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

TL;DR

This study reports the first experimental measurement of gamma-ray polarization in strong-field nonlinear Compton scattering, confirming key quantum electrodynamics predictions and demonstrating a new method for generating polarized gamma rays.

Contribution

It provides the first experimental verification of gamma-ray polarization in strong-field QED using all-optical methods, confirming theoretical predictions and highlighting quantum interference effects.

Findings

Gamma rays with ~50% linear polarization measured.

Results agree with SFQED calculations using the locally monochromatic approximation.

Divergence observed from locally constant field approximation predictions.

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.}