EMMI Workshop: Light-ion collisions 2026

Light-ion collisions provide a unique opportunity to study the onset of QGP collectivity and probe nuclear structure at high energy. In this talk, I will present recent STAR measurements in d+Au and $^{16}$O+$^{16}$O collisions at $\sqrt{s_{NN}}$=200 GeV, where controlled variations of the initial geometry help disentangle nucleon configurations, subnucleonic fluctuations, and final-state collective response [1]. Elliptic and triangular flow measurements show that both systems create medium droplets of comparable size but distinct geometry: the larger $v_2$ in d+Au reflects its elliptic shape, while comparable $v_3$ in both systems points to subnucleonic fluctuations. Hydrodynamic calculations tuned to Au+Au collisions describe these patterns, supporting QGP-like droplet formation and turning symmetric--asymmetric comparisons into a differential scan of initial geometry. I will also discuss new STAR measurements of $v_n-[p_T]$ correlations and $[p_T]$ fluctuations, which are sensitive to the intrinsic structure of $^{16}$O, including possible $\alpha$-clustering configurations. Complementary hard and electromagnetic probes extend this picture: recoil-hadron/jet correlations in $^{16}$O+$^{16}$O show significant high-event-activity suppression [2], while dileptons provide a penetrating thermometer of these small QGP droplets. Together, these results show that light-ion collisions can bridge QCD collectivity, initial-state dynamics, and the many-body structure of light nuclei [3,4].

[1] STAR Collaboration, "Engineering the shapes of quark-gluon plasma droplets by comparing anisotropic flow in small symmetric and asymmetric collision systems", arXiv: 2510.19645 [nucl-ex].
[2] STAR Collaboration, "Measurement of jet quenching in O+O collisions at 200 GeV by the STAR experiment at RHIC", arXiv: 2604.13935 [nucl-ex].
[3] S. Huang, J. Jia, and C. Zhang, "Symmetric-asymmetric collision comparison: Disentangling nuclear structure and subnucleonic structure effects for small system flow", Phys. Lett. B 870, 139926 (2025).
[4] C. Zhang, J. Chen, G. Giacalone, S. Huang, J. Jia, and Y.-G. Ma, "Ab-initio nucleon-nucleon correlations and their impact on high energy 16O+16O collisions", Phys. Lett. B 862, 139322 (2025).