The \bar{B} \to X_s \gamma \gamma decay: NLL QCD contribution of the Electromagnetic Dipole operator O_7

TL;DR

This paper computes next-to-leading order QCD corrections to the ar{B} o X_s \, ext{gamma gamma} decay involving the electromagnetic dipole operator O_7, providing analytical results and addressing infrared and collinear singularities.

Contribution

It offers the first calculation of O(\,\alpha_s) corrections to the double differential decay width for this decay process, including virtual and bremsstrahlung contributions with an approximation to handle singularities.

Findings

Analytical expressions for virtual corrections within a specific s_1, s_2 range.

Infrared and collinear singularities are canceled in the double differential width under approximation.

Uncanceled singularities appear when all powers of s_3 are included, indicating the need for non-perturbative inputs.

Abstract

We calculate the set of O(\alpha_s) corrections to the double differential decay width d\Gamma_{77}/(ds_1 \, ds_2) for the process \bar{B} \to X_s \gamma \gamma originating from diagrams involving the electromagnetic dipole operator O_7. The kinematical variables s_1 and s_2 are defined as s_i=(p_b - q_i)^2/m_b^2, where p_b, q_1, q_2 are the momenta of b-quark and two photons. While the (renormalized) virtual corrections are worked out exactly for a certain range of s_1 and s_2, we retain in the gluon bremsstrahlung process only the leading power w.r.t. the (normalized) hadronic mass s_3=(p_b-q_1-q_2)^2/m_b^2 in the underlying triple differential decay width d\Gamma_{77}/(ds_1 ds_2 ds_3). The double differential decay width, based on this approximation, is free of infrared- and collinear singularities when combining virtual- and bremsstrahlung corrections. The corresponding results are obtained analytically. When retaining all powers in s_3, the sum of virtual- and bremstrahlung corrections contains uncanceled 1/\epsilon singularities (which are due to collinear photon emission from the s-quark) and other concepts, which go beyond perturbation theory, like parton fragmentation functions of a quark or a gluon into a photon, are needed which is beyond the scope of our paper.