Light nuclei production as a probe of the QCD phase diagram

Abstract: It is generally believed that the quark-hadron transition at small values of baryon chemical potentials $\mu_B$ is a crossover but changes to a first-order phase transition with an associated critical endpoint (CEP) as $\mu_B$ increases. Such a $\mu_B$-dependent quark-hadron transition is expected to result in a double-peak structure in the collision energy dependence of the baryon density fluctuation in heavy-ion collisions with one at lower energy due to the spinodal instability during the first-order phase transition and another at higher energy due to the critical fluctuations in the vicinity of the CEP. By analyzing the data on the $p$, d and $^3$H yields in central heavy-ion collisions within the coalescence model for light nuclei production, we find that the relative neutron density fluctuation $\Delta \rho_n=\langle(\delta \rho_n)^2\rangle/\langle \rho_n\rangle^2$ at kinetic freeze-out indeed displays a clear peak at $\sqrt{s_{NN}}=8.8$ GeV and a possible strong re-enhancement at $\sqrt{s_{NN}}=4.86$ GeV. Our findings thus provide a strong support for the existence of a first-order phase transition at large $\mu_B$ and its critical endpoint at a smaller $\mu_B$ in the temperature versus baryon chemical potential plane of the QCD phase diagram.