Small systems and regulator dependence in relativistic hydrodynamics

Understand

Consistent theories of hydrodynamics necessarily include nonhydrodynamic modes, which can be viewed as a regulator necessary to ensure causality.

• Under many circumstances the choice of regulator is not relevant, but this is not always the case.
• In particular, for sufficiently small systems (such as those arising in pA or pp collisions) such dependence may be inevitable.
• We address this issue in the context of the modern version of M\"uller-Israel-Stewart theory of relativistic hydrodynamics.