Baryon diffusion coefficient of the strongly interacting medium

TL;DR

This paper proposes using the $p_T$ differential splitting of directed flow between protons and anti-protons as a sensitive observable to extract the baryon diffusion coefficient in hot, dense QCD matter created in heavy ion collisions, employing hydrodynamics and initial condition calibration.

Contribution

It introduces a novel method to determine the baryon diffusion coefficient using directed flow splitting data within a hydrodynamic framework, calibrated to experimental measurements.

Findings

The $p_T$ differential splitting of directed flow is sensitive to the baryon diffusion coefficient.

Recent STAR data favors a $C_B$ value between 0.5 and 1.5.

The method links flow splitting measurements to microscopic transport properties.

Abstract

We propose that the transverse momentum ($p_T$) differential splitting of directed flow ($\Delta v_1$) between proton and anti-proton can serve as a sensitive observable to extract the baryon diffusion coefficient ($\kappa_B$) of the hot and dense strongly interacting matter produced in relativistic heavy ion collisions. We use relativistic dissipative hydrodynamics framework with Glauber model based initial condition for the energy as well as baryon deposition that is calibrated to capture the rapidity dependence of charged particle multiplicity, net proton yield as well as the elusive $v_1$ splitting between proton and anti-proton. We employ the commonly used kinetic theory motivated ansatz: $\kappa_B= C_B \frac{n_B}{T} \left( \frac{1}{3} \text{coth}\left(\frac{\mu_B}{T} \right) - \frac{n_BT}{\epsilon+P} \right)$ where $n_B$, $\epsilon$, $P$, $T$ and $\mu_B$ are baryon number density, energy density, pressure, temperature and baryon chemical potential respectively while $C_B$ is an arbitrary constant which is largely unknown for the Quantum Chromodynamics (QCD) medium. We find that the variation of $\Delta v_1$ with $p_T$ is strongly influenced by the choice of $C_B$. Further, we find that the recent STAR measurement of the centrality dependence of the rapidity slope of $\Delta v_1$ prefers $0.5<C_B<1.5$.