Electron-positron pair production

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When an electron hits its antiparticle, a positron, both can annihilate into pure electromagnetic energy in the form of two photons. This reaction represents a striking example of Einstein‘s equivalence between mass and energy. In principle, the inverse process is possible as well: photons colliding with each other can transform their energy into mass by forming an electron-positron pair. Until now, this fundamental quantum process of matter creation purely from photon beams - which is named after Breit and Wheeler - could not directly be observed in experiment, though, since the requirements for obtaining a detectable signal are very high.Due to a sustained progress in high-intensity laser technology, the situation is currently changing. When an extremely intense optical laser pulse collides with a high-frequency photon beam (e.g. from bremsstrahlung), a strong-field version of the Breit-Wheeler process may occur: electron-positron pairs are created in photon-multiphoton collisions. In the relevant domain of nonperturbative parameters, the hitherto unobserved creation mechanism resembles a field-induced tunneling process by which the quantum vacuum decays into massive particles. However, the required field intensities can only be achieved by tight focusing of the laser light. Thereby a very complex spatio-temporal structure is imprinted on the electromagnetic fields.In view of planned experimental studies, the goal of the present project is to provide theoretical predictions for Breit-Wheeler pair production in focused high-intensity laser pulses. This poses a formidable challenge that shall be overcome by a suitable combination of scientific methods: analytical S-matrix calculations to obtain total production probabilities and momentum spectra of created particles are combined with numerical simulations based on a many-body description of the process via Vlasov equations. Both approaches can complement and amplify each other very well. In addition, during the second funding period, the closely related processes of multiphoton trident and Bethe-Heitler pair production in collisions of relativistic particle beams (electrons or bare ions) with focused laser pulses will be studied, with special attention being paid to similarities and differences with the strong-field Breit-Wheeler process.Aside from passing the computational challenges, we aim at answering conceptual questions, e.g. how radiation reaction can be incorporated consistently into the theoretical treatment of pair creation at ultrahigh intensities. Corresponding solutions can make important contributions to an improved understanding of the quantum vacuum under extreme-field conditions