Radiation Reaction near the Classical Limit in Aligned Crystals

TL;DR

This study experimentally tests the Landau-Lifshitz equation for radiation reaction by measuring emission spectra of high-energy positrons and electrons in aligned crystals, confirming the importance of radiation reaction effects in strong electromagnetic fields.

Contribution

It provides the first quantitative experimental validation of the Landau-Lifshitz equation in a classical regime with strong fields, including quantum corrections, using particle-crystal interactions.

Findings

Spectra agree with Landau-Lifshitz predictions with quantum corrections.

Lorentz force alone is inadequate for high-energy particles in strong fields.

Experimental results highlight the significance of radiation reaction effects.

Abstract

An accelerated charged particle emits electromagnetic radiation. If the driving force is sufficiently strong, the radiated energy becomes comparable to the kinetic energy of the particle and the back-action of the emitted radiation (radiation reaction) significantly alters the dynamics of the particle. The Landau-Lifshitz (LL) equation has been proposed as the classical equation to describe the dynamics of a charged particle in a strong electromagnetic field when the effects of radiation reaction are taken into account. Hitherto, the experimental problem in validating the LL equation has been to achieve sufficiently strong fields for radiation reaction to be important without quantum effects being prominent. Notwithstanding, here we provide a quantitative experimental test of the LL equation by measuring the emission spectrum for a wide range of settings for 50 GeV positrons crossing aligned silicon single crystals near the $(110)$ planar channeling regime as well as 40 GeV and 80 GeV electrons traversing aligned diamond single crystals near the $\langle100\rangle$ axial channeling regime. The experimental spectra are in remarkable agreement with predictions based on the LL equation of motion with small quantum corrections for recoil and, in case of electrons, spin and reduced radiation emission, as well as with a more elaborate quantum mechanical model. Our experiment clearly shows the inadequacy of the Lorentz force as the sole agent of force on the particles in the classical limit, due to its absence of radiative energy loss in describing the dynamics of high-energy charged particles in strong electromagnetic fields like those in aligned single crystals.