  We report the first measurement of the parity-violating asymmetry APV in the
elastic scattering of polarized electrons from 208Pb from the Lead Radius
Experiment PREX which ran in Hall A at the Thomas Jefferson National
Accelerator Facility (JLab). APV is sensitive to the radius of the neutron
distribution Rn. The Z boson that mediates the weak neutral interaction couples
mainly to neutrons and provides a clean, model-independent measurement of the
RMS radius Rn of the neutron distribution in the nucleus and is a fundamental
test of nuclear structure theory. The result, APV = 0.656 +/- 0.060(stat) +/-
0.014(syst) ppm, corresponds to a difference between the radii of the neutron
and proton distributions Rn - Rp = 0.33 +16 -18 fm and provides the first
electroweak observation of the neutron skin which is expected in a heavy,
neutron-rich nucleus.
