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

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient v₁, are presented for transverse momenta p_T, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range s_NN=7.7–200 GeV. The measurements underscore the importance of momentum conservation, and the characteristic dependencies on s_NN, centrality and 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 p_T dependencies of v₁ ^even, as well as an observed similarity between its excitation function and that for v₃, could serve as constraints for initial-state models. The v₁ ^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.