Electron–positron pair production in vacuum by a single focused laser pulse and by two counterpropagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic threedimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with ...

The cross section for the reaction 7 + C -+e+ + e+ C has been measured for a series of kinematic points giving nuclear momentum transfers in the region 0.2 0.7 F -2 , with incident -photon energies in the region for photoproduction of the first two pi-nucleon resonances. Measurements were made, using asymmetric detection geome-try, of e3.ectron and positron cross sections for each point. The ch...