# Paramagnetic squeezing of a uniformly expanding quark-gluon plasma in and out of equilibrium

## TL;DR

This paper develops a framework to describe the pressure anisotropy in a magnetized quark-gluon plasma, incorporating magnetization effects into anisotropic hydrodynamics, and provides analytical and numerical results for its evolution.

## Contribution

It introduces a novel anisotropic hydrodynamics approach accounting for magnetization in a quark-gluon plasma, extending to dissipative fluids with dynamical magnetic fields.

## Key findings

- Pressure anisotropy due to paramagnetism is modeled in an expanding plasma.
- Analytical expressions for viscosities in a magnetized medium are derived.
- Numerical solutions show the evolution of energy density and pressures over time.

## Abstract

The plasma of quarks and gluons created in ultrarelativistic heavy-ion collisions turns out to be paramagnetic. In the presence of a background magnetic field, this paramagnetism thus leads to a pressure anisotropy, similar to anisotropies appearing in a viscous fluid. In the present paper, we use this analogy, and develop a framework similar to anisotropic hydrodynamics, to take the pressure anisotropy caused, in particular, by the nonvanishing magnetization of a plasma of quarks and gluons into account. We consider the first two moments of the classical Boltzmann equation in the presence of an electromagnetic source in the relaxation-time approximation, and derive a set of coupled differential equations for the anisotropy parameter $\xi_0$ and the effective temperature $\lambda_0$ of an ideal fluid with nonvanishing magnetization. We also extend this method to a dissipative fluid with finite magnetization in the presence of a strong and dynamical magnetic field. We present a systematic method leading to the one-particle distribution function of this magnetized dissipative medium in a first-order derivative expansion, and arrive at analytical expressions for the shear and bulk viscosities in terms of the anisotropy parameter $\xi$ and effective temperature $\lambda$. We then solve the corresponding differential equations for $(\xi_0,\lambda_0)$ and $(\xi,\lambda)$ numerically, and determine, in this way, the proper time and temperature dependence of the energy density, directional pressures, speed of sound, and the magnetic susceptibility of a longitudinally expanding magnetized quark-gluon plasma in and out of equilibrium.