Decoding the flow evolution in Au+Au reactions at $1.23 A$ GeV using hadron flow correlations and dileptons

TL;DR

This paper studies the development of flow patterns in heavy ion collisions at 1.23 A GeV, proposing flow correlation measurements and dilepton elliptic flow as methods to probe the early-state equation of state at high densities.

Contribution

It introduces a novel approach to analyze flow evolution using flow correlations and dilepton elliptic flow to better understand the early-stage equation of state in heavy ion collisions.

Findings

Positive initial elliptic flow due to early pressure gradients

Spectator shadowing causes a transition to negative elliptic flow

Dilepton flow measurements can directly probe the early equation of state

Abstract

We investigate the development of the directed, $v_1$, and elliptic flow, $v_2$, in heavy ion collisions in mid-central Au+Au reactions at $E_\mathrm{lab}=1.23 A$ GeV. We demonstrate that the elliptic flow of hot and dense matter is initially positive ($v_2>0$) due to the early pressure gradient. This positive $v_2$ transfers its momentum to the spectators, which leads to the creation of the directed flow $v_1$. In turn, the spectator shadowing of the in-plane expansion leads to a preferred decoupling of hadrons in the out-of-plane direction and results in a negative $v_2$ for the observable final state hadrons. We propose a measurement of $v_1-v_2$ flow correlations and of the elliptic flow of dileptons as methods to pin down this evolution pattern. The elliptic flow of the dileptons allows then to determine the early-state EoS more precisely, because it avoids the strong modifications of the momentum distribution due to shadowing seen in the protons. This opens the unique opportunity for the HADES and CBM collaborations to measure the Equation-of-State directly at 2-3 times nuclear saturation density.