Chemical potential differentials in the QCD phase diagram from heavy-ion isobar collisions

TL;DR

This work uses isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}=200$ GeV to perform a Bayesian thermal analysis of identified-hadron yields, isolating net-charge differences $\Delta Q$ to minimize systematics. The analysis yields precise freeze-out temperature $T_{\text{Chem}}$ and chemical potentials $\mu_B$, $\mu_S$, and $\mu_Q$, along with their differences $\Delta\mu_i$ between the isobars, and derives ratios $\Delta\mu_i/\Delta\mu_j$. Comparisons with lattice-based $BQS$ expansions and the mCMF effective model show consistent signs and approximate magnitudes for the extracted quantities, while providing continuous derivatives $d\mu_i/d\mu_j$ that map trajectories across the QCD phase diagram. The results demonstrate that controlled isospin variations in heavy-ion collisions offer a precision tool for 4D QCD thermodynamics and connect phenomenology with first-principles QCD and neutron-star matter constraints.

Abstract

Temperature and baryon, charge, and strangeness chemical potentials characterize QCD matter under extreme conditions. Differences between these chemical potentials and their ratios probe conserved-charge correlations and the system's response in the multidimensional QCD phase diagram. We extract these quantities from STAR Ru+Ru and Zr+Zr isobar collisions using a Bayesian thermal analysis of hadron yields, which substantially reduces systematic uncertainties, and compare them with Taylor-expanded lattice-QCD and Chiral Mean Field model predictions. Isobar collisions thus emerge as a precision probe of four-dimensional QCD thermodynamics.

Figures (7)

• Figure 1: QCD phase diagram regions reproduced by experiment and covered by the theoretical descriptions discussed in this work.
• Figure 2: Bayesian analyses posterior showing temperature and chemical potential differences for 0-10% centrality class. Values in GeV.
• Figure 3: Strangeness and charge chemical potential change from Ru to Zr from experiment and theory with baryon stopping $\alpha=1/1.84$.
• Figure 4: Experimental vs theoretical results for the ratio of chemical potential differences. Results from $BQS$ and mCMF are computed as ($d\mu_{i}/d\mu_{j}$), and experimental results consist of data with error bars ($\Delta\mu_{i}/\Delta\mu_{j}$).
• Figure S1: Bayesian analysis posterior distributions for all pairs of parameters. Energies are expressed in GeV for $T$, $\mu_{B}$, $\mu_{S}$, $\Delta\mu_{S}$ and $\Delta\mu_{Q}$ ($\gamma_{S}$ is omitted as being dimension-less), and distances are in fm for $R$ and $\Delta R$. The lower‑triangular half of the matrix shows the posterior when $\Delta Q$ is included in the THERMUS fit, while the upper‑triangular half displays the posterior obtained without $\Delta Q\,.$ The diagonal histograms show the marginalized probability distributions both with $\Delta Q$ (light blue) and without $\Delta Q$ (dark red).