Bs->mu+mu-gamma decay rate at large q^2 from lattice QCD

Wedetermine, by means of lattice QCD calculations, the local form factors describing the Bs → μþμ−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 ¼ 2 þ 1 þ 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 ∈½mDs ;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 f f iffiffiffiffi invariant masses q2 p >4.16GeV, and use our results to determine the branching fraction for Bs →μþμ−γ, which has been recently measured by LHCb in the region ffiffiffiffiffi p >4.9GeV. The largest contribution to the uncertainty in the partial branching fractions at values of ffiffiffiffiffi q2 q2 p <4.8GeVisnowdueto resonance and other long-distance effects, including those from “charming penguins,” which we estimate by summing over the contributions from the JP ¼ 1− charmonium resonances.