# Experimental Evidence of Quantum Radiation Reaction in Aligned Crystals

## TL;DR

This paper presents the first experimental evidence of quantum radiation reaction effects in ultrarelativistic positrons within aligned silicon crystals, demonstrating multiple photon emissions and nonlinear quantum dynamics.

## Contribution

It provides the first experimental validation of quantum radiation reaction theory in a regime where quantum recoil and nonlinear photon emissions are significant.

## Key findings

- Positrons emit multiple high-energy photons, showing quantum recoil effects.
- Emission spectra indicate nonlinear quantum regime with multi-photon absorption.
- Quantum theory of radiation reaction explains experimental results.

## Abstract

Radiation reaction is the influence of the electromagnetic field emitted by a charged particle on the dynamics of the particle itself. Here we report experimental radiation emission spectra from ultrarelativistic positrons in silicon in a regime where both quantum and radiation-reaction effects dominate the dynamics of the positrons. We found that each positron emits multiple photons with energy comparable to its own energy, revealing the importance of quantum photon recoil. Moreover, the shape of the emission spectra indicates that photon emissions occur in a nonlinear regime where positrons absorb several quanta from the crystal field. Our theoretical analysis shows that only a full quantum theory of radiation reaction is capable of explaining the experimental results, with radiation-reaction effects arising from the recoils undergone by the positrons during multiple photon emissions. This experiment is the first fundamental test of quantum electrodynamics in a new regime where the dynamics of charged particles is determined not only by the external electromagnetic fields but also by the radiation-field generated by the charges themselves. Future experiments carried out in the same line will be able to, in principle, also shed light on the fundamental question about the structure of the electromagnetic field close to elementary charges.