Generation of quasi-monoenergetic positron beams in chirped laser fields

High energy photons can decay to electron-positron pairs via the nonlinear Breit-Wheeler process when colliding with an intense laser pulse. The energy spectrum of the produced particles is broadened because of the variation of their effective mass in the course of the laser pulse. Applying a suitable chirp to the laser pulse can narrow the energy distribution of the generated electrons and positrons. We present a scenario where a high-energy electron beam is collided with a chirped laser pulse to generate a beam of quasi-monoenergetic $\gamma$-photons, which then decay in a second chirped, UV pulse to produce a quasi-monoenergetic source of high-energy electrons and positrons.