Electron-seeded pair creation in intense laser pulses

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As the intensity frontier is pushed back in current and next-generation high power laser facilities (currently under construction), our understanding of how to convert light to higher frequencies in a controlled and efficient way and how to convert that radiation into matter and antimatter is increasing. The proposed research will contribute to this effort by establishing how these processes are generated in high-intensity, short laser pulses, allowing predictions from the standard model to finally be verified, or a deviation to be found. The process of electron-seeded pair-creation, which forms the subject of the proposal, is a central example of a high-intensity quantum phenomenon. Only a single experiment, E-144, which combined a 47GeV electron beam and a 10^18 W/cm^2 laser pulse, performed two decades ago at the Stanford Linear Accelerator Center, has measured this effect and only in the multiphoton regime. They reported observation of the sequential process of nonlinear Compton scattering to produce high-energy photons and their subsequent decay via the nonlinear Breit-Wheeler process, into electron-positron pairs. If this experiment could be performed again with the higher laser intensities available today, the process is predicted to be nonperturbative. These types of processes are of great interest because they are poorly understood and typically occur in difficult parts of the standard model e.g. confinement in QCD is non-perturbative.The aim of the proposed programme is to calculate electron-seeded pair-creation in a laser pulse. Although this process has been calculated in a constant and a monochromatic field, there has been no full calculation in a pulsed field. Inclusion of the pulsed nature is essential for accurate experimental predictions in high power laser experiments. In addition to the sequential process measured at E-144, there is also predicted to be a simultaneous process in which the photon remains virtual (often referred to as the ``trident process''). Such virtual processes are currently neglected by QED laser-plasma simulation codes, which are frequently used in the design and analysis of high-intensity experiments. A main objective of the research is to ascertain to what extent the approximations used in simulation, such as the field being instantaneously constant during the formation of quantum processes, are faithful to the predictions of QED when the duration of the laser pulse is decreased. This will allow for accurate predictions for future experimental campaigns. A further, and related, objective is to establish under what conditions a separation into sequential and simultaneous processes is at all well-defined as the extent of the laser pulse is reduced where quantum interference plays an ever-larger role. Whilst the approximation of lowest-order dressed processes such as photon decay and nonlinear Compton scattering is well-understood, how to approximate higher-order dressed processes such as electron-seeded pair-creation has yet to be properly investigated.By working with a project partner who is the principal investigator of an EPSRC-sponsored QED laser-plasma simulation campaign, knowledge-transfer from the research in the form of analytical results and expertise to plasma simulation will be ensured. The final aim of the project is the benchmarking of next-generation numerical codes with analytical results. A main beneficiary will be the high-intensity plasma simulation community and we expect our analysis of approximation to this second-order process to be highly relevant to the simulation of other second-order processes such as double nonlinear Compton scattering in short laser pulses, which become more important as the laser intensity used in experiment increases. In general, the proposed research underpins high power laser science and laser-plasma physics, in line with the UK research portfolio.