Speaker
Description
Exclusive $B$-decays are sensitive to contributions from new physics and can thus be used to test the Standard Model. At large hadronic recoil Soft-Collinear Effective Theory is the appropriate theory to describe the QCD dynamics and to resum logarithmic corrections to all orders in perturbation theory. However, since the relevant hadronic matrix elements are power suppressed, the factorisation of soft and collinear contributions is spoilt by endpoint divergences. We therefore resort to diagrammatic resummation techniques to derive the double-logarithmic series of the “soft-overlap” contribution to $B_c \to \eta_c$ transition form factors, assuming the scale hierarchy $m_b \gg m_c\gg \Lambda_{\rm QCD}$. We find that the leading double logarithms arise from a peculiar interplay of soft-quark “endpoint logarithms” from ladder diagrams with energy-ordered spectator-quark propagators, as well as standard Sudakov-type soft-gluon corrections. We elucidate the all-order systematics, and show that their resummation proceeds via a novel type of integral equations.
