Characterizing the initial state of hydrodynamics in pp and pA collisions

TL;DR

This paper investigates how to characterize the initial hydrodynamic state in proton-proton and proton-nucleus collisions, proposing entanglement entropy as a key measure for matching quantum initial conditions to classical hydrodynamics.

Contribution

It introduces entanglement entropy as a suitable initial state measure for hydrodynamic modeling in small collision systems, addressing the quantum-classical matching challenge.

Findings

Entanglement entropy is a better initial state measure than traditional 3D nucleon structure descriptors.

The study highlights the importance of quantum information measures in high-energy collision modeling.

Proposes a framework for connecting quantum initial conditions to classical hydrodynamics in small systems.

Abstract

The observation of seeming hydrodynamic-like behavior in proton-proton and proton-nucleus collisions presents us with the conceptual problem of how the initial state of such a hydrodynamic evolution should be characterized. This is an issue because, while nuclei can reasonably be approximated as ``large'' systems w.r.t. the characteristic Fermi momentum of their constituents, this is no longer true for nucleons. Hence, one would need to match a ``quantum'' theory, whose observables are described via highly non-commuting operators, to a ``classical'' hydrodynamics. Operationally assuming a ``fast'' thermalization, we survey what kind of object is best suited to such a matching condition. We show that it cannot be any of the objects usually associated with ``the 3D structure of the nucleon'' but rather a measure associated with entanglement entropy.