[Hybrid RBRC seminar] The 3D structure of anisotropic flow in small collision systems at the RHIC and LHC

by Wenbin Zhao

Thursday 26 Jan 2023, 12:30 → 13:30 US/Eastern

2-160 (https://bnl.zoomgov.com/j/1606067399?pwd=am02b0xIUXZ6TDR3d1dWL05TTDdaUT09)

STAR and PHENIX Collaborations have recently reported measurements of the anisotropic flow coefficients in p+Au, d+Au, and $^{3}$He+Au collisions at the RHIC. However, some tension between STAR and PHENIX data has not been fully resolved. Because the STAR and PHENIX Collaborations applied different pseudorapidity ranges to analyze the two-particle correlations, the flow rapidity correlations in these asymmetric systems could play a crucial role in understanding the difference in the data.
This talk will present full (3+1)D dynamical simulations of asymmetric nuclear collisions at RHIC and the LHC. We first explore the rapidity dependence of anisotropic flow in the RHIC small system scan at 200 GeV. By extrapolating from $^3$He+Au to d+Au and p+Au collisions, we find that the different amounts of longitudinal flow decorrelations result in larger $v_3$ with the STAR definition than those with the PHENIX definition in p+Au and d+Au collisions at 200 GeV. Furthermore, our calculation demonstrates that a considerable fraction of the $v_3(p_T)$ difference in STAR and PHENIX measurements can be explained using reference flow vectors from different rapidity regions. Therefore, the longitudinal flow decorrelation is crucial to understand the anisotropic flow measurements in asymmetric nuclear collisions.
Then we will talk about the "collectivity" in ultra-peripheral Pb+Pb collisions (UPCs) at the LHC. Intriguing experimental results on two-particle azimuthal correlations in UPCs have been measured at the LHC. Extrapolating from p+Pb collisions, we explore whether a quasi-real photon $\gamma^*$ interacting with the lead nucleus in an ultra-peripheral collision can create a many-body system exhibiting fluid behavior. Assuming the strong final-state interactions, we provide model results for charged hadron multiplicity, mean transverse momenta, and anisotropic flow coefficients and compare them with experimental data. The elliptic flow hierarchy between p+Pb and  the much lower energy $\gamma^*$+Pb collisions is dominated by the difference in longitudinal flow decorrelations and experimental data is well reproduced. Our theoretical framework provides a quantitative tool to study particle production and collectivity for all system sizes, ranging from central heavy-ion collisions to small asymmetric collision systems at RHIC and LHC and even at the future Electron-Ion Collider.