Evolution of Relativistic Pair Beams: Implications for Laboratory and TeV Astrophysics

TL;DR

This paper investigates the evolution of relativistic pair beams influenced by plasma instabilities, using a Fokker-Planck model and particle-in-cell simulations to understand their energy loss and broadening effects relevant to astrophysics and laboratory experiments.

Contribution

It introduces a combined analytical and simulation approach to model pair beam evolution, highlighting the roles of energy loss and momentum diffusion in plasma environments.

Findings

Instability causes energetic broadening of pair beams.

Broadening slows down the linear growth of the instability.

Results are applicable to laboratory and astrophysical observations.

Abstract

Missing cascades from TeV blazar beams indicate that collective plasma effects may play a significant role in their energy loss. It is possible to mimic the evolution of such highly energetic pair beams in laboratory experiments using modern accelerators. The fate of the beam is governed by two different processes, energy loss through the unstable mode and energetic broadening of the pair beam through diffusion in momentum space. We chalk out this evolution using a Fokker-Planck approach in which the drift and the diffusion terms respectively describe these phenomena in a compact form. We present particle-in-cell simulations to trace the complete evolution of the unstable beam-plasma system for a generic narrow Gaussian pair beam for which the growth rate is reactive. We show that the instability leads to an energetic broadening of the pair beam, slowing down the instability growth in the linear phase, in line with the analytical and numerical solutions of the Fokker-Planck equation. Whereas in a laboratory experiment the change in the momentum distribution is an easily measured observable as a feedback of the instability, the consequence of diffusive broadening in an astrophysical scenario can be translated to an increase in the opening angle of the pair beam.