Speaker
Description
The emergence and evolution of collective behavior in small collision systems remains a key area of interest in high-energy nuclear physics. To understand how collectivity evolves with system size RHIC has conducted a dedicated small system scans, including He$^3$+Au, d+Au, and p+Au collisions.
In 2021, the STAR collaboration expanded the scan by introducing a symmetric yet small system through O+O collisions at RHIC, offering a unqiue opportunity to study the interplay between initial-state geometry and fluctuations. In the same year, STAR revisited d+Au collisions at RHIC, utilizing the detector’s extended pseudorapidity coverage ($|\eta| <$ 1.5) and (2.1 $< |\eta| <$ 5.1) to systematically study azimuthal correlations, focusing on their dependence on relative pseudorapidity ($\Delta\eta$).
The smallest system displaying collectivity observed at RHIC to date is p+Au. To examine even smaller systems, STAR has initiated a search for collectivity in photon-induced ($\gamma$+Au) processes by triggering on ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, corresponding to a maximum photon-nucleon center-of-mass energy of $W_{\gamma,N} \approx$ 34.7 GeV. Additionally, in 2024, STAR conducted its first dedicated effort to probe collectivity in high-multiplicity $p+p$ collisions, collecting high-statistics data at low luminosity at RHIC.
This talk will present STAR’s latest measurements of azimuthal correlations in O+O and d+Au collisions, the status of the collectivity search in $\gamma$+Au processes.We will also discuss the challenges, prospects, and future directions for collectivity studies using high-statistics data from the 2023–2025 RHIC runs in various small systems such as $\gamma$+Au, $p+p$, and the anticipated p+Au collisions.
