Measurement of parity violation in polarized neutron capture on 3He

The weak interaction between nucleons remains an oustanding topic of theoretical and experimental research. At low energies, where nucleons bind into nuclei, non-perturbative QCD is so far unable to make accurate calculations of the Hadronic Weak Interaction. An alternative approach to studying these systems is the meson-exchange model proposed by Desplanques, Donoghue, and Holstein, which describes hadronic weak interactions in terms of meson-nucleon coupling constants. An experimental solution to the problem of measuring weak interactions between strongly interacting particles is to search for parity-violating observables. This work presents the results of the measurement of the parity-violating directional asymmetry from the capture of polarized neutrons on gaseous 3He. This measurement was conducted with a pulsed neutron beam at the Spallation Neutron Source at Oak Ridge National Laboratory. In this experiment, neutrons are polarized, captured on3 He, and then the spin-dependent asymmetry is measured. One year of data was recorded and analyzed to eliminate false asymmetries and extract the precise value of the proton asymmetry. This experiment was optimized through simulation of the detector signals and calculation of the geometry factors. A multi-variable, weighted custom simulation was written for this experiment and run on a computing cluster. The inputs used to construct the simulation came from measurements of the neutron phase space, ENDF cross-sections, and PSTAR ionization data. Here, we present an independent calculation of two nuclear asymmetries in n+3He→p+t: αpv=(7.8±10.3±1.6)·e−9 and αpc=(4.2±0.5)·e−7. This calculation has the highest precision of any measured nuclear asymmetry. These results are consistent with existing few-body experiments, and provide a unique constraint on the phase space for any future theoretical analysis of the hadronic weak interation.