Dilepton production in nucleon-nucleon collisions reexamined

We present a fully relativistic and gauge invariant framework for calculating the cross sections of dilepton production in nucleon-nucleon ($NN$) collisions which is based on the meson-exchange approximation for the $NN$ scattering amplitudes. Predictions of our model are compared with those of other covariant models that have been used earlier to describe this reaction. Our results are also compared with those of the semiclassical models which are employed to get the input elementary cross sections in the transport model calculations of the dilepton production in nucleus-nucleus collisions. It is found that cross sections obtained within the semiclassical and quantum mechanical models differ noticeably from each other.