Selected highlights from STAR experiment

TL;DR

The paper surveys STAR’s multi-probe study of QCD matter created in relativistic heavy-ion and polarized proton collisions, with emphasis on partonic collectivity, electromagnetic probes, heavy flavor and jets, antimatter nuclei, nuclear structure, and spin phenomena. It highlights detector advances by the STAR-China consortium, including UPC and flow-imaging capabilities, and reports new multi-energy constraints on QGP temperatures from thermal dielectrons, as well as sequential quarkonium suppression and jet-structure studies that illuminate transport properties of the QGP. It also presents groundbreaking work on antimatter nuclei production, three-dimensional imaging of nuclear shapes through flow responses, and spin physics, including gluon helicity, Sivers/Collins effects, and global/local polarization in heavy-ion collisions. Collectively, these results deepen our understanding of QCD under extreme conditions and set the stage for high-precision measurements with upcoming high-statistics data, advancing both heavy-ion and nuclear-structure frontiers.

Abstract

In this paper, we review recent highlights in heavy-ion collisions and proton-proton collisions at top energies from STAR experiment at the Relativistic Heavy Ion Collider (RHIC) with key contributions from Chinese groups, including the Quark-Gluon Plasma (QGP) bulk properties, electromagnetic probes, heavy flavor and jets, antimatter hyper-nucleus, nuclear structure, global polarization, and nucleon spin structure. These data serve as important ingredients in the physics of Quantum Chromodynamics (QCD).

Figures (18)

• Figure 1: The elliptic flow coefficient $v_2$ as a function of transverse momentum $p_{\rm T}$ for (a) light hadrons ($\pi$ and $p$) and (b) multi-strange hadrons ($\phi$ and $\Omega$) in minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV. Results are adapted from STAR:2015gge.
• Figure 2: The elliptic flow $v_2$, scaled by the number of constituent quarks ($n_q$), as a function of the scaled transverse kinetic energy $(m_T - m_0)/n_q$ for $D^0$, $\Xi^-$, $\Lambda$, and $K_S^0$ in 10–40% Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. Results adapted from STAR:2017kkh. Here, $m_T = \sqrt{p_{\rm T}^2 + m_0^2}$ is the transverse mass.
• Figure 3: The ratios of $v_2$ (a) and $v_3$ (b) between p+Au ($^{3}$He+Au) and d+Au collisions with a similar number of particles produced in the collisions ($\langle \mathrm{N_{ch}} \rangle$). The solid lines are constant fits to the measurements, and the dashed lines are eccentricity ratios from model calculations with (model $c$) and without (model $a$) subnucleonic fluctuations. The vertical bars indicate statistical uncertainties, while the systematic uncertainties are indicated by the boxes. The figure is taken from STAR:2022pfn.
• Figure 4: Panel (a) shows the fully corrected inclusive dielectron spectra (black dots) and physics backgrounds (dashed and shaded lines) in Au+Au 0--80% collisions at $\sqrt{s_{\rm NN}} = 54.4$ and $27$ GeV. Panel (b) shows the STAR thermal dielectron spectra compared with NA60 thermal dimuon spectra. Dashed curves show the temperature fits; dot-dashed curves indicate the expected vacuum $\rho$ from $p{+}p$Aguilar-Benitez:1991hzq and $e^{+}$+$e^{-}$Derrick:1985jx collisions. Vertical bars and boxes denote statistical and systematic uncertainties, and downward arrows indicate statistical uncertainties exceeding 100%. The figure is taken from Ref. STAR:2024bpc.
• Figure 5: Temperatures as a function of baryon chemical potential. Temperatures measured by STAR are compared to those from NA60 NA60:2008dcb (diamond markers) and HADES HADES:2019auv (inverted triangle markers). Chemical freeze-out temperatures from statistical thermal models (SH, GCE, SCE) Andronic:2017pugSTAR:2017sal (circles) and the lattice QCD critical temperature $T_{\mathrm{C}}\xspace$HotQCD:2018pds (yellow band) are also shown. All temperatures are plotted at their corresponding chemical freeze-out $\mu_B$. Vertical bars and boxes denote statistical and systematic uncertainties, respectively. The figure is taken from Ref. STAR:2024bpc.