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  • Letter
  • Open Access

Far-from-equilibrium slow modes and momentum anisotropy in an expanding plasma

Jasmine Brewer1,2,*, Weiyao Ke3,6, Li Yan4, and Yi Yin5,†

  • 1Theoretical Physics Department, CERN, CH-1211 Genève 23, Switzerland
  • 2Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3PU, United Kingdom
  • 3Theoretical Division, Los Alamos National Laboratory, Los Alamos New Mexico 87545, USA
  • 4Institute of Modern Physics, Fudan University, Handan Road 220, Yangpu District, Shanghai, 200433, China
  • 5Quark Matter Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
  • 6Key Laboratory of Quark and Lepton Physics (MOE) & Institute of Particle Physics, Central China Normal University, Wuhan 430079, China

  • *Corresponding author: jasmine.brewer@physics.ox.ac.uk
  • †yiyin@impcas.ac.cn
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Phys. Rev. D 109, L091504 – Published 28 May, 2024

DOI: https://doi.org/10.1103/PhysRevD.109.L091504

Abstract

The momentum distribution of particle production in heavy-ion collisions encodes information about thermalization processes in the early-stage quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium evolution of an expanding plasma with an anisotropic momentum-space distribution. We identify slow and fast degrees of freedom in the far-from-equilibrium plasma from the evolution of moments of this distribution. At late times, the slow modes correspond to hydrodynamic degrees of freedom and are naturally gapped from the fast modes by the inverse of the relaxation time, τR−1. At early times, however, there are an infinite number of slow modes with a gap inversely proportional to time, τ−1. From the evolution of the slow modes we generalize the paradigm of the far-from-equilibrium attractor to vector and tensor components of the energy-momentum tensor, and even to higher moments of the distribution function that are not part of the hydrodynamic evolution. We predict that initial-state momentum anisotropy decays slowly in the far-from-equilibrium phase and may persist until the relaxation time.

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Phys. Rev. D 109, L091504– Published 28 May, 2024
  • Received 19 December 2022
  • Accepted 30 April 2024
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by CERN.

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