Abstract
A simple kinematical argument suggests that the classical approximation may be inadequate to describe the evolution of a system with an anisotropic particle distribution. In order to verify this quantitatively, we study the Boltzmann equation for a longitudinally expanding system of scalar particles interacting with a ϕ 4 coupling, that mimics the kinematics of a heavy ion collision at very high energy. We consider only elastic 2 → 2 scatterings, and we allow the formation of a Bose-Einstein condensate in overpopulated situations by solving the coupled equations for the particle distribution and the particle density in the zero mode. For generic CGC-like initial conditions with a large occupation number, the solutions of the full Boltzmann equation cease to display the classical attractor behavior sooner than expected; for moderate coupling, the solutions appear never to follow a classical attractor solution.
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Epelbaum, T., Gelis, F., Jeon, S. et al. Kinetic theory of a longitudinally expanding system of scalar particles. J. High Energ. Phys. 2015, 117 (2015). https://doi.org/10.1007/JHEP09(2015)117
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DOI: https://doi.org/10.1007/JHEP09(2015)117