Initial-state-driven spin correlations in high-energy nuclear collisions

TL;DR

This paper proposes that initial nucleon spin fluctuations in high-energy nuclear collisions can produce measurable polarization of final particles, offering a new perspective beyond traditional thermal vorticity explanations.

Contribution

It introduces a novel initial-state mechanism for spin polarization, estimating significant net polarization effects and proposing an observable to detect initial spin fluctuations.

Findings

Initial-state fluctuations induce ~1% polarization in central collisions.

Net polarization exceeds thermal vorticity contributions in noncentral collisions.

A two-particle correlation observable can reveal initial spin fluctuations.

Abstract

In the study of spin-polarization phenomena in heavy-ion collisions, it is typically assumed that final-state particles are polarized through thermal vorticity and shear. In this sense, polarization is a final-state effect. Here, we propose a different mechanism. We postulate that the collision of spin-carrying nucleons generates an initial transverse spin density, inducing a net polarization of the QCD fireball along a random direction. If the net spin is conserved throughout the evolution of the fireball, the final-state particles should exhibit measurable polarization. Within a wounded nucleon picture, we estimate that initial-state fluctuations induce a net polarization of $\Lambda$ baryons which is around $1\%$ in central collisions and over $10\%$ in noncentral collisions, significantly exceeding the contributions from thermal vorticity and shear. We introduce a two-particle angular correlation observable designed to reveal initial net-spin fluctuations, and emphasize the main signatures to look for in experiments. We argue that the discovery of these phenomena would have profound implications for nuclear structure and our understanding of spin in relativistic hydrodynamics.