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Another Look at ``soft'' lepton pair production in nucleon-nucleon bremsstrahlung

We compare different formalisms for the calculation of lepton pair emission in hadron-hadron collisions and discuss the consequences of the approximations inherent to each of them. Using a Lorentz-covariant and gauge-invariant formalism proposed by Lichard, we calculate lepton pair emission via bremsstrahlung in proton-proton and neutron-proton reactions at energies between 1 and 5 GeV to leading order in the virtual photon four-momentum. These new results suggest that some previous bremsstrahlung calculations based on varieties of the soft-photon approximation might have somewhat overestimated the DLS low-mass dilepton cross sections. We find that the respective intensities of dilepton production through $p p$ and $n p$ bremsstrahlung are energy-dependent and become comparable at $E_{\rm kin} \agt $ 2 GeV. We also calculate the polar angle anisotropy of the lepton spectrum.