TMD soft function from large -momentum effective theory

We study Euclidean formulations of the transverse-momentum-dependent (TMD) soft function, which is a cross section for soft gluon radiations involving color charges moving in two conjugate lightcone directions in quantum chromodynamics. We show it is related to a special form factor of a pair of color sources traveling with nearly-lightlike velocities, which can be matched to TMD physical observables in semi-inclusive deep-inelastic scattering and Drell-Yan process in the framework of large momentum effective theory. It can also be extracted by combining a large-momentum form factor of light meson and its leading TMD wave function. These formulations are useful for initiating nonperturbative calculations of this useful quantity. © 2020