Collective longitudinal polarization in relativistic heavy-ion collisions at very high energy

TL;DR

This paper investigates the longitudinal polarization of particles in high-energy relativistic heavy-ion collisions using hydrodynamics, revealing its slow decrease with energy and its potential to inform about the Quark Gluon Plasma's cooling rate.

Contribution

It introduces the study of longitudinal polarization in such collisions and shows its dependence on temperature gradients, offering a new observable for QGP properties.

Findings

Longitudinal polarization decreases slowly with energy.

Polarization is proportional to temperature gradient at hadronization.

Measurement of polarization can reveal QGP cooling dynamics.

Abstract

We study the polarization of particles in relativistic heavy-ion collisions at very high energy along the beam direction within a relativistic hydrodynamic framework. We show that this component of the polarization decreases much slower with center-of-mass energy compared to the transverse component, even in the ideal longitudinal boost-invariant scenario with non-fluctuating initial state, and that it can be measured by taking advantage of its quadrupole structure in the transverse momentum plane. In the ideal longitudinal boost-invariant scenario, the polarization is proportional to the gradient of temperature at the hadronization and its measurement can provide important information about the cooling rate of the Quark Gluon Plasma around the critical temperature.