Jet functions for next-to-leading power factorization
We discuss the factorization of scattering processes near partonic threshold at next-to-leading power (NLP) in the threshold variable $1-z$, with $z \equiv q^2/\hat{s}$. We review the general structure of power-suppressed contributions both in Soft-Collinear Effective Theory (SCET) and in a direct QCD approach, and discuss the definition of NLP jet functions as gauge-invariant operator matrix elements in QCD. As a controlled check of the resulting factorization formula, we verify it explicitly at one and two loops for the massive electromagnetic form factor in the limit $m^2 \ll s$, using the method of regions. We conclude by outlining the two challenges that remain for a systematic resummation of NLP logarithms: the treatment of endpoint divergences in SCET convolutions, and the extension of jet functions to radiative processes capable of describing an arbitrary number of soft-gluon emissions -- the latter being the last missing ingredient for exponentiation, given that the purely soft sector is already understood in terms of generalised webs via the replica trick.