Speaker
Description
The expansion dynamics and transport properties of the quark-gluon plasma (QGP) are essential for understanding the evolution of matter created in relativistic heavy-ion collisions [1–5]. In particular, comparisons between different collision systems provide a powerful way to isolate the system-size dependence of collective expansion and its fluctuations, thereby improving sensitivity to the transport properties of the medium. We present STAR measurements of the $p_{\rm T}$-differential radial-flow fluctuation observable $v_0(p_{\rm T})$, which quantifies long-range correlations between particle-yield fluctuations in a given $p_{\rm T}$ interval and mean-$p_{\rm T}$ fluctuations, in Au+Au and O+O collisions at $\sqrt{s_{NN}}=200$ GeV, with Au+Au serving as the large-system reference. Using a two-particle correlation method with separated pseudorapidity subevents, we measure inclusive charged particles over $0.2 < p_{\rm T} < 10.0~\mathrm{GeV}/c$, identified pions and kaons over $0.2 < p_{\rm T} < 2.0~\mathrm{GeV}/c$, and protons over $0.5 < p_{\rm T} < 3.0~\mathrm{GeV}/c$. Both systems show a characteristic sign change and rise in $v_0(p_{\rm T})$. Identified-particle results exhibit mass ordering at low $p_{\rm T}$. The normalized differential response $v_0(p_{\rm T})/v_0$, where $v_0$ denotes the $p_{\rm T}$-integrated fluctuation magnitude, provides a way to investigate centrality dependence at low-to-intermediate $p_{\rm T}$, while its extension to high $p_{\rm T}$ provides sensitivity to possible jet-quenching contributions. A comparison of O+O and Au+Au collisions at similar multiplicity provides sensitivity to the system-size dependence of radial-flow fluctuations. Model variations indicate strong sensitivity to bulk viscosity, especially for light hadrons, while weak sensitivity to shear viscosity. These results demonstrate the potential of $v_0(p_{\rm T})$ as a differential probe of radial flow fluctuations, providing constraints on the system-size dependence of radial-flow fluctuations and on the properties of the QGP medium.
References
[1] B. Schenke, C. Shen, and D. Teaney, “Transverse momentum fluctuations and their correlation with elliptic flow in nuclear collision,” Phys. Rev. C 102, 034905 (2020). DOI: 10.1103/PhysRevC.102.034905. arXiv: 2004.00690 [nucl-th].
[2] J. Jia, “Sources of radial-flow fluctuations in the quark-gluon plasma,” Phys. Rev. Lett. 136, 112301 (2026). DOI: 10.1103/d6mc-sq8m. arXiv: 2507.14399 [nucl-th].
[3] T. Parida, R. Samanta, and J.-Y. Ollitrault, “Probing collectivity in heavy-ion collisions with fluctuations of the $p_T$ spectrum,” Phys. Lett. B 857, 138985 (2024). DOI: 10.1016/j.physletb.2024.138985. arXiv: 2407.17313 [hep-ph].
[4] ALICE Collaboration, “Long-range transverse-momentum correlations and radial flow in Pb--Pb collisions at the LHC,” Phys. Rev. Lett. 136, 032302 (2026). DOI: 10.1103/l36g-6f46. arXiv: 2504.04796 [nucl-ex].
[5] ATLAS Collaboration, “Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector,” Phys. Rev. Lett. 136, 032301 (2026). DOI: 10.1103/ldcn-r2lq. arXiv: 2503.24125 [nucl-ex].
