Electron-seeded pair creation in intense laser pulses

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.

The project studies electron-seeded pair-creation, which comprises the sub-processes of high-frequency photon generation and electron-positron production. The beneficiaries are i)
laser-plasma simulators and experimentalists; ii) high-power laser scientists; iii) scientists involved in spectroscopy, high-frequency imaging and non-intrusive detection; iv) users of
antimatter beams.

i) In order to simulate the behaviour of plasmas at the highest laser intensity, it is necessary to include quantum processes such as electron-seeded pair-creation. Currently, there is no agreed method how to achieve this. The project will provide a solution to this problem, which is expected to be in the form of analytical expressions for pair-creation rates. These rates will be implemented, as part of the project, in QED laser-plasma simulation codes, which will be benchmarked against analytical results. The benefit is the ability to accurately design and analyse higher-intensity laser-plasma experiments.

ii) The results of the project will increase the visibility and attractiveness of physics involving high-power lasers. The UK and the CLF (Central Laser Facility) is a world-leader in this
field, which is evidenced by the HiLASE facility at the ELI-beams centre purchasing in 2014 a DiPOLE optical amplifier from CALTA (Centre for Advanced Laser Technology and Applications) for £2.2M. A more recent example is the DiPOLE instrument to be installed in the HIBEF target area of the European XFEL facility at DESY, which was purchased in 2015 for £8M.

iii) High-frequency photon or so-called ``gamma'' sources are useful in spectroscopy throughout the life, physical and environmental sciences. Gamma sources of high frequencies such as the UK-based Diamond Light Source synchrotron or of high brilliance such as the LCLS-II X-ray free electron laser in Berkeley, sell beamtime to research groups to perform microscopic studies in their field. The gamma radiation produced in laser-electron collisions has recently been shown to sit between these sources, producing a higher brilliance than synchrotrons and a higher frequency than X-ray free electron lasers. As the process of high-frequency photon generation in collisions of electron beams and laser pulses becomes better understood, which this project will contribute to, it is expected that this technology can replace established synchrotron sources. Moreover, it has recently been
shown that this high-frequency radiation is very well suited for non-intrusive inspection of e.g. lined cargo containers. This high-frequency radiation is also employed in nuclear resonance fluorescence imaging, one application of which is the measuring the enrichment of uranium hexafluoride cylinders used in nuclear power stations. It is expected that as laser science improves, more applications of this young technology will be found.

iv) Electron-positron pairs recently generated in laser-plasma collisions have been proposed to become the most intense sources of antimatter available in the lab. Currently, antimatter production is very expensive (1g of positrons was predicted a decade ago to cost around £17.5B), and intense laser-matter interactions is a potentially more economical mechanism to replace standard methods involving particle accelerators. These sources have recently been used to study the behaviour of leptonic astrophysical jets which accompany still-mysterious gamma-ray bursts. Antimatter beams have also been suggested as a possible way to treat cancer, in a similar way to ion therapy. Moreover, it has been noted that positrons can be used to detect weaknesses and abnormalities in industrial materials before they develop into serious defects. Since the technology here is also still relatively new, the possible uses of positrons are expected to greatly increase in number.