Speaker
Description
At next-to-leading power, predictions in Soft-Collinear Effective Theory often suffer from a technical problem, known as endpoint-divergent convolution integrals. While this problem has recently been overcome for some processes, these methods are currently limited to examples with non-recursive refactorsation identities. As I will show in this talk, exclusive heavy-to-light transition form factors do not belong to this class. We therefore resort to diagrammatic resummation techniques to derive the double-logarithmic series of the so-called “soft-overlap” contribution to $B_c \to \eta_c$ form factors at large hadronic recoil (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.