The Strong Field QED approach of the vacuum interaction processes at ELI-NP
The commissioning of the high power laser facility Extreme Light Infrastructure - Nuclear Physics (ELI-NP) at Bucharest-Magurele (Romania) allows the in-depth study of nonlinear interactions in Strong Field Quantum Electrodynamics (SF-QED). The present paper analyzes the SF-QED processes possible to study at ELI-NP. Carrying out such experiments will allow finding answers to many fundamental QED questions. After a brief review of the first experiment (E-144 SLAC) which confirmed the existence of nonlinear QED interactions of high-energy electrons with photons of a laser beam, we presented the fundamental QED processes that can be studied at ELI-NP in the multi-photon regime along with the characteristic parameters of the laser beam used in the QED interaction with electrons. To prepare an experiment at ELI-NP, it is necessary to analyze both the kinematics and the dynamics of the interactions. Therefore, we first reviewed the kinematics of linear QED processes and then the corresponding Feynman diagrams. For nonlinear, non-perturbative multi-photon QED interactions, the Feynman diagram technique must be adapted from linear to nonlinear processes. This is done by switching to quantum fields described by Dirac-Volkov dressed states, of particles in an intense electromagnetic (EM) field. This allows the evaluation of the amplitude of the physical processes and finally the determination of the cross-sections of these processes. SF-QED processes of multi-photon interactions with strong laser fields can be investigated taking into account the characteristics of the ELI-NP facility in the context of QED vacuum pair production of electron-positron pairs and energetic gamma rays. Finally, we present some similar experimental projects from other research centers, in different stages of implementation.