Speaker
Description
Investigating collective behavior due to the formation of a fluid-like medium in small collision systems has been a significant focus in the field. A tell-tale signature of this would be the medium's response to the initial state in small collision systems, as predicted by fluid-dynamic models.
Recent RHIC studies of small systems have shown a hierarchy of elliptic anisotropy coefficients ($v_2$$^{3}$He+Au) $\sim$ $v_2$(d+Au) $>$ $v_2$(p+Au)) suggesting fluid-dynamic behavior even in the smallest systems. This raises the question: could a photo-nuclear collision, such as $\gamma$+Au also exhibit signatures of collectivity? Notably, signatures of collectivity have been investigated in high-multiplicity, high energy $\gamma$+p/Pb collisions at the LHC.
In this work, we explore anisotropic flow in $\gamma$+Au processes at RHIC by triggering ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. At this collision energy, the maximum photon-nucleon center of mass energy $W_{\gamma N}^{\rm max}$ $\approx$ 34.7 GeV [1], an energy between d+Au collisions at $\sqrt{s_{NN}}$ = 19 GeV and $\sqrt{s_{NN}}$ = 39 GeV, previously performed at RHIC. For both $\gamma$+Au and d+Au collisions, a similar multiplicity range is accessible at STAR, making d+Au a suitable baseline system for comparison. Furthermore, the STAR detector’s extended rapidity coverage, with mid and forward rapidity upgrades ($|\eta| < 1.5$ and $2.1 < |\eta| < 5.1$) enables the triggering and analysis of photo-nuclear processes. Preliminary measurements of $v_2$ and $v_3$ in $\gamma$+Au collisions have been conducted at multiplicities and energy levels comparable to those observed in d+Au collisions, where collective behavior has already been established. These results will provide new insights into collectivity in small collision systems, emphasizing the role of initial-state effects and collective behavior in understanding the evolution of the fluid-like medium created in various collision systems at RHIC.
References:
[1] A.J. Baltz et al, The physics of ultraperipheral collisions at the LHC, Physics Reports, 458(1):1–171, 2008.
