Unveiling the Collins-Soper kernel in inclusive DIS at threshold

Abstract: We revisit the factorization of inclusive deep inelastic scattering near the kinematic threshold to explicitly track off-lightcone effects. Particle production develops around two opposite near-lightcone directions like in transverse-momentum-dependent processes, and the Collins-Soper kernel emerges as a universal function in the rapidity evolution of the relevant parton correlators. We clarify outstanding issues regarding soft radiation and rapidity divergences, and uncover a new way to calculate the Collins-Soper kernel on the lattice with collinear operators.