DILEPTON PRODUCTION IN NUCLEON-NUCLEON COLLISIONS AND THE LOW-ENERGY THEOREM

We consider the process of dilepton production in nucleon-nucleon (NN) collisions NN --> NNl+l- (virtual bremsstrahlung). The reaction amplitude in the rest frame of the virtual photon is presented in powers of the photon invariant mass M(gamma). It is shown that the leading term O(M(gamma)-1) is expressed through the on-mass-shell NN T-matrix. If in addition the photon 3-momentum in the lab (or c.m.) frame is small enough the next term O(M(gamma)0 becomes model independent as well. The amplitude is presented in a covariant form which allows one to calculate it in any reference frame and to compare it with the well-known low-energy theorem for the real photon NN bremsstrahlung. The decomposition of the exclusive cross section of the virtual NN bremsstrahlung in terms of response functions is carried out as well and the ratios of the response functions are calculated using the leading term in the amplitude.