NPDGamma experiment

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NPDGamma is an experiment that looks for a tiny parity-violating effect in how neutrons are captured by parahydrogen. Polarized cold neutrons with energies around 2 to 15 meV strike a liquid parahydrogen target. When a neutron is captured, a deuterium nucleus is formed and a 2.2 MeV gamma ray is emitted and detected by a surrounding array of detectors—four rings with twelve detectors each around the beam.

To study how the gamma emission depends on the neutron spin, the neutron beam polarization is flipped rapidly. Researchers count how many gamma rays are detected in a given direction for the two neutron spin states. The asymmetry is the difference in counts divided by the total counts. This asymmetry, A_gamma, has both parity-violating and parity-conserving parts; the parity-violating part is related to the long-range weak interaction constant h_pi (f_pi).

The project’s first phase ran at Los Alamos in 2006 but didn’t collect enough data to test theory (the expected A_gamma is about 10^-8). The current phase is at Oak Ridge’s Spallation Neutron Source, with data starting in spring 2012. In 2018, the collaboration reported a successful measurement of parity violation in this process, providing new information about the weak force inside nucleons.