Bs to l+ l- gamma as a Test of Lepton Flavor Universality

Preprint number: CP3-Origins-2017-30 DNRF90
Authors: Diego Guadagnoli (Université de Savoie), Méril Reboud (Université de Savoie & École Normale Supérieure de Lyon), and Roman Zwicky (University of Edinburgh & CP3-Origins)
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We discuss a number of strategies to reduce the BR(Bs to l+ l- gamma) theoretical error, and make such a measurement a new probe of the interactions that are interesting in the light of present-day flavor discrepancies. In particular, for low di-lepton invariant mass we propose to exploit the close parenthood between BR(Bs to l+ l- gamma) and the measured BR(Bs to phi (to K+ K-) gamma). For high q^2, conversely, we exploit the fact that the decay is dominated by two form-factor combinations, plus contributions from broad charmonium that we model accordingly. We construct the ratio R_gamma, akin to R_K and likewise sensitive to lepton-universality violation. Provided the two rates in this ratio are integrated in a suitable region that minimises bremsstrahlung contributions while maximising statistics, the ratio is very close to unity and the form-factor dependence cancels to an extent that makes it a new valuable probe of lepton-universality violating contributions in the effective Hamiltonian. We finally speculate on additional ideas to extract short-distance information from resonance regions, which are theoretically interesting but statistically limited at present.