Speaker
Description
Understanding the applicability of fluid-dynamical models to describe the hot and dense matter produced in the early stages of hadronic collisions is a fundamental problem in the field. In particular, it is not clear to what degree this hydrodynamization process requires proximity to a local equilibrium state. In this contribution, we study this problem in kinetic theory considering an ultrarelativistic gas undergoing strong longitudinal expansion, assuming Bjorken flow. We solve the Boltzmann equation using a relativistic generalization of the method of moments [1], where the single-particle distribution is reconstructed via a moment expansion. We first demonstrate that this traditional moment expansion actually diverges for this system and that this series must be resummed [2]. We then compare the exact resummed solution of the single-particle distribution function with solutions of the Boltzmann equation in the hydrodynamic limit and verify that the system displays considerable deviations from local equilibrium, even though the energy-momentum tensor is well described by fluid dynamics. We further show the 14-moment approximation [3], widely employed in simulations of heavy ion collisions, does not provide a good approximation for the momentum distribution of this system. We quantity this effect computing the emission of photons in the quark-gluon plasma [4] and verify if this deviation from equilibrium can be observed.
[1] G. S. Denicol, H. Niemi, E. Molnár, and D. H. Rischke. Phys. Rev. D 85, 114047 (2012). [Erratum: Phys. Rev. D 91, 039902 (2015)].
[2] C. V. P. de Brito, D. Wagner, G. S. Denicol, and D. H. Rischke. arXiv:2411.06267 [nucl-th].
[3] W. Israel and J. M. Stewart. Ann. Phys. 118, 341 (1979).
[4] C. Shen, J.-F. Paquet, U. Heinz, and C. Gale. Phys. Rev. C 91, 014908 (2015).
| Category | Theory |
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