UCLA

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient v 1 , 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 N N = 7.7 – 200 GeV. The measurements underscore the importance of momentum conservation, and the characteristic dependencies on s N N , 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 v 1 even , as well as an observed similarity between its excitation function and that for v 3 , could serve as constraints for initial-state models. The v 1 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.