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We discuss a number of strategies to reduce the $\mathcal B(B^0_s \to\ell^{+} \ell^{-} \gamma)$ theoretical error, and make such a measurement a newprobe of the interactions that are interesting in the light of present-dayflavor discrepancies. In particular, for low di-lepton invariant mass wepropose to exploit the close parenthood between $\mathcal B(B^0_s \to \ell^{+}\ell^{-} \gamma)$ and the measured $\mathcal B(B^0_s \to \phi (\to K^+ K^-)\gamma)$. For high $q^2$, conversely, we exploit the fact that the decay isdominated by two form-factor combinations, plus contributions from broadcharmonium that we model accordingly. We construct the ratio $R_\gamma$, akinto $R_K$ and likewise sensitive to lepton-universality violation. Provided thetwo rates in this ratio are integrated in a suitable region that minimisesbremsstrahlung contributions while maximising statistics, the ratio is veryclose to unity and the form-factor dependence cancels to an extent that makesit a new valuable probe of lepton-universality violating contributions in theeffective Hamiltonian. We finally speculate on additional ideas to extractshort-distance information from resonance regions, which are theoreticallyinteresting but statistically limited at present.

$B^0_s \to \ell^+ \ell^- \gamma$ as a Test of Lepton Flavor Universality