Electric dipole response of 208Pb from proton inelastic scattering: Constraints on neutron skin thickness and symmetry energy

The electric dipole (E1) response of Pb-208 has been precisely determined by measuring Coulomb excitation induced by proton scattering at very forward angles. The electric dipole polarizability, defined as inverse energy-weighted sum rule of the E1 strength, has been extracted as alpha(D) = 20.1 +/- 0.6 fm(3). The data can be used to constrain the neutron skin thickness of Pb-208 to Delta r(np) = 0.165 +/- (0.009)(expt) +/- (0.013)(theor) +/- (0.021)(est) fm, where the subscript expt refers to the experimental uncertainty, theor to the theoretical confidence band and est to the uncertainty associated with the estimation of the symmetry energy at the saturation density. In addition, a constraint band has been extracted in the plane of the symmetry energy (J) and its slope parameter (L) at the saturation density.