Collective Flow Measurements from the PHENIX Experiment

TL;DR

This paper reports on collective flow measurements from the PHENIX experiment, analyzing higher order anisotropy, particle type, energy dependence, and jet modification effects in heavy-ion collisions.

Contribution

It provides new measurements of higher order flow anisotropy and its relation to initial geometry and viscosity, enhancing understanding of quark-gluon plasma properties.

Findings

Higher order flow anisotropy consistent with initial geometric fluctuations

v2, v3, v4 nearly unchanged from 39 GeV to 200 GeV collisions

Direct photon v2 is small at high pT but significant at low pT

Abstract

Recent collective flow measurements including higher moment event anisotropy from the PHENIX experiment are presented, and the particle type, beam energy dependence and the relation with jet modification are discussed. The measured higher order event anisotropy with event plane defined at forward rapidities and the long range correlation with large $\eta$ gaps are both consistent with initial geometrical fluctuation of the participating nuclei. In 200 GeV Au+Au collisions, higher order event anisotropy, especially simultaneous description of v$_2$ and v$_3$, is found to give an additional constraining power on initial geometrical condition and viscosity in the hydrodynamic calculations. v$_2$, v$_3$ and v$_4$ are almost unchanged down to the lower colliding energy at 39 GeV in Au+Au. The measured two particle correlation with subtraction of the measured v$_n$ parameters shows a significant effect on the shape and yield in the associate particle $\Delta\phi$ distribution with respect to the azimuthal direction of trigger particles. However some medium responses from jet suppression or jet modification seems to be observed. Direct photon v$_2$ has been measured in 200 GeV Au+Au collisions. The measured v$_2$ is found to be small at high p$_{\rm T}$ as expected from non-suppressed direct photon R$_{\rm AA}$ $\simeq$ 1, which can be understood as being dominated by prompt photons from initial hard scattering. On the other hand, at lower p$_{\rm T}$ $<$ 4 GeV/c it is found to be significantly larger than zero, which is comparable to other hadron v$_2$, where thermal photons are observed.