Form factors and branching ratio for the B→lνγ decay

Abstract

Form factors parameterizing radiative leptonic decays of heavy mesons (B+→γl+νl) against photon energy are computed in the language of dispersion relations. The states contributing to the absorptive part in the dispersion relation are the multiparticle continuum, estimated by the quark triangle graph, and resonances with quantum numbers 1- and 1+, which includes B* and BA * and their radial excitations, which model the higher state contributions. Constraints provided by the asymptotic behavior of the structure dependent amplitude, Ward identities and gauge invariance are used to provide useful information for the parameters needed. The couplings gBB*γ and fBBA*γ are predicted as we restrict ourselves to the first radial excitation; otherwise using these as an input the radiative decay coupling constants for the radial excitations are predicted. The value of the branching ratio for the process B+→γμ+νμ is found to be in the range 0.5×10-6. A detailed comparison is given with other approaches.