Observation of quantum effects on radiation reaction in strong fields

TL;DR

This work presents the first high-significance observation of quantum radiation reaction in strong fields, using an all-optical wakefield setup to collide electron beams with an intense laser and measure concomitant electron and gamma spectra. A novel Bayesian framework, combined with neural-network-predicted pre-collision spectra and partial-parameter inference, enables robust model selection between classical, quantum-continuous, and quantum-stochastic descriptions; the data favor the quantum descriptions over the classical one, due to lower predicted energy losses, with quantum-continuous and quantum-stochastic performing comparably. The approach achieves >600 high-quality collisions and integrates electron and photon diagnostics within a single inference, providing strong evidence for quantum corrections in radiation reaction and offering a general framework for model discrimination in laser-particle experiments. The results have broad implications for high-field QED, laser-driven accelerators, and photon sources, informing both fundamental physics and practical applications such as ICS-based imaging and gamma-ray generation.

Abstract

Radiation reaction, the force experienced by an accelerated charge due to radiation emission, has long been the subject of extensive theoretical and experimental research. Experimental verification of a quantum, strong-field description of radiation reaction is fundamentally important, and has wide-ranging implications for astrophysics, laser-driven particle acceleration, next-generation particle colliders and inverse-Compton photon sources for medical and industrial applications. However, the difficulty of accessing regimes where strong field and quantum effects dominate inhibited previous efforts to observe quantum radiation reaction in charged particle dynamics with high significance. We report the first high significance (> 5σ) observation of strong-field radiation reaction on electron spectra where quantum effects are substantial. We obtain the first, quantitative, strong evidence favouring the quantum-continuous and quantum-stochastic models over the classical model; the quantum models perform comparably. The lower electron energy losses predicted by the quantum models accounts for their improved performance. Model comparison was performed using a novel Bayesian framework which has widespread utility for laser-particle collision experiments, including those utilising conventional accelerators, where some collision parameters cannot be measured directly.

Figures (16)

• Figure 1: Experimental set-up, qualitative comparisons of measured hits and nulls and simulated radiation reaction models. a) Experimental setup: one laser pulse, focused into a gas jet, drove a wakefield accelerator. A second, tightly focused, counter-propagating laser pulse collided with the electron beam which emitted gamma photons. The electron spectrometer consisted of a dipole magnet which dispersed the electron beam through a wire array onto two LANEX scintillating screens (green). A Caesium Iodide (CsI) profile screen and stack characterised the transverse profile and spectrum of the emitted gamma radiation, respectively. b) Simulated post-collision electron spectra (normalised by integration) and photon spectra illustrating the classical, quantum stochastic an quantum-continuous model predictions for $a_0=10$ (bottom) and $a_0=20$ (top). The electron beam and laser pulse collided []40 after focus. The transverse and longitudinal laser intensity profiles were gaussian, with respective full-width half-maxima (FWHM) of []2.47 and []30. c) Measured electron spectra for hits with high gamma profile yields are shown above those measured for and moderate yields, together with corresponding gamma profile signals. Nulls have been randomly selected.
• Figure 2: Shot selection and summary statistics a) The shot selection procedure is illustrated. Background-subtracted total counts measured by the gamma profile diagnostic are shown as a function of $Q\langle\gamma^2\rangle$ for all shots, where $Q$ and $\gamma$ denote electron beam total charge and Lorentz factor, respectively. A constant, $C_\gamma=1\times10^{5}$ has been added to the normalised total counts for all shots to allow the data to be shown on a logarithmic scale. Nulls (blue, 608 shots) consist of combined misses and beam-off shots. The latter lie within $1\sigma$ (cyan, dashed) of the scaling of background gamma yield with $Q\langle\gamma^2\rangle$ (cyan, continuous). The small fraction of nulls which lie above this threshold are beam-off shots. Hits (red, 687 shots) lie $3\sigma$ (orange, dashed) above the background scaling. The grey points cannot be categorised as hits or nulls and thus are excluded from the analysis. The shots analysed using the Bayesian framework are encircled (black). b) Simulated post-collision electron spectra, normalised by integration, predicted by different radiation reaction models for a collision between a electron beam (pre-collision spectrum shown) with a gaussian temporal profile with full-width half-maxima (FWHM) []141 and a laser pulse with $a_0 = 14$ and gaussian transverse and longitudinal intensity profiles with FWHM []2.47 and []45, respectively. The collision was offset temporally from the laser focus by []60. The mean energy, $\langle E \rangle$ and peak height above the $70^{th}$ percentile electron energy, $P_{70}$, which indicates the prominence of the high energy peak in the spectrum, are shown.
• Figure 3: Model-independent analysis of electron energy loss and photon yield. Distributions of a) $\langle E \rangle$ and b) $P_{70}$, for measured hits (red) and nulls (blue). Hit and null distributions have been normalised to the total number of shots in each. Hit and null distributions of c) mean $\langle E \rangle$, denoted $\langle \tilde{E} \rangle$, and d) mean $P_{70}$, denoted $\tilde{P}_{70}$, obtained by bootstrapping hit and null distributions in a) and b), respectively. Bottom: e) Mean $\langle E \rangle$ for the 687 hits and 607 nulls analysed, binned logarithmically by gamma profile yield normalised to $Q\langle\gamma^2\rangle$; different bins contain different numbers of shots. f) Similar to e), for $P_{70}$.
• Figure 4: Bayesian inference results for the highest gamma-yield shot normalised to $Q\langle\gamma^2\rangle$ (shot 2). Measured data (red) and predictions for the classical (green), quantum-continuous (blue), quantum-stochastic (magenta) models, which inferred $\langle \tilde{a}_{0}\rangle=6.2\pm1.0$ and $\sigma_{a_0}=1.2\pm0.3$, $\langle \tilde{a}_{0}\rangle=6.8\pm0.9$ and $\sigma_{a_0}=0.4\pm0.1$ and $\langle \tilde{a}_{0}\rangle=6.7\pm0.9$ and $\sigma_{a_0}=0.4\pm0.1$, respectively. a) Measured and inferred post-collision electron spectra. For the former, $\langle E \rangle =\qty[parse-numbers = false]{(564.1\pm0.0(10.3))}{\mega\electronvolt}$$P_{70}=\qty[parse-numbers = false]{0.83 \pm 0.00 (0.01)}{\per\giga\electronvolt}$. The distribution of pre-collision electron spectra predicted by the neural network (orange), for which $\langle E\rangle=\qty[parse-numbers = false]{(574.1 \pm 3.9 (10.7))}{\mega\electronvolt}$, $P_{70}=\qty[parse-numbers = false]{1.29 \pm 0.05 (0.02)}{\per\giga\electronvolt}$, and its median (black). b) Measured and inferred photon energy deposition in each scintillation crystal as a function of propagation distance in the CsI photon spectrometer. The mean photon energy measured was [separate-uncertainty]63.3(58).
• Figure 5: Bayesian comparison of radiation reaction models. Bayes factors for individual shots (circles) and combined over ten shots (triangles) are shown. Weak (white), substantial (light shading) and strong (dark shading) evidence favouring model 1 (red) or model 2 (blue) are categorised according to the half-log scale convention outlined by Kass and Raftery Kass_1995. The dashed grey line indicates equal performance of compared models.