Electric dipole polarizability in 208Pb: Insights from the droplet model

We study the electric dipole polarizability alpha(D) in Pb-208 based on the predictions of a large and representative set of relativistic and nonrelativistic nuclear mean-field models. We adopt the droplet model as a guide to better understand the correlations between alpha(D) and other isovector observables. Insights from the droplet model suggest that the product of aD and the nuclear symmetry energy at saturation density J is much better correlated with the neutron skin thickness Delta r(np) of 208Pb than the polarizability alone. Correlations of aDJ with Delta r(np) and with the symmetry energy slope parameter L suggest that alpha(D)J is a strong isovector indicator. Hence, we explore the possibility of constraining the isovector sector of the nuclear energy density functional by comparing our theoretical predictions against measurements of both alpha(D) and the parity-violating asymmetry in Pb-208. We find that the recent experimental determination of aD in 208Pb in combination with the range for the symmetry energy at saturation density J = [31 +/- (2)(est)] MeV suggests Delta r(np) (Pb-208) = 0.165 +/- (0.009)(expt) +/- (0.013)(theor) +/- (0.021)(est) fm and L = 43 +/- (6)(expt) +/- (8)(theor) +/- (12)(est) MeV.