Probing in-medium effect via giant dipole resonance in the extended quantum molecular dynamics model

Abstract

Rather than using the geometric method employed in the original Extended Quantum Molecular Dynamics (EQMD) model, this article employs a stochastic approach to analyze the collision term and examine the width of the isovector giant dipole resonance (GDR) in ${}^{208}$Pb. Based on the ``soft" EQMD model, which we recently developed, the response and strength functions are self-consistently determined for various symmetry energy coefficients and in-medium reduction factor values. The results confirm that the peak position and GDR width in ${}^{208}$Pb are highly sensitive to the symmetry energy and the in-medium nucleon-nucleon ({\it NN}) cross section. This provides an opportunity to study the nuclear equation of state (EoS) and the medium effect. A significant reduction in free {\it NN} elastic cross sections within the medium is necessary to accurately reproduce the GDR width, as demonstrated by a comparison with the evaluation data.

Figures (7)

• Figure 1: Comparison of the NN elastic cross sections utilized in the original EQMD and the parameterization of elastic cross-sections given by Ref. Cugnon. Note the different scales in the inset.
• Figure 2: The GDR oscillation of $^{208}$Pb as a function of time (a) and the corresponding strength function (b). The peak positions and GDR widths calculated with and without low-energy cut are plotted as a square and a triangle in the inset. And the shaded area is the evaluation data of GDR peak position and width.
• Figure 3: The time evolution of the GDR oscillation for ${}^{208}$Pb with different symmetry energy coefficients (a) and the symmetry energy coefficient dependence of the GDR peak position (b). The corresponding peak positions are plotted as squares in the insert and the shaded area represents the evaluated data.
• Figure 4: The time evolution of the radial distribution of the density of the ground state $^{208}$Pb nucleus with binary collisions during 1000 fm/$c$.
• Figure 5: The responses of GDR oscillations with constant nucleon-nucleon cross section and the more realistic energy-dependent nucleon-nucleon cross section in panel (a). The corresponding strength functions in panel (b). The inset displays the corresponding decay width and the evaluated data.