Attractive vs. repulsive interactions in the Bose-Einstein condensation dynamics of relativistic field theories

TL;DR

This paper investigates how attractive and repulsive self-interactions influence the nonequilibrium dynamics, Bose-Einstein condensation, and formation of stable charge clumps in relativistic quantum fields with large low-momentum occupancies.

Contribution

It provides a comparative analysis of attractive versus repulsive interactions on condensation dynamics using classical simulations and a systematic 1/N expansion in scalar field theories.

Findings

Repulsive interactions lead to a universal inverse particle cascade and condensation.

Attractive interactions suppress the inverse cascade and increase particle annihilation rates.

Stable localized charge clumps (Q-balls) can form when a conserved charge is present.

Abstract

We study the impact of attractive self-interactions on the nonequilibrium dynamics of relativistic quantum fields with large occupancies at low momenta. Our primary focus is on Bose-Einstein condensation and nonthermal fixed points in such systems. As a model system we consider O(N)-symmetric scalar field theories. We use classical-statistical real-time simulations, as well as a systematic 1/N expansion of the quantum (2PI) effective action to next-to-leading order. When the mean self-interactions are repulsive, condensation occurs as a consequence of a universal inverse particle cascade to the zero-momentum mode with self-similar scaling behavior. For attractive mean self-interactions the inverse cascade is absent and the particle annihilation rate is enhanced compared to the repulsive case, which counteracts the formation of coherent field configurations. For N >= 2, the presence of a nonvanishing conserved charge can suppress number changing processes and lead to the formation of stable localized charge clumps, i.e. Q-balls.