Exclusive Chromomagnetism in heavy-to-light FCNCs

TL;DR

This paper calculates matrix elements of the chromomagnetic operator relevant for rare B and D meson decays using light-cone sum rules, revealing a large CP-even phase and discussing complex analytic structures.

Contribution

It introduces a novel application of light-cone sum rules to compute chromomagnetic operator matrix elements, including analysis of complex thresholds and long-distance effects.

Findings

Large CP-even phase identified with long-distance interpretation

Comparison shows differences with QCD factorisation at end-point divergences

Analytic structure includes complex anomalous thresholds

Abstract

We compute matrix elements of the chromomagnetic operator, often denoted by $ {\cal O}_8$, between $B/D$-states and light mesons plus an off-shell photon by employing the method of light-cone sum rules (LCSR) at leading twist-2. These matrix elements are relevant for processes such as $B \to K^* l^+l^-$ and they can be seen as the analogues of the well-known penguin form factors $T_{1,2,3}$ and $f_T$. We find a large CP-even phase for which we give a long-distance (LD) interpretation. We compare our results to QCD factorisation for which the spectator photon emission is end-point divergent. The analytic structure of the correlation function used in our method admits a complex anomalous threshold on the physical sheet. The meaning and handling within the sum rule approach of the anomalous threshold is discussed.