Directed Flow Indicates a Crossover Deconfinement Transition in Relativistic Nuclear Collisions

TL;DR

This study analyzes directed flow in heavy-ion collisions across various energies, finding that a crossover deconfinement transition in the equation of state best explains experimental data, indicating a stiffer quark-gluon sector at high densities.

Contribution

It demonstrates that the crossover EoS aligns better with experimental directed flow data than first-order or purely hadronic EoS, highlighting the nature of the deconfinement transition.

Findings

Crossover EoS is favored by experimental data.

First-order phase transition EoS shows a wiggle in proton v1 slope.

Hadronic EoS cannot reproduce the positive midrapidity slope.

Abstract

Analysis of directed flow ($v_1$) of protons, antiprotons and pions in heavy-ion collisions is performed in the range of incident energies $\sqrt{s_{NN}}$ = 2.7--27 GeV. Simulations have been done within a three-fluid model employing a purely hadronic equation of state (EoS) and two versions of the EoS involving deconfinement transitions: a first-order phase transition and a smooth crossover transition. High sensitivity of the directed flow, especially the proton one, to the EoS is found. The crossover EoS is favored by the most part of considered experimental data. A strong wiggle in the excitation function of the proton $v_1$ slope at the midrapidity obtained with the first-order-phase-transition EoS and a smooth proton $v_1$ with positive midrapidity slope, within the hadronic EoS unambiguously disagree with the data. The pion and antiproton $v_1$ also definitely testify in favor of the crossover EoS. The results obtained with deconfinement EoS's apparently indicate that these EoS's in the quark-gluon sector should be stiffer at high baryon densities than those used in the calculation.