Magnetic field-induced squeezing effect at RHIC and at the LHC

TL;DR

This study investigates how strong, long-lived magnetic fields in off-central heavy-ion collisions influence the anisotropic expansion and flow of quark-gluon plasma at RHIC and LHC energies, revealing a significant suppression of momentum anisotropy.

Contribution

The paper introduces (3+1)-D ideal hydrodynamics simulations with external magnetic fields to quantify their impact on QGP anisotropic flow, highlighting effects of long-lived magnetic fields.

Findings

Long-lived magnetic fields suppress QGP momentum anisotropy by up to 20%.

Magnetic fields leave detectable imprints on the elliptic flow of charged pions.

Magnetic effects are negligible for rapidly decaying fields.

Abstract

In off-central heavy-ion collisions, the quark-gluon plasma (QGP) is exposed to the strongest magnetic fields ever created in the universe. Due to the paramagnetic nature of the QGP at high temperatures, the spatially inhomogeneous magnetic field configuration exerts an anisotropic force density that competes with the pressure gradients resulting from purely geometric effects. In this paper, we simulate (3+1)-dimensional ideal hydrodynamics with external magnetic fields to estimate the effect of this force density on the anisotropic expansion of the QGP in collisions at RHIC and at the LHC. While negligible for quickly decaying magnetic fields, we find that long-lived fields generate a substantial force density that suppresses the momentum anisotropy of the plasma by up to $20\%$ at the LHC energy, and also leaves its imprint on the elliptic flow $v_2$ of charged pions.