Long-time drift of the isospin degree of freedom in heavy ion collisions

The drift of the isospin degree of freedom (IDOF) is experimentally visualized through the wide-range angular distribution and the beam energy dependence of the isospin composition of light charged particles (LCP) produced in heavy ion collisions. It is evidenced that the net isospin transport is presented in the whole process from early dynamic emission to the statistical evaporation lasting to very late stage. Due to the long-time accumulation of the isospin effect, the angular distribution of the relative neutron excess summing over the LCPs is utilized as a sensitive probe to the symmetry energy. With $S$ fixed at 28.3 MeV, a soft density dependence with $L=33\text{–}61$ MeV (0.95 confidential level) at ${\rho }_0$ is circumstantially deduced using an improved quantum molecular dynamics transport model attached with a statistical emission afterburner.

Figure 1

The inclusive isotopic ratio for $Z=1$ (upper) and $Z=2$ (lower) isotopes as a function of laboratory angle at 30 MeV/u (left) and of beam energy (right).

Figure 2

(a) The energy spectra of $\alpha$ particles fitted by two moving sources. (b) LCP yield as a function of ${\theta }_{\mathrm{lab}}$ for the emissions from IVS (black square) and CNS (open circle), respectively; see text. (c) The average emission time $(〈\tau 〉-{\tau }_1)$ seen by the LCPs as a function of ${\theta }_{\mathrm{lab}}$ in units of $({\tau }_2-{\tau }_1)$.

Figure 3

(a) Angular distribution of the relative neutron richness (symbols) summed over the LCPs in comparison with the ImQMD+GEMINI simulations (curves) using $\gamma =0.4$, 0.5, 0.75, and 2, respectively. (b) The slope $k$ of the linear fit to the data (filled band, $1\sigma$ width) below ${90}^{\circ }$ compared with simulations (open circles).