Angular momentum conservation in heavy ion collisions at very high energy

TL;DR

This paper explores how angular momentum conservation influences the behavior of quark-gluon plasma in high-energy heavy ion collisions, affecting azimuthal anisotropy, vorticity, and hadron polarization, with implications for LHC observations.

Contribution

It introduces the role of angular momentum conservation in enhancing elliptic flow and vorticity effects in quark-gluon plasma, providing analytical expressions for polarization signatures.

Findings

Angular momentum increases elliptic flow (v2) at high energies.

Vorticity in the plasma leads to hadron polarization.

Enhanced effects are expected at LHC compared to RHIC.

Abstract

The effects of angular momentum conservation in peripheral heavy ion collisions at very high energy are investigated. It is shown that the initial angular momentum of the quark-gluon plasma should enhance the azimuthal anisotropy of particle spectra (elliptic flow) with respect to the usual picture where only the initial geometrical eccentricity of the nuclear overlap region is responsible for the anisotropy. In hydrodynamical terms, the initial angular momentum entails a non trivial dependence of the initial longitudinal flow velocity on the transverse coordinates. This gives rise to a non-vanishing vorticity in the equations of motion which enhances the expansion rate of the supposedly created fluid compensating for the possible quenching effect of viscosity. A distinctive signature of the vorticity in the plasma is the generation of an average polarization of the emitted hadrons, for which we provide analytical expressions. These phenomena might be better observed at LHC, where the initial angular momentum density will be larger and where we envisage an increase of the elliptic flow coefficient v_2 with respect to RHIC energies.