Directed flow of photons in Cu + Au collisions at RHIC

Abstract

The Cu + Au collisions are expected to provide interesting new aspects in the understanding of anisotropic flow in heavy ion collisions due to asymmetric initial geometry on the transverse plane as well as due to the significant difference in number of participants in the two colliding nuclei. We calculate directed flow coefficient v 1 of thermal photons from 200 A GeV Cu + Au collisions at RHIC using a hydrodynamic model with fluctuating initial conditions and compare the results with the elliptic and triangular flow parameters. The photon v 1(p T) is found to be non-zero and shows a different nature compared to the elliptic and triangular flow parameters as a function of p T. We see a negative v 1 for p T < 2 GeV/c, which becomes positive and rises slowly with larger p T (>2 GeV/c) values. The QGP radiation completely dominates the v 1 in the region 1 < p T < 6 GeV/c whereas, the contribution from hadronic phase is only marginal. In addition, the v 1 is found to be more sensitive to the initial formation time of the plasma compared to the elliptic and triangular flow parameters. Photon v 1 from Au + Au collisions is also calculated and compared with the v 1 from Cu + Au collisions. These results indicate that fluctuations play the most significant role in the determination of photon v 1 as the results from symmetric and symmetric collisions are found to be close to each other. We suggest that a simultaneous measurements of photon v n coefficients (n = 1, 2, 3) can provide valuable information about the initial state produced in heavy ion collisions.

Anisotropic flow of photons in relativistic heavy ion collisions

“…[25] that photon v 1 does not depend on the centrality of the collisions. We have also seen that the directed flow of photons is totally dominated by the QGP radiations [26]. Thus, the experimental determination of photon v 1 is expected to provide valuable information regarding photon anisotropic flow parameter.…”

“…On the other hand, the inclusion of shear viscosity in the hydrodynamical model calculation is found to reduce the elliptic flow especially towards the higher p T values. It has also been shown recently that the directed flow parameter v 1 of photons can play a crucial role in our understanding of photon observables in heavy ion collisions [25,26]. The photon v 1 is found to show distinct p T dependent behaviour compared to the elliptic and the triangular flow parameters.…”

“…As a result, it is expected to provide interesting new aspects in the understanding of anisotropic flow in heavy-ion collisions. Two different geometries of mid-central Cu+Au collisions are shown in figure 23 [114]. The STAR Collaboration at RHIC has shown in an interesting study that the directed flow of hadrons from Cu+Au collisions is larger than that from Au+Au collisions at mid-rapidity [115].…”

“…A recent study has shown that directed flow of thermal photons from Cu+Au collisions at RHIC shows different nature compared to the elliptic and triangular flow parameters [114] as a function of p T for 20-30% centrality bin. The photon directed flow is large, nonzero and is negative (positive) at smaller (larger) values of transverse momentum (see figure 24).…”

“…The v 1 is found to be negative for smaller p T and becomes positive at higher p T (> 2 GeV) values. In addition, the v 1 is found to be completely dominated by the QGP radiation [26].…”