The process γ+γ→ℓ+ℓ in coherent heavy ion collisions

Using a covariant formalism we calculate to leading order the 2-photon contribution to lepton pair production in coherent heavy ion collisions with-in the framework of EPA. The most significant result of our calculation is the presence of a prominent narrow threshold enhancement in thee ⁺ e ^? invariant mass distribution. The position of this peak is essentially energy independent, however, its height and width tend to increase with increasing energy, mass and charge of the colliding ions. Results are presented for configurations at the Bevalac and SPS machines which are then extrapolated down to the GSI energy region.