It is generally believed that the quark-hadron transition at small values of baryon chemical potentials µB is a crossover but changes to a first-order phase transition with an associated critical endpoint (CEP) as µB increases. Such a µB-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 ∆ρn = (δρn) 2 / ρn 2 at kinetic freeze-out indeed displays a clear peak at √ sNN = 8.8 GeV and a possible strong re-enhancement at √ sNN = 4.86 GeV. Our findings thus provide a strong support for the existence of a first-order phase transition at large µB and its critical endpoint at a smaller µB in the temperature versus baryon chemical potential plane of the QCD phase diagram.

Background: Temperature (T ) in heavy-ion collisions is an important parameter. Previously, many works have focused on the temperature of the hot emitting source. But there are few systematic studies of the temperature among heavy fragments in peripheral collisions with incident energies near the Fermi energy to a few A GeV, though it is very important to study the property of neutron-rich nucleus in heavy-ion collisions.Purpose: This work focuses on the study of temperature associated with the final heavy fragments in reactions induced by both the neutron-proton symmetric and the neutron-rich projectiles, and with incident energy ranges from 60A MeV to 1A GeV. Methods: Isobaric yield ratio (IYR) is used to determine the temperature of heavy fragments. Cross sections of measured fragments in reactions are analyzed, and a modified statistical abrasion-ablation (SAA) model is used to calculate the yield of fragment in 140A MeV 64 Ni + 9 Be and 1A GeV 136 Xe + 208 Pb reactions. Results: Relatively low T of heavy fragments are obtained in different reactions (T ranges from 1 to 3 MeV). T is also found to depend on the neutron richness of the projectile. The incident energy affects T very little.μ/T (the ratio of the difference between the chemical potential of neutron and proton to temperature) is found to increase linearly as N/Z of projectile increases. It is found that T of the 48 Ca reaction, for which IYRs are A < 50 isobars, is affected greatly by the temperature-corrected B(T ). But T of reactions using IYRs of heavier fragments are only slightly affected by the temperature-corrected B(T ). The SAA model analysis gives a consistent overview of the results extracted in this work. Conclusions: T from IYR, which is for secondary fragments, is different from that of the hot emitting source. T and μ are essentially governed by the sequential decay process.

In the framework of a modified Fisher model, using the isobaric yield ratio method, we investigate the fragments produced in the 140 𝐴 MeV 40,48 Ca+ 9 Be and 58,64 Ni+ 9 Be projectile fragmentation reactions. Using different approximation methods, 𝑎sym/𝑇 (the ratio of symmetry-energy coefficient to temperature) of symmetric and neutron-rich fragments are extracted. It is found that 𝑎sym/𝑇 of fragments depend on the reference nucleus and the neutron excess of fragments. The 𝑎sym/𝑇 of the isobar decreases when the neutron-excess of the isobar increases, while for a fragment with the same neutron-excess, 𝑎sym/𝑇 increases as the mass of the fragment increases but saturate when the mass of the fragment becomes larger.