Parity non-conservation in the gamma decay of 181Ta

Hadronic Parity Violation: A New View Through the Looking Glass

“…The basic requirements of a neutron spin rotation experiment are a source of low-energy polarized neutrons, a target containing the material of interest, and a spin analyzer downstream of the target, from which the value of φ PV can be deduced. Two cases are particularly interesting from the standpoint of testing HWI models: PV neutron spin rotations in 1 H and 4 He. These two cases have complementary dependences on the weak meson-nucleon couplings h i M .…”

“…The np case is dominated by the weak pion exchange contribution and thus yields similar information to the NPDGamma experiment described above. The 4 He PV spin rotation has a significant contribution from h 1 π but is also sensitive to the h 0 ρ coupling. Experimentally, the 4 He case is more tractable, owing to a much longer mean free path for low-energy neutrons than hydrogen.…”

“…The 4 He PV spin rotation has a significant contribution from h 1 π but is also sensitive to the h 0 ρ coupling. Experimentally, the 4 He case is more tractable, owing to a much longer mean free path for low-energy neutrons than hydrogen. Like NPDGamma, the np spin rotation experiment requires a parahydrogen target to avoid neutron depolarization due to spin-exchange collisions with the hydrogen molecules, and only neutron energies below 15 meV would be useful for the experiment.…”

“…What confounds the beautiful simplicity of the measurement principle is the extremely small size of the PV rotations expected on the basis of the DDH model: For 4 He, the predicted effect is φ PV = − 1 × 10 −7 rad/m (76), whereas for the proton, the effect should be approximately five times larger (70). These PV spin rotations are eight orders of magnitude smaller than those induced by the Earth's magnetic field in a typical measurement apparatus!…”

“…The distance at which the repulsive core of the strong NN potential becomes dominant, r 0.4 fm, corresponds to a mass scale 500 MeV, which, for our purposes, we take to be of order HAD . The size of hadronic matrix elements relevant to the HWI is governed by the ratio of F 2 π to v 2 , which is typically normalized to the quantity g π , as in Equation 4.…”