Electron-seeded pair creation in intense laser pulses

随着当前和下一代高功率激光设施（目前正在建设中）的强度前沿被推回，我们对如何以可控和有效的方式将光转换为更高频率以及如何将辐射转换为物质和反物质的理解正在增加。这项研究将通过确定这些过程是如何在高强度、短激光脉冲中产生的，从而最终验证标准模型的预测，或者发现偏差，从而为这一努力做出贡献。电子种子对创造过程是高强度量子现象的一个中心例子，也是这项研究的主题。20年前，在斯坦福大学直线加速器中心，只有一个实验E-144测量到了这种效应，而且只是在多光子状态下。E-144将47 GeV的电子束和10^18 W/cm ^2的激光脉冲结合在一起。他们报告了观测到的非线性康普顿散射产生高能光子的连续过程，以及它们随后通过非线性布雷特-惠勒过程衰变为电子-正电子对的过程。如果这个实验可以用今天可用的更高的激光强度再次进行，预计这个过程是非微扰的。这些类型的过程是非常感兴趣的，因为他们是知之甚少，通常发生在标准模型的困难部分，例如在QCD的约束是非微扰的。虽然这一过程已经在恒定和单色场中计算，但在脉冲场中还没有完整的计算。在高功率激光实验中，考虑脉冲特性是精确实验预测的关键。除了在E-144测量的顺序过程之外，还预测有一个光子保持虚拟的同步过程（通常称为“三叉戟过程”）。这种虚拟过程目前被QED激光等离子体模拟程序所忽略，而QED激光等离子体模拟程序经常用于高强度实验的设计和分析。研究的一个主要目的是确定在何种程度上的近似模拟中使用的，如在量子过程的形成过程中的字段是瞬间恒定的，是忠实于QED的预测时，激光脉冲的持续时间减少。这将为未来的实验活动提供准确的预测。另一个相关的目标是确定在什么条件下，当激光脉冲的范围减小时，在量子干涉起着越来越大的作用时，顺序和同时过程的分离是完全明确的。虽然最低阶的修饰过程如光子衰变和非线性康普顿散射的近似是很好理解的，但如何近似高阶的修饰过程如电子种子对产生还没有得到适当的研究。通过与EPSRC赞助的QED激光等离子体模拟活动的首席研究员合作，将确保以分析结果和专门知识的形式从研究向等离子体模拟转移知识。该项目的最终目标是用分析结果对下一代数字代码进行基准测试。一个主要的受益者将是高强度等离子体模拟社区，我们希望我们的分析近似这个二阶过程是高度相关的模拟其他二阶过程，如双非线性康普顿散射在短激光脉冲，这变得更加重要的激光强度在实验中使用的增加。总的来说，拟议的研究支持高功率激光科学和激光等离子体物理学，与英国的研究组合一致。

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.