Measurement of the Neutron Radius of Pb 208 through Parity Violation in Electron Scattering

We report the first measurement of the parity-violating asymmetry ${A}_{\mathrm{PV}}$ in the elastic scattering of polarized electrons from $^{208}\mathrm{Pb}$. ${A}_{\mathrm{PV}}$ is sensitive to the radius of the neutron distribution (${R}_{n}$). The result ${A}_{\mathrm{PV}}=0.656\ifmmode\pm\else\textpm\fi{}0.060(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.014(\mathrm{syst})\text{ }\text{ }\mathrm{ppm}$ corresponds to a difference between the radii of the neutron and proton distributions ${R}_{n}\ensuremath{-}{R}_{p}={0.33}_{\ensuremath{-}0.18}^{+0.16}\text{ }\text{ }\mathrm{fm}$ and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.

• NS
  National Science Foundation
• UD
  U.S. Department of Energy

  Awards: DE-AC05-060R23177, DE-AC05
• AN
  Agence Nationale de la Recherche
• CN
  Centre National de la Recherche Scientifique

  Award: CNRS/IN2P3
• IN
  Institut National de Physique Nucléaire et de Physique des Particules

  Award: CNRS/IN2P3
• TJ
  Thomas Jefferson National Accelerator Facility

  Award: DE-AC05-060R23177