QCD Factorization for Semi-Inclusive Deep-Inelastic Scattering at Low   Transverse Momentum
  We demonstrate a factorization formula for semi-inclusive deep-inelastic scattering with hadrons in the current fragmentation region detected at low transverse momentum. To facilitate the factorization, we introduce the transverse-momentum dependent parton distributions and fragmentation functions with gauge links slightly off the light-cone, and with soft-gluon radiations subtracted. We verify the factorization to one-loop order in perturbative quantum chromodynamics and argue that it is valid to all orders in perturbation theory. 
Xiangdong Ji, Jian-ping Ma, and Feng Yuan