Accurate Determination of the Neutron Skin Thickness of Pb 208 through Parity-Violation in Electron Scat

Abstract

We report a precision measurement of the parity-violating asymmetry APV in the elastic scattering of longitudinally polarized electrons from Pb208. We measure APV=550±16(stat)±8(syst) parts per billion, leading to an extraction of the neutral weak form factor FW(Q2=0.00616 GeV2)=0.368±0.013. Combined with our previous measurement, the extracted neutron skin thickness is Rn-Rp=0.283±0.071 fm. The result also yields the first significant direct measurement of the interior weak density of Pb208: ρW0=-0.0796±0.0036(exp)±0.0013(theo) fm-3 leading to the interior baryon density ρb0=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.