Experimental Determination of Gamma-Ray Polarization in Strong-Field Nonlinear Compton Scattering

Abstract

The polarization of gamma rays produced in strong-field quantum electrodynamics (SFQED) is a fundamental and long-standing prediction, the verification of which has remained elusive, limiting both foundational tests and applications. Here, we report the first experimental measurement of gamma-ray polarization generated via all-optical nonlinear Compton scattering. Colliding a laser-wakefield-accelerated electron beam with an intense counter-propagating laser pulse reflected from a plasma mirror, we produce bright gamma rays in the strong-field regime ($a_0 > 1$). For gamma rays with $a_0 \approx 3$, a linear polarization degree of $\sim 50\%$ is measured via the azimuthal asymmetry of photoneutrons from a deuterium target, and independently verified by a Compton polarimeter.The results show excellent agreement with SFQED calculations employing the locally monochromatic approximation, while diverging from predictions based on the locally constant field approximation, highlighting the importance of quantum interference effects in this regime. Our work provides experimental evidence for polarization dynamics in SFQED, supports a key prediction of nonperturbative QED, and paves the way for compact, laser-driven sources of polarized gamma rays.}

Figures (4)

• Figure 1: Experimental set-up. Schematic of the all-optical experimental setup for studying polarized nonlinear Compton scattering. A drive laser pulse generates an ultrarelativistic electron beam via LWFA in a gas jet. The residual laser is reflected and focused by a plasma mirror to collide with the electrons. The scattered electrons are analyzed by a magnetic spectrometer. The emitted gamma rays are characterized using an Imaging Plate (IP) for beam profile and a pixelated LYSO (LYSO-PX) calorimeter for spectrum. Their polarization is determined via two interchangeable detection modules (bottom insets): a) a heavy-water ($\mathrm{D_2O}$) target monitored by two bubble detectors positioned parallel (side) and perpendicular (bottom) to the horizontal linear polarization of the incident laser, and b) a solid carbon converter for Compton scattering asymmetry analysis using IPs.
• Figure 2: Distinguishing Compton scattering from bremsstrahlung background.a-f, Single-shot measurements under two conditions: bremsstrahlung-only (a-c) and bremsstrahlung with NCS (d-f). a, d, Electron energy spectra (false-colour DRZ images with unified scale). White curves: lineouts showing the energy distribution. b, e, Gamma-ray beam spatial profiles on an IP. c, f, Gamma-ray spectral signals recorded by the LYSO-PX calorimeter. The white curves represent the longitudinal signal profiles obtained by vertical integration. To facilitate direct comparison of signal yields, these profiles are normalized to the global maximum within each measurement modality: the electron spectra (a and d) are normalized to the peak intensity of a, while the calorimeter signals (c and f) are normalized to the peak intensity of f.
• Figure 3: Spectral decomposition proving nonlinear Compton scattering.a, Inferred effective laser intensity $a_0$ and electron participant fraction $R$ (defined as the ratio of electrons undergoing Compton scattering to those producing bremsstrahlung) for three independent interaction events (labeled Event 1--3). The stars mark the best-fit parameters derived from the maximum likelihood analysis. b, Signal reconstruction for Event 1. The experimental LYSO profile (red circles) is compared with best-fit models assuming nonlinear (NCS, purple dashed line) or linear (LCS, green dashed-dotted line) Compton scattering superimposed on the bremsstrahlung background. c, Deconvolved gamma-ray spectra for Event 1. The extracted NCS component (blue dashed line) extends well beyond the linear cutoff energy (vertical black dashed line, $\approx 8.4$ MeV), while the red curve depicts the bremsstrahlung background.
• Figure 4: Determination and validation of gamma-ray polarization.a, Comparison of the experimentally measured spatial profile of the gamma-ray signal (integrated IP signal, grey line) with Geant4 simulations (colored solid lines, assuming a known gamma-ray spectrum with varying polarization degrees of 0.4, 0.5, 0.6). b, Neutron doses measured by bubble detectors positioned parallel (Para., red) and perpendicular (Perp., blue) to the laser polarization axis. The solid bars represent the total measured dose (signal + background), while the hatched bars denote the isolated NCS component obtained after explicitly subtracting the bremsstrahlung background. Error bars represent the standard deviation. c, Validation of the retrieved polarization. The plot displays the experimental polarization degree (derived from the neutron yield asymmetry) and average energy for three shots, compared against theoretical predictions based on the LCFA and LMA models for the emitted photon polarization.