Bs→μ+μ−γ decay rate at large q2 from lattice QCD

Abstract

We determine, by means of lattice QCD calculations, the local form factors describing the Bs→μ+μ−γ decay, in the so-called electroquenched approximation. For this analysis we make use of the gauge configurations produced by the ETM Collaboration with Nf=2+1+1 flavor of Wilson-Clover twisted-mass fermions at maximal twist. To obtain the Bs meson form factors, we perform simulations for several heavy-strange meson masses mHs in the range mHs∈[mDs,2mDs], and extrapolate to the physical Bs meson point mBs≃5.367  GeV making use of the HQET scaling laws. We cover the region of large dimuon invariant masses q2>4.16  GeV, and use our results to determine the branching fraction for Bs→μ+μ−γ, which has been recently measured by LHCb in the region q2>4.9  GeV. The largest contribution to the uncertainty in the partial branching fractions at values of q2<4.8  GeV is now due to resonance and other long-distance effects, including those from “charming penguins,” which we estimate by summing over the contributions from the JP=1− charmonium resonances. Published by the American Physical Society 2024