Comparing matching prescriptions between pre-equilibrium and hydrodynamic models in high-energy nuclear collisions

TL;DR

This paper introduces a new entropy-based matching prescription for hybrid high-energy nuclear collision simulations, reducing interface discontinuities and maintaining consistent final-state observables compared to the standard energy-conservation method.

Contribution

It presents a novel entropy-based matching method at the pre-equilibrium and hydrodynamic interface, improving the consistency of hybrid simulation models.

Findings

The entropy-based matching reduces pressure discontinuities at the interface.

Final-state hadronic observables remain stable across different matching prescriptions.

The new method significantly decreases the bulk-to-thermodynamic pressure ratio at the start of hydrodynamics.

Abstract

State-of-the-art simulations of high-energy nuclear collisions rely on hybrid setups, involving in particular a pre-equilibrium stage to let the system evolve from a far-from-equilibrium initial condition towards a near-equilibrated state after which fluid dynamics can be applied meaningfully. A known issue is the mismatch between the equation of state in the fluid-dynamical evolution and the effective one in the previous stage, which leads to discontinuities at the interface between the two models. Here we introduce a new matching prescription at this interface, based on the entropy, and we compare it with the standard one relying on local energy conservation. We study the behavior of various quantities at the switching time between the models and investigate a number of final-state hadronic observables. For the latter, we show that they are not modified significantly by the choice of matching prescription, provided an appropriate normalization is chosen for the initial state. In turn, our approach reduces sizeably the ratio of bulk over thermodynamic pressure at the beginning of the fluid-dynamical stage.