Unfolding Baryon Number Fluctuations from Correlations of Light Nuclei Production in Heavy-Ion Collisions

Abstract

Event-by-event fluctuations of the baryon number, which is mostly carried by protons and neutrons, in relativistic heavy-ion collisions provide a sensitive probe for locating the conjectured critical point in the quantum chromodynamics (QCD) phase diagram. Since current experiments have limited access to neutron fluctuations because detectors are largely insensitive to neutrons, measurements of (net-)proton fluctuations are often used as a proxy for (net-)baryon number fluctuations. Although direct measurements of neutron fluctuations are challenging, their information are encoded in the production and correlations of light nuclei, when they are formed through coalescence of nucleons at kinetic freeze-out. Here, we propose to unfold neutron fluctuations from correlations among light nuclei produced in heavy-ion collisions. Model calculations validate this approach and show that baryon number fluctuations can be unfolded up to the third order. For fourth and higher-order cumulants, however, the uncertainties become sizable, indicating that further methodological developments and refinements are required.

Figures (3)

• Figure 1: Cumulant ratios of proton number and baryon number distributions as a function of collision energy $\sqrt{s_{NN}}$ in central Au+Au collisions. The upper panel shows $C_2 / C_1$, and the lower panel shows $C_3 / C_2$. The orange bands and stars represent the cumulant ratios of proton number distributions from the AMPT model and experiments. The grey bands represent the cumulant ratios of baryon number distributions from the AMPT model. The blue bands and diamonds represent the cumulant ratios of baryon number distributions restored by our method, both in the AMPT model and experiments. Error bands are obtained directly from the statistical fluctuations in numerical calculations, while experimental data errors were computed from the data via the error propagation formula: $\frac{\sigma_{XY}}{\langle XY \rangle} \approx \sqrt{\left( \frac{\sigma_X}{\langle X \rangle} \right)^2 + \left( \frac{\sigma_Y}{\langle Y \rangle} \right)^2}$. The experimental data are taken from the STAR Collaboration STAR:2021iopSTAR:2023ebz.
• Figure 2: The fourth-to-second cumulant ratio of baryon number distribution as a function of collision energy $\sqrt{s_{NN}}$ in the AMPT model. The gray band represents the result given by AMPT, while the blue band represents the result unfolded with light nuclei production.
• Figure 3: Energy dependence of isospin fluctuation. The grey band is obtained in the AMPT model and orange stars represent the results unfolded from experimental data STAR:2021iopSTAR:2023ebz. The errors were calculated the same way as in Fig. \ref{['pic: C2C1C3C2']}.