Constraining the density slope of nuclear symmetry energy at subsaturation densities using electric dipole polarizability in 208Pb

Nuclear structure observables usually most effectively probe the properties of nuclear matter at subsaturation densities rather than at saturation density. We demonstrate that the electric dipole polarizibility α_D in ²⁰⁸Pb is sensitive to both the magnitude E_sym(ρ_c) and density slope L (ρ_c) of the symmetry energy at the subsaturation cross density ρ_c=0.11 fm^-3. Using the experimental data of α_D in ²⁰⁸Pb from RCNP (Research Center for Nuclear Physics, Osaka University) and the recent accurate constraint of E_sym(ρ_c) from the binding energy difference of heavy isotope pairs, we extract a value of L (ρ_c)=47.3 ±7.8 MeV. The implication of the present constraint of L (ρ_c) to the symmetry energy at saturation density, the neutron skin thickness of ²⁰⁸Pb and the core-crust transition density in neutron stars is discussed.