Lepton Pair Production in the Electromagnetic Fields of Relativistic Collisions.

Electromagnetic lepton pair production is studied in an ab initio approach with no free parameters. A semi -classical approximation to the relative motion of the two incident particles is made and the resulting second-order diagram is calculated. This is done by using a Monte Carlo computer program which evaluates the necessary seven-dimensional integration. The current matrix elements which occur in the calculation are generated directly in the frame where they are needed, rather than using a previous approach of generating them in the Breit frame and boosting them. We find the approximations made in this earlier approach are not adequate. We apply our calculations to a variety of systems. Electron -positron production in pi^{- }-p collisions at p_pi = 17 GeV is studied. A majority of the predicted pairs are found to arise from this two-photon mechanism if the spin-flip part of the proton current is included. This mechanism has previously been overlooked. The current matrix element to this proton to delta transition contribution is explicitly given. The method is used to predict proton-proton pair production for scattering, which is of fundamental interest as a basic mechanism for any nucleus-nucleus collision. The systems ^3He-^3He, ^3He-^4He, and ^4He-^4He are proposed as a clean way of experimentally separating the spin flip from the non-flip processes, and predictions are made for these systems.