We report a precision measurement of the parity-violating asymmetry $A_{ PV}$ in the elastic scattering of longitudinally polarized electrons from $^{ 208}$Pb. We measure $A_{ PV}=550\pm 16 { \rm (stat)}\pm 8\ { \rm (syst)}$ parts per billion, leading to an extraction of the neutral weak form factor $F_W(Q^2 = 0.00616\ { \rm GeV}^2) = 0.368 \pm 0.013$. Combined with our previous measurement, the extracted neutron skin thickness is $R_n-R_p=0.283 \pm 0.071$~fm. The result also yields the first significant direct measurement of the interior weak density of $^{ 208}$Pb: $\rho^0_W = -0.0796\pm0.0036\ { \rm (exp.)}\pm0.0013\ { \rm (theo.)}\ { \rm fm}^{ -3}$ leading to the interior baryon density $\rho^0_b = 0.1480\pm0.0036\ { \rm (exp.)}\pm0.0013\ { \rm (theo.)}\ { \rm fm}^{ -3}$. 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.
Categories: nucl-ex