External magnetic field induced paramagnetic squeezing effect in heavy-ion collisions at the LHC

TL;DR

This study uses hydrodynamic simulations to show that strong, enduring magnetic fields in non-central heavy-ion collisions can significantly influence the anisotropic expansion of quark-gluon plasma, affecting observable flow patterns.

Contribution

It introduces a detailed modeling of the paramagnetic squeezing effect on QGP expansion considering realistic magnetic field profiles and susceptibilities.

Findings

Enduring magnetic fields reduce QGP momentum anisotropy by up to 10%.

Rapidly decaying magnetic fields have negligible impact.

Magnetic fields leave a measurable imprint on elliptic flow $v_{2}$.

Abstract

In non-central heavy-ion collisions, the quark-gluon plasma (QGP) encounters the most intense magnetic field ever produced in nature, with a strength of approximately 10$^{19 - 20}$ Gauss. Recent lattice-QCD calculations reveal that the QGP exhibits paramagnetic properties at high temperatures. When an external strong magnetic field is applied, it generates an anisotropic squeezing force density that competes with pressure gradients resulting from the purely QGP geometric expansion. In this study, we employ (3+1)-dimensional ideal hydrodynamics simulations to estimate the paramagnetic squeezing effect of this force density on the anisotropic expansion of QGP in non-central Pb+Pb collisions at the Large Hadron Collider (LHC). We consider both up-to-date magnetic susceptibility and various magnetic field profiles in this work. We find that the impact of rapidly decaying magnetic fields is insignificant, while enduring magnetic fields produce a strong force density that diminishes the momentum anisotropy of the QGP by up to 10% at the intial stage, leaving a visible imprint on the elliptic flow $v_{2}$ of final charged particles. Our results provide insights into the interplay between magnetic fields and the dynamics of QGP expansion in non-central heavy-ion collisions.