Probing initial baryon stopping and equation of state with rapidity-dependent directed flow of identified particles

TL;DR

This paper investigates how the rapidity-dependent directed flow of identified particles in heavy-ion collisions reveals information about initial baryon stopping and the nuclear matter equation of state, using a comprehensive hybrid model across various energies.

Contribution

It introduces a (3+1)-D hybrid framework that successfully reproduces directed flow data and demonstrates the sensitivity of baryon and meson flow to initial conditions and the equation of state.

Findings

The model accurately reproduces $v_1(y)$ for mesons and baryons across energies.

Baryon $v_1(y)$ strongly constrains initial baryon stopping.

Directed flow probes the nuclear matter equation of state at finite chemical potentials.

Abstract

Using a (3+1)-dimensional hybrid framework with parametric initial conditions, we study the rapidity-dependent directed flow $v_1(y)$ of identified particles, including pions, kaons, protons, and lambdas in heavy-ion collisions. Cases involving Au+Au collisions are considered, performed at $\sqrt{s_{\rm NN}}$ ranging from 7.7 to 200 GeV. The dynamics in the beam direction is constrained using the measured pseudo-rapidity distribution of charged particles and the net proton rapidity distribution. Within this framework, the directed flow of mesons is driven by the sideward pressure gradient from the tilted source, and that of baryons mainly due to the initial asymmetric baryon distribution with respect to the beam axis driven by the transverse expansion. Our approach successfully reproduces the rapidity- and beam energy-dependence of $v_1$ for both mesons and baryons. We find that the $v_1(y)$ of baryons has strong constraining power on the initial baryon stopping, and together with that of mesons, the directed flow probes the equation of state of the dense nuclear matter at finite chemical potentials.