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Fluid-dynamical theories are always constructed in terms of an expansion around a given, yet arbitrary, local equilibrium state. This is implemented by the choice of the so-called matching conditions which define the temperature, chemical potential, and velocity of a viscous fluid. Matching conditions are an essential feature of nonequilibrium systems and their consequences to the emergence of hydrodynamic behavior have not been explored. In particular, the interplay between matching conditions and fluid-dynamical attractors [1] are far from understood.
We investigate for the first time how fluid-dynamical attractors in Bjorken flow are affected by choices of matching conditions, considering several formulations of fluid dynamics and kinetic theory. We show that the effect considerably worsens the agreement between solutions of first-order [2] and second-order fluid dynamics [3] and kinetic theory. These results directly affect the modeling of ultrarelativistic heavy-ion collisions where a fluid dynamical approximation is thought to be valid even at early times when the system is far from equilibrium.
[1] M. Heller and M. Spalinski, Phys. Rev. Lett. 115 (2015) 7, 072501.
[2] F. S. Bemfica, M. M. Disconzi and J. Noronha, Phys.Rev.D 98, no.10, 104064 (2018).
[3] W. Israel, J. M. Stewart, Ann. of Phys., 118 (2), 341-372 (1979); G.S.Denicol, H. Niemi, E. Molnar and D. H. Rischke, Phys. Rev. D 85, 114047 (2012).