Production of Light Nuclei in Au+Au Collisions at $\sqrt{s_{\rm NN}} = 7.7-27$ GeV from STAR BES-II

TL;DR

The paper investigates light-nuclei production in Au+Au collisions at BES-II energies ($\\sqrt{s_{\\rm NN}}=7.7$–27 GeV) to discriminate thermal and coalescence formation mechanisms. It leverages STAR BES-II data with enhanced detectors to measure corrected $p_T$ spectra, $dN/dy$, $\\langle p_T\\rangle$, and coalescence parameters $B_A$, applying Blast-Wave extrapolations and comparing to thermal-model expectations. Results show that while several yield ratios align with thermal predictions, $^3$He/p is overestimated by the thermal model; coalescence analyses reveal $B_A \\propto (1/V_{\\rm eff})^{A-1}$ and a decreasing $^{A-1}\\sqrt{B_A}$ from low to intermediate energy that plateaus up to 27 GeV, signaling an expanding and then stabilizing effective volume for coalescence. Overall, the findings constrain light-nuclei production mechanisms and inform chemical and kinetic freeze-out conditions in QCD matter.

Abstract

The studies of the production of light nuclei, such as deuteron and helium nuclei, in heavy-ion collisions are essential for understanding the dynamics of nuclear matter under extreme conditions. The yields and ratios of light nuclei serve as an effective method to distinguish between the thermal and coalescence models of light nuclei formation. Within the coalescence framework, the energy dependence of the coalescence parameters reflects the effective volume of the collision system, while in the thermal model yields are governed by chemical freeze-out conditions. The significantly larger datasets from the STAR Beam Energy Scan Phase II (BES-II), combined with enhanced detector capabilities, allow more precise and comprehensive measurements than phase I. In these proceedings, we present measurements of light nuclei production, including p, $\rm \bar{p}$, d, $\rm \bar{d}$, $\rm ^3He$, in Au+Au collisions at BES-II energies of $\sqrt{s_{\rm NN}} = 7.7 - 27$ GeV. The results include centrality-dependent transverse momentum spectra and yields ($\mathrm{d}N/\mathrm{d}y$), along with coalescence parameters $B_\mathrm{A}$ and particle yield ratios. The physics implications of these results are discussed.

Figures (3)

• Figure 1: Transverse momentum spectra of light nuclei and protons in central ($0$--$5\%$) Au+Au collisions at mid-rapidity in the STAR BES-II program at $\sqrt{s_{\rm NN}} = 7.7 - 27$ GeV. The rapidity selections are $|y| < 0.1$ for (anti-)protons, $|y| < 0.3$ for (anti-)deuterons, and $|y| < 0.5$ for $\rm ^3He$. The dashed lines represent the corresponding Blast-Wave fits. Statistical and systematic uncertainties are shown as vertical lines and shaded boxes, respectively. The spectra are scaled for clarity.
• Figure 2: (a) Mass number ($A$) dependence of $\mathrm{d}N/\mathrm{d}y$, scaled by the spin degeneracy factor $2J + 1$, for primordial protons, deuterons, and $^3$He in Au+Au collisions at BES-II energies of $\sqrt{s_{\rm NN}} = 7.7$--27 GeV. (b) Energy dependence of the average transverse momentum ($\langle p_{\rm T} \rangle$) for protons and light nuclei in 0-10% central Au+Au collisions at $\sqrt{s_{\rm NN}} = 3$--27 GeV.
• Figure 3: (a) Light nuclei-to-proton ratios in Au+Au collisions at various energies. Red circles for $\rm d/p$, blue squares for $\rm ^3He/p$, green crosses for $\rm \bar{d}/\bar{p}$, and magenta diamonds for $\rm ^4He/p$. Dashed lines show the thermal model predictions. (b) Energy dependence of $^{A-1}\sqrt{B_A}$ at mid-rapidity in 0--10% central Au+Au collisions from the STAR BES-II program. The coalescence parameters $B_A$ are evaluated at $p_{\rm T}/A = 0.65$ GeV/$c$. Circles, squares, and crosses represent $B_2({\rm d})$, $B_3({\rm ^3He})$, and $B_2({\rm \bar{d}})$, respectively. Statistical uncertainties are represented by vertical lines, and systematic uncertainties by shaded boxes.