Constraining the symmetry energy at subsaturation densities using isotope binding energy difference and neutron skin thickness

We show that the neutron skin thickness Δrnp of heavy nuclei is uniquely fixed by the symmetry energy density slope L(ρ) at a subsaturation cross density ρc≈0.11 fm−3 rather than at saturation density ρ0, while the binding energy difference ΔE between a heavy isotope pair is essentially determined by the magnitude of the symmetry energy Esym(ρ) at the same ρc. Furthermore, we find a value of L(ρc) leads to a negative Esym(ρ0)–L(ρ0) correlation while a value of Esym(ρc) leads to a positive one. Using data on Δrnp of Sn isotopes and ΔE of a number of heavy isotope pairs, we obtain simultaneously Esym(ρc)=26.65±0.20 MeV and L(ρc)=46.0±4.5 MeV at 95% confidence level, whose extrapolation gives Esym(ρ0)=32.3±1.0 MeV and L(ρ0)=45.2±10.0 MeV. The implication of these new constraints on the Δrnp of 208Pb and the core–crust transition density in neutron stars is discussed.