Constraining symmetry energy from pygmy dipole resonances in ⁶⁸Ni and ¹³²Sn*

Abstract We conduct a new investigation of the correlation at saturation (subsaturation) density between the density dependence of symmetry energy and the percentage of the energy-weighted sum rule (EWSR) exhausted by pygmy dipole resonances (PDR) in 68 Ni and 132 Sn. The calculations are performed within the Skyrme HF (or HF+BCS) plus random phase approximation (RPA) (or quasiparticle RPA) problem using SAMi-J effective interactions. The effect of pairing on the dipole strength distribution of 68 Ni and density dependence of the symmetry energy is discussed. Slope parameter L and symmetry energy J at saturation (subsaturation) density are 41.8−90.2 MeV (39.3−64.1 MeV) and 28.0−32.5 MeV (23.0−23.8 MeV), respectively. They are consistent with the currently accepted values except for J at subsaturation density, which is slightly lower than the values obtained from nuclear mass difference measurements and electric dipole polarizability data.