Flavour-Dependent Chemical Freeze-Out of Light Nuclei in Relativistic Heavy-Ion Collisions

TL;DR

This work assesses flavour-dependent chemical freeze-out in heavy-ion collisions by applying Hadron Resonance Gas (HRG) models with 1CFO and 2CFO scenarios, implemented via Thermal-FIST, to Au+Au and Pb+Pb data across RHIC and LHC energies. By fitting hadron yields with and without light nuclei and by incorporating post-chemical-equilibrium evolution (HRG-PCE) and energy-dependent resonance widths, the study evaluates light-nuclei yield ratios such as $d/p$, $\bar{d}/\bar{p}$, $t/p$, $t/d$, and $^4\text{He}/^3\text{He}$. The main finding is that a flavour-dependent (2CFO) freeze-out, especially when combined with HRG-PCE, provides a more accurate description of light-nuclei yields and their ratios across energies, though certain ratios in Pb+Pb at 5.02 TeV show exceptions where 1CFO can be favorable. The results imply a sequential chemical freeze-out with strange hadrons freezing earlier than light hadrons and light nuclei, highlighting the significance of including light nuclei in thermal fits and of post-freeze-out evolution for interpreting final-state observables in heavy-ion collisions.

Abstract

We study the production of light nuclei in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7 - 200 GeV and Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 2.76 and 5.02 TeV within a flavour-dependent freeze-out framework, assuming different flavoured hadrons undergo separate chemical freeze-out. Using the Thermal-FIST package, thermal parameters extracted from fits to various sets of hadron yields, including and excluding light nuclei, are used to calculate the ratios of the yields of light nuclei, namely, $d/p$, $\bar{d}/\bar{p}$, $t/p$, and $t/d$. A comparison with data from the STAR and ALICE collaborations shows that a sequential freeze-out scenario provides a better description of light nuclei yield ratios than the traditional single freeze-out approach. These results suggest the flavour-dependent chemical freeze-out for final state light-nuclei production persists in heavy-ion collisions at both RHIC and LHC energies.

Figures (4)

• Figure 1: Ratios of experimental data to thermal model fit to $\pi$, $K$, $K_s^{0}$, $p$, $\phi$, $\Lambda$, $\Xi$, and $\Omega$ in 0--10% centrality of Au+Au and Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7 – 5020 GeV ($K^0_s$, $\Lambda$, $\Xi$, and $\Omega$ yields at $\sqrt{s_\mathrm{NN}}$ = 200 GeV were measured in 0--5% centrality). The 1CFO yield calculations are shown as open black circles while the 2CFO calculations are shown as solid red squares. The solid green diamonds indicate the case where the charged kaons were included in the strange hadrons set.
• Figure 2: Ratios of experimental data to thermal model fit to $\pi$, $K$, $K_s^{0}$, $p$, $\phi$, $\Lambda$, $\Xi$, $\Omega$, $d$, and $t$ (or $^3\text{He}$) in 0--10% centrality of Au+Au and Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7 - 5020 GeV ($K^0_s$, $\Lambda$, $\Xi$, and $\Omega$ yields at $\sqrt{s_\mathrm{NN}}$ = 200 GeV were measured in 0--5% centrality). Thermal model calculations are shown for: 1CFO in HRG (open black circles), 2CFO in HRG (open red squares), 1CFO in HRG-PCE (solid black circles), and 2CFO in HRG-PCE (solid red squares).
• Figure 3: Chemical freeze-out parameters $T_\text{ch}$, $\mu^{(0)}_B$, and $R^{(0)}$ in the 2CFO scenario, extracted from thermal fits to light hadrons (red circles), light hadrons + nuclei (green double diamonds), and strange hadrons (blue squares).
• Figure 4: Comparison of light nuclei yield ratios—$d/p$, $\bar{d}/\bar{p}$, $t/p$, $t/d$, $^4\text{He}/^3\text{He}$, and $^3\text{H}_{\Lambda}/^3\text{He}$—with thermal model calculations at $T = T_\text{ch}$ and $T = T_{kin} = 100$ MeV, open and full markers, respectively. The calculations use chemical freeze-out parameters from Eq. \ref{['eq:part']} obtained under both the 1CFO and 2CFO scenarios, blue and red markers, respectively, without (left) and with (right) the inclusion of light nuclei in the thermal fits.