Renormalisation group evolution of the shape function $g_{17}$ in $\bar{B}\to X_s \gamma$ and $\bar{B} \to X_s \ell^+\ell^-$ at subleading power

TL;DR

This paper derives and solves the RG evolution of the shape function $g_{17}$ at subleading power in inclusive B-decays, enabling more precise NLO analyses of photon interference effects and simplifying operator renormalization in HQET.

Contribution

It provides the first RG evolution of the shape function $g_{17}$ at subleading power and introduces a simplified approach to operator renormalization in HQET for B-decays.

Findings

First RG evolution of $g_{17}$ at subleading power.

Simplified renormalization method for operators in HQET.

Derived a reduced RG equation for soft functions in exclusive B-decays.

Abstract

We derive and solve the renormalisation-group (RG) equation of the shape function $g_{17}(\omega,\omega_1;\mu)$, which appears at subleading power in the factorization of the inclusive decays $\bar{B} \to X_s \gamma$ and $\bar B \to X_s \ell^+\ell^-$. Our results provide the first ingredient for a next-to-leading order analysis of the respective resolved-photon ${Q}^{c}_{1} - {Q}_{7\gamma}$ interference contribution, whose current uncertainties are among the largest ones in both inclusive penguin modes. As a by-product of our study, we find that the analytic properties of the soft anomalous dimension as well as the jet functions in a factorization theorem allow for a simplified renormalisation of operators in Heavy-Quark Effective Theory that are composed of fields smeared along distinct light-cone directions. Their hadronic matrix elements become relevant in various inclusive and exclusive $B$-decays beyond leading power in the heavy-quark and large-energy expansion. Using these insights, we derive and solve a simpler ``reduced'' RG equation of an amplitude-level soft function that describes the long-distance QCD dynamics for penguin contributions to exclusive $\bar{B}_{d,s} \to \gamma \gamma$ decays, and which has recently been discussed in the literature.