Effects of a first-order QCD phase transition on light nucleus production

Abstract

Using an extended Polyakov-looped Nambu--Jona-Lasinio (PNJL) model to describe the baryon density fluctuations of quark matter along the isentropic trajectories corresponding to different $s/ρ_B$ values extracted from Au+Au collisions at energies $\sqrt{s_{NN}} = 7.7-200$ GeV, we investigate the effects of the first-order phase transition on the light nucleus yield ratio $N_t \times N_p/N_d^2$. The results indicate that the second-order scaled density moment $y_2$, used to quantify density fluctuations, rapidly increases to form a peak when the isentropic trajectories pass through the phase coexistence region. We extract the yield ratios $N_t\times N_p/N_d^2$ at chemical freeze-out from the isentropic trajectories at different collision energies and found significant enhancements at 19.6 GeV and 27 GeV. This is similar to the trends observed by the STAR experiment, suggesting that the enhancements in the yield ratios $N_t\times N_p/N_d^2$ observed in the STAR experiment could be explained by the density fluctuations generated in the first-order phase transition region.

Figures (5)

• Figure 1: Phase diagram from an extended PNJL model for quark matter in $T-\rho_B$ plane. The gray dash-dotted line represents the boundary of the two-phase coexistence region, and the shaded region is the mechanical (spinodal) instability regions with $(\partial P /\partial \rho_B)_T < 0$. The black dashed line and dot are respectively for the chiral crossover and CEP that connect the chiral crossover and coexistence region. The different color solid lines represent isentropic trajectories of the $s/\rho_B$ values listed in Table II.
• Figure 2: Upper panel: the contour map of $y_2$ in the full phase diagram based on the extended PNJL model with the coupling constant $G_{SV}=-3500\Lambda^{-8}$. The different color solid lines represent isentropic trajectories of the $s/\rho_B$ values for different collision energies listed in Table II. Besides, the isentropic lines $s/\rho_B$ = 5, 10 are shown for comparison. Lower panel: $y_2$ as functions of $\rho_B$ along the isentropic trajectories of the $s/\rho_B$ values for different collision energies listed in Table II.
• Figure 3: Collision energy dependence of the yield ratio $N_t\times N_p/N_d^2$, where the yield ratio is approximated by $y_2/(2\sqrt{3})$. The violet open circles correspond to the results at chemical freeze-out during the isentropic process at different energies, while the orange open squares represent the results at kinetic freeze-out. The results from 0%-10% central Au + Au collisions at RHIC are also shown for comparison. Red solid stars with error bars are the final results with extrapolation to the full $p_T$ range and the dash-dotted fitted lines are the coalescence baselines from the coalescence-inspired fit. The colored bands and dashed fitted lines denote $p_T$ acceptance dependence, for which the statistical and systematic uncertainties are added in quadrature.
• Figure 4: Coexistence (dash-dotted) and spinodal (solid) lines in $T-\rho_B$ plane from the realistic 3-flavor PNJL model for different values of the scalar-vector coupling constant $G_{SV}$. Other parameters are set to the values in Table II.
• Figure 5: Isentropic trajectories of the entropy per baryon ($s/\rho_B$) for the different collision energies in Table III, calculated for different values of the scalar-vector coupling constant $G_{SV}$.