Accurate Determination of the Neutron Skin Thickness of through Parity-Violation in Electron Scattering

Abstract

We report a precision measurement of the parity-violating asymmetry A PV in the elastic scattering of longitudinally polarized electrons from Pb 208. We measure A PV= 550±16 (stat)±8 (syst) parts per billion, leading to an extraction of the neutral weak form factor F W (Q 2= 0.00616 GeV 2)= 0.368±0.013. Combined with our previous measurement, the extracted neutron skin thickness is R n− R p= 0.283±0.071 fm. The result also yields the first significant direct measurement of the interior weak density of Pb 208: ρ W 0=− 0.0796±0.0036 (exp)±0.0013 (theo) fm− 3 leading to the interior baryon density ρ b 0= 0.1480±0.0036 (exp)±0.0013 (theo) fm− 3. The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.