Charge asymmetry and photon energy spectrum in the decay Bs→l+l−γ

We consider the structure-dependent amplitude of the decay Bs→l+l−γ (l=e,μ) in a model based on the effective Hamiltonian for bs̄→l+l− containing the Wilson coefficients C7,C9 and C10. The form factors characterising the matrix elements 〈γ|s̄γμ(1∓γ5)b|B̄s〉 and 〈γ|s̄σμν(1∓γ5)b|B̄s〉 are taken to have the universal form fV≈fA≈fT≈fBsMBsRs/(3Eγ) suggested by recent work in QCD, where Rs is a parameter related to the light cone wave function of the Bs meson. Simple expressions are obtained for the charge asymmetry A(xγ) and the photon energy spectrum dΓ/dxγ(xγ=2Eγ/MBs). The decay rates are calculated in terms of the decay rate of Bs→γγ. The branching ratios are estimated to be Br(Bs→e+e−γ)=2.0×10−8 and Br(Bs→μ+μ−γ)=1.2×10−8, somewhat higher than earlier estimates.