Measurement of parity violation in np capture: the NPDGamma experiment

The NPDGamma experiment will measure the parity-violating directional gamma ray asymmetry A(gamma) in the reaction (n) over right arrow + p --> d + gamma. Ultimately, this will constitute the first measurement in the neutron-proton system that is sensitive enough to challenge modern theories of nuclear parity violation, providing a theoretically clean determination of the weak pion-nucleon coupling. A new beamline at the Los Alamos Neutron Science Center (LANSCE) delivers pulsed cold neutrons to the apparatus, where they are polarized by transmission through a large volume polarized He-3 spin filter and captured in a liquid para-hydrogen target. The 2.2 MeV gamma rays from the capture reaction are detected in an array of CsI(Tl) scintillators read out by vacuum photodiodes operated in current mode. We will complete commissioning of the apparatus and carry out a first measurement at LANSCE in 2004-05, which would provide a statistics-limited result for A gamma accurate to a standard uncertainty of +/- 5 x 10(-8) level or better, improving on existing measurements in the neutron-proton system by a factor of 4. Plans to move the experiment to a reactor facility, where the greater flux would enable us to make a measurement with a standard uncertainty of +/- 1 x 10(-8), are actively being pursued for the longer term.