Measurements of azimuthal anisotropy and charged-particle multiplicity in + Au collisions at = 200, 62.4, 39, and 19.6 GeV

Aidala, C.*; Hasegawa, Shoichi   ; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 246 of others*

We present measurements of the elliptic flow, , as a function of transverse momentum (), pseudorapidity (), and centrality in Au collisions at 200, 62.4, 39, and 19.6 GeV. The beam-energy scan of Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero signal at all four collision energies, which, at midrapidity and low , is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the Monte Carlo generator) show good agreement with the data at midrapidity to forward (-going) rapidities and low . At backward (Au-going) rapidities and GeV/, the data diverges from calculations of relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity () as a function of in central Au collisions at the same energies. We find that in Au collisions at 200 GeV the scales with over all in the PHENIX acceptance. At 62.4, and 39 GeV, scales with at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the scaling may be a further indication of nonflow effects dominating at backward rapidity.