# Quantum radiation reaction in aligned crystals beyond the local constant field approximation

## TL;DR

This paper presents experimental data on photon emission by high-energy positrons in aligned silicon crystals, revealing the limitations of existing models and proposing a new theoretical approach that accurately describes the observed quantum radiation reaction effects.

## Contribution

The study introduces a novel theoretical framework that accounts for multiple photon emissions, recoil, and field variation, surpassing the local constant field approximation.

## Key findings

- Experimental spectra show deviations from traditional models.
- The new theory aligns well with observed data.
- Quantum radiation reaction effects are significant in this regime.

## Abstract

We report on experimental spectra of photons radiated by 50 GeV positrons crossing silicon single crystals of thicknesses 1.1 mm, 2.0 mm, 4.2 mm, and 6.2 mm at sufficiently small angles to the (110) planes that their motion effectively is governed by the continuum crystal potential. The experiment covers a new regime of interaction where each positron emits several hard photons, whose recoil are not negligible and which are formed on lengths where the variation of the crystal field cannot be ignored. As a result neither the single-photon semiclassical theory of Baier et al. nor the conventional cascade approach to multiple hard photon emissions (quantum radiation reaction) based on the local constant field approximation are able to reproduce the experimental results. After developing a theoretical scheme which incorporates the essential physical features of the experiments, i.e., multiple emissions, photon recoil and background field variation within the radiation formation length, we show that it provides results in convincing agreement with the data.