Coulomb-field-induced conversion of a high-energy photon into a pair assisted by a counterpropagating laser beam

The laser-induced modification of a fundamental process of quantum electrodynamics, the conversion of a high-energy gamma photon in the Coulomb field of a nucleus into an electron-positron pair, is studied theoretically. Although the employed formalism allows for the general case where the gamma photon and laser photons cross at an arbitrary angle, we here focus on a theoretically interesting and numerically challenging setup, where the laser beam and gamma photon counterpropagate and impinge on a nucleus at rest. For a peak laser field smaller than the critical Schwinger field and gamma photon energy larger than the field-free threshold, the total cross section is verified to be almost unchanged with respect to the field-free case, whereas the differential cross section is drastically modified by the laser field. The modification of the differential cross section is explained by classical arguments. We also find the laser-dependent maximal energy of the produced pair and point out several interesting features of the angular spectrum.