pertaining to the $K^+π^-$ S-wave contribution to the final state are presented and consideration is given to effects arising from the mass of the muons. The measurements of the $CP$-averaged observables and the branching fractions continue to exhibit the pattern of tensions with the Standard Model predictions that have been seen in previous analyses that use part of the dataset considered in this study. The extracted $CP$-asymmetry observables show no significant deviations from zero.

💡 Research Summary

The LHCb collaboration presents the most precise measurement to date of the rare decay B⁰ → K⁰(→K⁺π⁻) μ⁺ μ⁻ using the full Run 1 and Run 2 data set corresponding to an integrated luminosity of 8.4 fb⁻¹. The analysis introduces several methodological advances: the invariant mass of the Kπ system, m(Kπ), is treated as an explicit fit variable, allowing simultaneous extraction of the S‑wave contribution and its interference with the dominant P‑wave K⁰ resonance; the muon mass is incorporated into the differential decay rate, which proves significant for q² below ~6 GeV²/c⁴; and a five‑dimensional unbinned maximum‑likelihood fit (q², m(Kπ), cosθℓ, cosθK, φ) is performed, yielding the full set of CP‑averaged angular observables, the corresponding CP‑asymmetry observables, and the differential branching fraction relative to the normalisation channel B⁰ → K⁺π⁻ J/ψ(→μ⁺μ⁻). Two independent analysis frameworks are used to cross‑validate the results. Backgrounds are suppressed with dedicated boosted‑decision‑tree classifiers and a series of vetoes targeting peaking contributions from charmonium and other hadronic decays; the remaining combinatorial background is modelled with Chebyshev polynomials. Acceptance effects are corrected using Legendre‑polynomial expansions derived from simulation, with separate functions for B⁰ and B̅⁰ and for three data‑taking periods. The normalisation branching fraction is taken as (4.88 ± 0.22) × 10⁻⁵. The measured CP‑averaged observables continue to show the well‑known tensions with Standard Model predictions, most notably in the P′₅ and F_L observables, while all CP‑asymmetry observables are compatible with zero within uncertainties of 0.02–0.05. Systematic uncertainties, dominated by simulation corrections, PID efficiencies, and background modelling, contribute roughly 20 % of the total error budget. These results constitute a comprehensive benchmark for future studies of b → s ℓ⁺ℓ⁻ transitions, providing crucial input for global fits and for the upcoming higher‑precision data from the LHCb Upgrade and Belle II experiments.