Pre-hydrodynamic initialization of relativistic hydrodynamics

Determine a theoretically justified description of the pre-hydrodynamic stage in relativistic heavy-ion collisions and derive a principled procedure for initializing hydrodynamic fields in relativistic hydrodynamics, specifying how and when to set the initial conditions for energy density, pressure, flow velocity, and dissipative currents prior to hydrodynamic evolution.

Background

Relativistic hydrodynamics requires initial conditions for its first-order-in-time evolution equations. While the late-stage transition from hydrodynamics to hadronic transport is well established, the early-time dynamics prior to hydrodynamization remains theoretically unsettled. The lack of a validated pre-hydrodynamic framework introduces systematic uncertainties in parameter extraction and model-to-data comparisons, even when Bayesian methods are employed.

The paper motivates constraining initial conditions via nonlinear causality but notes that a first-principles prescription for initializing the hydrodynamic fields themselves is not yet established, highlighting a foundational gap in current modeling of heavy-ion collisions.

References

On the other hand, it has not been established so far how to describe the pre-hydrodynamic stage, namely how to initialize hydrodynamic fields in relativistic hydrodynamics.

— Constraint on initial conditions of one-dimensional expanding fluids from nonlinear causality  (2412.02405 - Hoshino et al., 2024) in Section 1 (Introduction)

Various studies of jet energy loss rates have been carried out in both the weak-coupling and strong-coupling scenarios, but the hydrodynamization process via which the energy lost by the jet partons and deposited in the medium becomes a perturbation to the hydrodynamic fluid is not fully understood.

— Computationally Efficient Description of Medium Response to Jets in Heavy Ion Collisions  (2609.10679 - Casalderrey-Solana et al., 9 Sep 2026) in Section 2, subsection “Linearized Hydrodynamics on Bjorken Flow”