Strong-field physics prospects at 50 keV at EuXFEL

Some studies of applying a 50 keV XFEL for strong-field physics, modelled by the interaction with an electron beam, were presented. The process of electron-positron pair production in strong fields deviates from the well-known perturbative result in weak field backgrounds. In a strong field background, an electron can directly emit a photon, generating a pair via the trident process. Using 50 keV photons, only about 5 MeV kinetic electron energy is required to reach the trident threshold, which is available by electron guns or laser acceleration (in solids, or wakefield). The trident process can also be used to test models for dark matter candidates. Using the proposed massive “dark photon”, the assumed mass and coupling to ordinary could be determined more precisely than with hadron experiments. Certain exclusion regions can be scanned, but the trident experiment could also be used to detect dark matter (instead of excluding certain mass/coupling ranges) because there is full control over the QED background. A second scheme is the interaction of hard x-rays with electrons in the presence of an intense, infrared few-cycle laser light field. It allows to study of laser-assisted Compton scattering, Breit Wheeler pair production and trident, where during the peaks of the few-cycle IR laser field, spectral features are introduced. A (quasi-) continuous X-ray beam (as in a synchrotron) is not sufficient for strong-field studies as high intensity is required. A user community, similar to established synchrotron or XFEL users, may not exist yet. Many colleagues work on theoretical models, but more experimentalists will emerge with upcoming experimental capabilities at XFELs. The LUXE experiment at DESY, and the detection of QED vacuum birefringence at HED/HIBEF, EuXFEL, are examples of such developments.