Revealing signals of higher-order nonlinear showers in particle-laser collisions

TL;DR

This work investigates how higher-order nonlinear strong-field QED processes can become observable in laboratory-scale laser–particle collisions as the system transitions toward RN-like nonperturbative dynamics. By combining Ptarmigan simulations with LCFA-based calculations, it identifies measurable signatures—notably skewness in light-front momentum distributions from nonlinear phototrident and deviations in electron spectra from nonlinear Compton showers—that signal higher-order contributions. The study shows that photon bandwidth can confound these signals, necessitating quasi-monoenergetic sources to reveal higher-order effects, and demonstrates how multiplicity statistics and spectral features evolve with the intensity parameter $a_0$ and background field, including a distinctive turning behavior in nonlinear trident yields. Overall, the results provide concrete observables to test the validity of current calculational frameworks in strong-field QED at higher orders and aim to illuminate the boundary between perturbative and non-perturbative regimes in upcoming experiments.

Abstract

Several high power laser facilities are reaching field strengths where leading order strong-field quantum electrodynamical (QED) processes can be measured in the non-perturbative regime for the first time. At very high, as yet unobtainable in the laboratory, field strengths, the contribution of higher-order processes is predicted to dominate, implying a breakdown of current calculational methods. Focusing on nonlinear showers and considering currently available experimental parameters, we find that if the momentum spectrum of the \emph{incident} particles is well known, asymmetries in the \emph{outgoing} particle spectrum may provide a useful signature of higher orders of nonlinear phototrident, trident and Compton scattering. These signatures could be used by experiment to test how accurate the current calculational framework is when applied to strong-field QED at higher orders.

Figures (14)

• Figure 1: 'Nonlinear' Compton scattering includes all orders of interaction between the laser field (dashed lines) and the electron/positron.
• Figure 2: Nonlinear Compton Shower. Diagrams in ellipsis contain contributions that are higher orders in $\alpha$ (i.e. contain loops).
• Figure 3: Representation of the factorisation of double nonlinear Compton scattering in sequential first-order nonlinear scattering which includes a sum over the spin state $\sigma$ of the propagating particle. The 'rest' includes non-propagating contributions and interference terms.
• Figure 4: The total probability for nonlinear Compton shower expanded as a series in pulse duration $\Phi$. At each order, the dominant contribution when $a_{0}\gg1$ is given by $\textsf{P}^{[k]}$ i.e. the sequential channel from $n$-fold iterations of single nonlinear Compton scattering joined by a real propagating particle. Numerical simulation includes the dominant contribution but does not in general explicitly include loop interference or off-shell tree-level contributions.
• Figure 5: The average change in energy parameter, $\Delta \eta$, of an electron or positron with initial energy parameter, $\eta$, when it nonlinear Compton scatters in an eight-cycle sine-squared plane wave pulse. (Calculated using the locally constant field approximation, LCFA.)