Gribov's Theorem on Soft Emission and the Reggeon-Reggeon-Gluon Vertex   at Small Transverse Momentum
  In spite of the fact that the Gribov's theorem about the region of applicability of the soft emission factorization cannot be referred literally to the case of massless charged particles, it can be used for the calculation of soft emission amplitudes in such a case also. We demonstrate this for the gluon production amplitude in the multi-Regge kinematics with small transverse momentum. 
V.S. Fadin, R. Fiore, M.I. Kotsky