Beam energy dependence of rapidity-even dipolar flow in Au+Au collisions

TL;DR

This paper reports new measurements of directed flow in Au+Au collisions across a range of energies, highlighting the role of initial fluctuations and hydrodynamics, with implications for understanding the quark-gluon plasma.

Contribution

It provides the first comprehensive analysis of the energy dependence of rapidity-even dipolar flow, linking initial-state fluctuations to hydrodynamic behavior in heavy-ion collisions.

Findings

Directed flow depends on energy, centrality, and transverse momentum.

Rapidity-even dipolar flow shows similar excitation patterns to triangular flow.

Results constrain initial-state models and aid in extracting shear viscosity.

Abstract

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient \vone, are presented for transverse momenta $\mathrm{p_T}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $\mathrm{\sqrt{s_{_{NN}}}} = 7.7 - 200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $\mathrm{\sqrt{s_{_{NN}}}}$, centrality and $\mathrm{p_T}$ are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and $\mathrm{p_T}$ dependencies of $\mathrm{v^{even}_{1}}$, as well as an observed similarity between its excitation function and that for $\mathrm{v_3}$, could serve as constraints for initial-state models. The $\mathrm{v^{even}_{1}}$ excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity.