Rare FCNC radiative leptonic decays $B\to\gamma\ell^+\ell^-$

TL;DR

This paper analyzes long-distance QCD effects in rare $B o\gamma\, ext{ extless}ell^+ extless}ell^-$ decays, focusing on form factors and charm-loop effects, to improve understanding of potential new physics signals.

Contribution

It introduces a relativistic dispersion approach for calculating $B o\gamma$ form factors and discusses the impact of charmonium resonance phases on decay observables.

Findings

Charmonium resonance phases significantly affect differential distributions.

Shape of $A_{FB}$ between $\psi$ and $\psi'$ provides phase information.

Fixing phases reduces theoretical uncertainties in $A_{FB}$ at $q^2=5-9$ GeV$^2$.

Abstract

We present our recent results for long-distance QCD effects in the FCNC radiative leptonic decays $B\to\gamma\ell^+\ell^-$, $\ell=\{e,\mu\}$. One encounters here two distinct types of long-distance effects: those encoded in the $B\to\gamma$ transition form factors induced by the $b\to q$ quark currents, and those related to the charm-loop effects. We calculate the $B\to\gamma$ form factors in a broad range of the momentum transfers making use of the relativisitc dispersion approach based on the constituent quark picture which has proven to provide reliable predictions for many weak-transition form factors. Concerning the description of the charm-loop contributions, we point out two observations: First, the precise description of the charmonium resonances, in particular, the relative phases between $\psi$ and $\psi'$, has impact on the differential distributions and on the forward-backward asymmetry, $A_{\rm FB}$, in a broad range of $q^2\ge 5$ GeV$^2$. Second, the shape of $A_{\rm FB}$ in $B\to\gamma\ell^+\ell^-$ and in $B\to V\ell^+\ell^-$ ($V$ the vector meson) {\it in the $q^2$-region between $\psi$ and $\psi'$} provides an unambiguous probe of the relative phases between $\psi$ and $\psi'$. Fixing the latter will lead to a strong reduction of the theoretical uncertainties in $A_{\rm FB}$ at $q^2=5-9$ GeV$^2$ where it has the sensitivity to physics beyond the SM.