[Paper Review] On-shot diagnostic of electron beam-laser pulse interaction based on stochastic quantum radiation reaction

Matteo Tamburini|arXiv (Cornell University)|Jul 6, 2020

Laser-Matter Interactions and Applications35 references4 citations

TL;DR

This paper proposes a novel on-shot diagnostic method for ultrarelativistic electron beam-laser pulse interactions using the stochastic nature of quantum radiation reaction. By exploiting transverse momentum asymmetry induced in the electron beam due to incoherent photon emission, the method enables real-time, shot-by-shot measurement of laser field strength and beam-laser overlap, offering a robust benchmarking tool for strong-field QED models despite experimental shot-to-shot fluctuations.

ABSTRACT

Ultrarelativistic electron beam-laser pulse scattering experiments are the workhorse for the investigation of QED and of possible signatures of new physics in the still largely unexplored strong-field regime. However, shot-to-shot fluctuations both of the electron beam and of the laser pulse parameters render it difficult to discern the dynamics of the interaction. Consequently, the possibility of benchmarking theoretical predictions against experimental results, which is essential for validating theoretical models, is severely limited. Here we show that the stochastic nature of quantum emission events provides a unique route to the on-shot diagnostic of the electron beam-laser pulse interaction, therefore paving the way for accurate measurements of strong-field QED effects.

Motivation & Objective

• Address the challenge of shot-to-shot fluctuations in electron beam and laser pulse parameters that hinder accurate benchmarking of strong-field QED models.
• Overcome the limitations of existing experiments in distinguishing between competing radiation reaction models due to uncertainty in interaction conditions.
• Demonstrate that stochastic quantum radiation reaction induces a measurable transverse asymmetry in electron beam momentum and spatial distribution.
• Develop a diagnostic tool that provides on-shot information on laser field strength and beam-laser overlap using only standard spatial beam profile measurements.
• Establish a practical, experimentally accessible method to validate theoretical predictions in strong-field QED using current and near-future high-intensity laser facilities.

Proposed method

• Simulate ultrarelativistic electron beam-laser pulse collisions using a stochastic radiation reaction model based on quantum electrodynamics (QED) in strong fields.
• Model the laser pulse as a counterpropagating, linearly polarized plane wave with normalized amplitude ξ, using a smooth temporal envelope f(φ) and phase δ.
• Track electron trajectories and momentum evolution under the influence of the laser field, including radiation reaction via the stochastic emission of photons.
• Calculate the final electron beam transverse momentum distribution and spatial distribution after ballistic propagation over distances of several centimeters.
• Introduce an asymmetry parameter 𝒜 = (σₓ − σᵧ)/(σₓ + σᵧ) to quantify the transverse deformation, where σₓ and σᵧ are beam widths along x and y axes.
• Compare results from stochastic radiation reaction with continuous radiation reaction models to isolate the unique signature of quantum stochasticity.

Experimental results

Research questions

• RQ1 Can the stochastic nature of quantum radiation reaction be used to extract on-shot information about the laser field strength and beam-laser overlap during electron beam-laser pulse interactions?
• RQ2 How does the transverse momentum and spatial distribution of an ultrarelativistic electron beam change due to incoherent photon emission in strong laser fields?
• RQ3 To what extent is the induced transverse asymmetry sensitive to variations in laser intensity and beam-laser misalignment?
• RQ4 Can this asymmetry be distinguished from classical or continuous radiation reaction effects in experimental measurements?
• RQ5 Is the diagnostic method robust under realistic experimental conditions, such as low beam energy spread and beam misalignment?

Key findings

• The stochastic radiation reaction model induces a distinct transverse momentum asymmetry in the electron beam, particularly in the direction orthogonal to the laser polarization, which is absent in continuous radiation reaction models.
• Even a 1 μm transverse beam-laser offset produces a measurable asymmetry in the final electron beam spatial distribution, demonstrating high sensitivity to overlap conditions.
• For ξ = 21, the transverse beam width along the polarization axis (x) increases, while the width along the orthogonal direction (y) decreases significantly due to cumulative photon emission effects.
• The asymmetry parameter 𝒜 shows a strong dependence on the normalized laser amplitude ξ, with clear variation across ξ = 15, 18, and 21, enabling quantitative field strength diagnostics.
• In focused laser pulses with w₀ = 4 μm, the transverse asymmetry is enhanced in the beam periphery due to spatially varying field strength, increasing diagnostic sensitivity.
• Misalignment of 1–4 μm along the x or y axis produces characteristic left-right or up-down asymmetries in the spatial beam profile, allowing direct inference of beam-laser misalignment.