Robert Michaels, Thomas Jefferson National Accelerator Facility
Argonne Physics Division Seminar - 19 Dec 2011

The Lead Radius Experiment PREX

The Lead Radius Experiment PREX ran in the Spring of 2010 in Hall A at the Thomas Jefferson National Accelerator Facility (JLab). The experiment measures the parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons from a 208Pb nucleus at an energy of 1.06 GeV and a scattering angle of 5 degrees. 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. This measurement is a fundamental test of nuclear structure theory, and our result establishes the existence of the neutron skin, i.e. that Rn > Rp. A precise measurement of Rn pins down the density-dependence of the symmetry energy of neutronrich nuclear matter, which has impacts on neutron star structure, heavy ion collisions, and atomic parity violation experiments. The experiment involves all aspects of the JLab accelerator, from the polarized source to the detector, and capitalizes on JLabâs unique strengths for carrying out high-precision parity experiments. In addition to the 2010 data, several technical challenges will be described, as well as prospects for future measurements at JLab from 208Pb and other nuclei such as 48Ca.

Argonne Physics Division Seminar Schedule