Global angular momentum generation in heavy-ion reactions within a hadronic transport approach

TL;DR

This paper uses a microscopic transport model to study how global angular momentum is generated in heavy-ion collisions, confirming previous geometric models and analyzing conservation and system size effects across energies.

Contribution

It applies the SMASH transport approach to quantify angular momentum transfer in heavy-ion collisions, providing detailed phase-space evolution and conservation analysis.

Findings

Angular momentum transfer peaks in mid-central collisions.

Impact parameter range for maximum transfer is 4-6 fm.

Optimal setups can recover angular momentum conservation.

Abstract

In 2017, the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) has measured finite global angular momentum in heavy-ion collisions through a spin polarization measurement of $\Lambda$ hyperons. This measurement revealed a high angular momentum of the heavy ions and provided experimental evidence for vorticity in the quark-gluon plasma (QGP) for the first time. In order to investigate the underlying mechanisms, a dynamic description of the transfer of angular momentum is required. In this work, the microscopic non-equilibrium transport approach SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is applied to study the generation of global angular momentum by the interaction of two nuclei. As SMASH provides access to the whole phase-space evolution of every particle at any given time, it allows to assess the fraction of angular momentum generated in the fireball by all participants. We confirm the previous modeling by Becattini \textit{et al} within a geometric Glauber model approach, which found that the angular momentum transfer reaches a unique maximum in mid-central collisions during time evolution. The corresponding impact parameter is around $b=4-6$ fm for all beam energies from $\sqrt{s_{\rm NN}}=2.41-200$ GeV. Even though angular momentum is not conserved locally in the transport approach a priori, we identify the contributions to the conservation violation and propose optimal setups for different energy regimes that recover conservation, based upon the test particle method and the treatment of Fermi motion. Furthermore, the system size and centrality dependence are investigated.