Observation of 1.5% Parity-Nonconservingγ-Ray Asymmetry

Abstract

The angular distribution of the 501-keV y radiation from low-temperature polarized I80mjj£ ^g^g found to exhibit a very large parity-nonconserving forward-backward asymmetry. At a polarization of 71 %, the measured asymmetry is -(1.49± 0.25) %. This asymmetry corresponds to an£2/M2 mixing ratio of magnitude 0.038±0.007 in fairly good agreement with circular-polarization experiments. We also deduce the £'3/M2 mixing ratio to be +6.0±0.5, agreeing with angular-correlation measurements,

Hadronic parity violation

“…First we argued here that if the existing experimental results yielding nonzero values for PNC observables are treated in a consistent formalism, the agreement among them is really quite good: The p + p, 18 F, 19 F, and p+ 4 He results combine to suggest a ratio of isoscalar-to-isovector strengths about a factor of six larger than the "best value" benchmark of DDH. The tension that had existed among the results appears to have arisen from comparisons that did not utilize a common set of strong meson-nucleon vertices.…”

“…Although the Tanner experiment lacked the sensitivity required to observe a signal, his effort was followed by others, with experiments designed to isolate tiny weak effects within systems where strong and electromagnetic interactions dominate continuing until today. The presence of weak effects is apparent from results such as the 2% photon asymmetry in the electromagnetic decay of an isomer of 180 Hf [4] A γ ( 180 Hf * → 180 Hf + γ) = −(1.66 ± 0.18) × 10…”

“…This constraint would require elementary NN experiments to be performed at scattering energies 2 MeV, which appears to be impractical except, potentially, in future spin rotation experiments in parahydrogen. While measuring NN scattering observables at very low energies may be prohibitively difficult, one might propose alternative measurements of near-threshold PNC observables in few-nucleon system, such as neutron spin rotation in 4 He. Unfortunately the effective center-of-mass momentum relevant to NN partial-wave analyses is generally not determined by the external kinematics in such reactions, but rather by the nuclear Fermi momentum, typically ∼ 200 MeV> m π .…”

“…Indeed the natural scale of hadronic PV effects is five orders of magnitude smaller, ∼ G F F 2 π ∼ 10 −7 , where F π ∼ 92. 4 MeV is the pion decay constant. In the more than half century since the discovery of PNC, many searches for hadronic PNC effects have been performed, both with and without the use of nuclear amplification, creating a substantial body of results.…”

Relativistic, model-independent, multichannel 2→2 transition amplitudes in a finite volume

“…It has been over half a century since Lee and Yang first suggested the possibility of parity non-conservation in the weak interaction [45], which was confirmed expermentially shortly thereafter by Wu et al [46][47][48][49][50]. Modern day experimental [51][52][53][54][55][56][57][58][59][60] and theoretical [61][62][63][64][65] studies have given attention to parity violating two-nucleon processes, where the strong interactions are most precisely understood.…”