The (Q_7,Q_{1,2}) contribution to B -> X_s gamma at O(alpha_s^2)

TL;DR

This paper calculates a crucial QCD correction to the B -> X_s gamma decay rate at zero charm quark mass, updating theoretical predictions for the Standard Model branching ratio.

Contribution

It provides the first evaluation of the O(alpha_s^2) correction at m_c=0, complementing previous heavy charm quark limit results, enhancing the precision of theoretical predictions.

Findings

Predicted branching ratio B^SM_{s gamma} = (3.36 ± 0.23) x 10^-4

Updated phenomenological analysis with both charm quark limits

Improved understanding of QCD corrections in B decays

Abstract

Interference between the photonic dipole operator Q_7 and the current-current operators Q_{1,2} gives one of the most important QCD corrections to the B -> X_s gamma decay rate. So far, the O(alpha_s^2) part of this correction has been known in the heavy charm quark limit only (m_c >> m_b/2). Here, we evaluate this part at m_c=0, and use both limits in an updated phenomenological study. Our prediction for the CP- and isospin-averaged branching ratio in the Standard Model reads B^SM_{s gamma} = (3.36 +_ 0.23) * 10^-4 for E_gamma > 1.6 GeV.