B0 s → ℓ+ℓ−γ as a test of lepton flavor universality

• DOI: 10.1007/jhep11(2017)184
• Publication Date: 2017-11-30T07:19:03Z
• Authors: Diego Guadagnoli; Méril Reboud; Roman Zwicky
• Keywords: High Energy Physics - Phenomenology; [PHYS.HPHE]Physics [physics]/High Energy Physics - Phenomenology [hep-ph]; Nuclear and particle physics. Atomic energy. Radioactivity; Beyond Standard Model; 0103 physical sciences; Heavy Quark Physics; hep-ph; Effective Field Theories; QC770-798; 530; 01 natural sciences

Abstract

We discuss a number of strategies to reduce the ℬ(B

s

0 → ℓ + ℓ − γ) 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 ℬ(B

s

0 → ℓ + ℓ − γ) and the measured ℬ(B

s

0 → ϕ(→K+K−)γ). 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

γ , 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.