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

Abstract

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient \vone, are presented for transverse momenta ${\mathrm{p}}_{\mathrm{T}}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $\sqrt{{\mathrm{s}}_{{}_{\mathrm{NN}}}}=7.7-200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $\sqrt{{\mathrm{s}}_{{}_{\mathrm{NN}}}}$, centrality and ${\mathrm{p}}_{\mathrm{T}}$ 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 ${\mathrm{p}}_{\mathrm{T}}$ dependencies of ${\mathrm{v}}_1^{\mathrm{even}}$, 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 ${\mathrm{v}}_1^{\mathrm{even}}$ 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.

8 pages, four figures, submitted for publication