Electron-seeded pair creation in intense laser pulses

随着当前和下一代高功率激光设备（目前正在建设中）的强度边界被推后，我们对如何以受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。拟议的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，从而使标准模型的预测最终得到验证，或者发现偏差，从而有助于这项工作。电子种子对产生的过程是高强度量子现象的一个核心例子，它构成了提案的主题。只有一个单独的实验，E-144，结合了47GeV的电子束和10^18 W/cm^2的激光脉冲，20年前在斯坦福直线加速器中心进行，测量了这种效应，而且只是在多光子状态下。他们报告了对非线性康普顿散射产生高能光子的连续过程的观察，以及它们随后通过非线性布雷特-惠勒过程衰变成电子-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，这个过程预计是非摄动的。这些类型的过程非常有趣，因为它们很少被理解，并且通常发生在标准模型的困难部分，例如，QCD中的约束是非摄动的。该计划的目的是计算激光脉冲中电子种子对的产生。虽然这个过程已经在常数场和单色场中计算过，但在脉冲场中还没有完整的计算。在高功率激光实验中，包含脉冲性质对于准确的实验预测是必不可少的。除了在E-144测量的顺序过程外，还预测会有一个光子保持虚态的同时过程（通常被称为“三叉戟过程”）。在高强度实验设计和分析中经常使用的QED激光等离子体模拟代码中，这种虚拟过程目前被忽略了。研究的一个主要目的是确定当激光脉冲持续时间减少时，模拟中使用的近似（例如在量子过程形成过程中场是瞬时常数）在多大程度上忠实于QED的预测。这将为未来的实验活动提供准确的预测。进一步的，相关的，目标是建立在什么条件下，当激光脉冲的范围减小，量子干涉起着越来越大的作用时，分离成顺序和同时的过程是完全明确的。虽然最低阶修饰过程（如光子衰减和非线性康普顿散射）的近似已经很好理解，但如何近似高阶修饰过程（如电子种子对产生）仍有待适当研究。通过与epsrc赞助的QED激光等离子体模拟活动的项目合作伙伴合作，将确保以分析结果和专业知识的形式从研究中转移到等离子体模拟。该项目的最终目标是用分析结果对下一代数值代码进行基准测试。一个主要的受益者将是高强度等离子体模拟社区，我们希望我们对这一二阶过程的近似分析与其他二阶过程的模拟高度相关，如短激光脉冲中的双重非线性康普顿散射，随着实验中使用的激光强度的增加，这将变得更加重要。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，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.