17 Mars – Thesis defense - Léo Esnault

14 h Amphi 3 - Building A9 (Talence campus)

Gamma-ray production in laser-matter interaction and electron-positron pair creation by photon-photon collisions.

Electron-positron pair creation by the mean of two real photon collision (linear Breit-Wheeler process) is one of the most basic quantum electrodynamics process, and is believed to underlie a large amount of high energy astrophysical phenomena, such as the Universe opacity to TeV photons or the production of pair plasmas near compact objects (AGN, pulsars). However, this process has never been directly observed in the laboratory since its prediction in 1934, mainly because of the absence of high flux MeV-range photon sources.
Due to the continuous development of high-power and high-intensity laser systems, the production of such photon sources become however conceivable. Despite the diversity of methods to produce energetic photons by lasers, previous estimates seems to show that the radiation sources produced whether by the slowing-down of electrons in matter (Bremsstrahlung), or by the multiphoton inverse Compton scattering process (sometimes called synchrotron-like) are among the most credible sources for the electron-positron pair production by two real photon collisions. The interaction of such multi-MeV photons with matter could however creates other electron-positron pairs, which could constitute a background noise for the detection of this process.
The goal of this thesis is to optimize the production of MeV-range gamma photons by the mean of the interaction of a laser with various kind of targets (simple of structured solid targets) in order to prepare photon-photon collision experiments on existing or currently building laser systems. A semi-analytical model allow firstly to optimize linear Breit-Wheeler pair production in term of the photons sources parameters. Particularly it is shown that, concerning Bremsstrahlung photon sources, the current laser systems already permit to reach the optimum photon energy distributions, opening the possibility to design such experiments at high repetition rate. This study is completed with numerical simulations modeling the laser-target interaction (via a Particle-In-Cell code), the gamma photon generation (via a Monte Carlo code) and their collisions (via a Barnes-Hut Monte Carlo code). The electron-positron pair background noise created by concurrent processes is also studied and compared to the expected linear Breit-Wheeler signal, helping to prepare future experiments in current laser facilities.

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