Effect of the Coulomb interaction on the liquid-gas phase transition of nuclear matter

We investigate the role of the Coulomb interaction on the liquid-gas phase transition of nuclear matter with three different values of proton fraction ($Y_p=0.5,0.3$, and 0.1), relevant for heavy-ion physics as well as various astrophysical scenarios, within the framework of quantum molecular dynamics. We perform simulations for a wide range of density and temperature with and without the Coulomb interaction and calculate the two-point correlation functions of nucleon density fluctuations for all the configurations to determine the phase transition region. We also determine the critical end point of the liquid-gas phase transition for all three values of proton fraction considered. We observe that the Coulomb interaction reduces the transition temperature by $\gtrsim 2$ MeV for nuclear matter with $Y_p=0.5$ and $0.3$ and by $\sim 1$ MeV for nuclear matter with $Y_p=0.1$. However, the critical density is found to be more or less insensitive to the Coulomb interaction.

Figure 1

Snapshots from simulations showing distribution of nucleons at $0.1{\rho }_0,Y_p=0.3,T=0$ with (a) and without (b) the Coulomb interaction. Green (red) spheres represent neutrons (protons).

Figure 2

Two-point correlation function ${\xi }_{NN}$ around the liquid-gas phase transition region at $T=0$ with (a) and without (b) Coulomb interaction for symmetric nuclear matter.

Figure 3

Two-point correlation function ${\xi }_{NN}$ at $\rho =0.4{\rho }_0$ with (a) and without (b) Coulomb interaction for symmetric nuclear matter. The figures in the bottom panel (c) and (d) are zoomed versions of the corresponding figures in the top panel (a) and (b).

Figure 4

Phase diagram for symmetric nuclear matter with (a) and without (b) Coulomb interaction. The dashed line indicates the phase transition line.

Figure 5

Same as Fig. 2 but for asymmetric nuclear matter with $Y_p=0.3.$

Figure 6

Two-point correlation functions at $\rho =0.35{\rho }_0$ with (a)–(c) and without (d)–(f) Coulomb interaction for asymmetric nuclear matter with $Y_p=0.3$.

Figure 7

Same as Fig. 4, for $Y_p$=0.3.

Figure 8

Two-point correlation as in Fig. 5 for $Y_p=0.1$.

Figure 9

Two-point correlation functions at $\rho =0.25{\rho }_0$ with (a)–(c) and without (d)–(f) Coulomb interaction for asymmetric nuclear matter for $Y_p=0.1$.

Figure 10

Phase diagrams as in Fig. 2, but for $Y_p=0.1$.