# Probing QCD critical fluctuations from light nuclei production in relativistic heavy-ion collisions

## TL;DR

This study uses a coalescence model to analyze light nuclei production in heavy-ion collisions, revealing a non-monotonic neutron density fluctuation behavior that peaks near the QCD critical endpoint, indicating potential critical phenomena detection.

## Contribution

It introduces a novel method linking light nuclei yield ratios to neutron density fluctuations, providing a new probe for QCD critical phenomena in heavy-ion collisions.

## Key findings

- Neutron density fluctuations peak at 8.8 GeV collision energy.
- Yield ratio sensitive to density fluctuations at kinetic freeze-out.
- Results suggest proximity to the QCD critical endpoint.

## Abstract

Based on the coalescence model for light nuclei production, we show that the yield ratio $\mathcal{O}_\text{p-d-t} = N_{^3\text{H}} N_p / N_\text{d}^2$ of $p$, d, and $^3$H in heavy-ion collisions is sensitive to the neutron relative density fluctuation $\Delta n= \langle (\delta n)^2\rangle/\langle n\rangle^2$ at kinetic freeze-out. From recent experimental data in central Pb+Pb collisions at $\sqrt{s_{NN}}=6.3$~GeV, $7.6$~GeV, $8.8$~GeV, $12.3$~GeV and $17.3$~GeV measured by the NA49 Collaboration at the CERN Super Proton Synchrotron (SPS), we find a possible non-monotonic behavior of $\Delta n$ as a function of the collision energy with a peak at $\sqrt{s_{NN}}=8.8$~GeV, indicating that the density fluctuations become the largest in collisions at this energy. With the known chemical freeze-out conditions determined from the statistical model fit to experimental data, we obtain a chemical freeze-out temperature of $\sim 144~$MeV and baryon chemical potential of $\sim 385~$MeV at this collision energy, which are close to the critical endpoint in the QCD phase diagram predicted by various theoretical studies. Our results thus suggest the potential usefulness of the yield ratio of light nuclei in relativistic heavy-ion collisions as a direct probe of the large density fluctuations associated with the QCD critical phenomena.