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Register a new userParity-violating asymmetry in $gamma d to vec{n}p$ with a pionless effective theory
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Nuclear parity violation is studied with polarized neutrons in the photodisintegration of the deuteron at low energies. A pionless effective field theory with di-baryon fields is used for the investigation. Hadronic weak interactions are treated by parity-violating di-baryon-nucleon-nucleon vertices, which have undetermined coupling contants. A parity-violating asymmetry in the process is calculated for the incident photon energy up to 30 MeV. If experimental data for the parity-violating asymmetry become available in the future, we will be able to determine the unknown coupling contants in the parity-violating vertices.
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With the use of pionless effective field theory including dibaryon fields, we study the $gamma d to vec{n} p$ reaction for the laboratory photon energy $E_gamma^{lab}$ ranging from threshold to 30 MeV. Our main goal is to calculate the neutron polarization $P_{y}$ defined as $P_{y} = (sigma_+ - sigma_-)/(sigma_+ + sigma_-)$, where $sigma_+$ and $sigma_-$ are the differential cross sections for the spin-up and spin-down neutrons, respectively, along the axis perpendicular to the reaction plane. We also calculate the total cross section as well as the differential cross section $sigma(theta)$, where $theta$ is the colatitude angle. Although the results for the total and differential cross sections are found to agree reasonably well with the data, the results for $P_{y}$ show significant discrepancy with the experiment. We comment on this discrepancy.
We consider a pionless effective theory with dibaryon fields for the description of the weak process involving two nucleons. We construct leading order Lagrangians that contain nucleon-dibaryon weak coupling constants. We calculate the physical observable in the photodisintegration of the deuteron at threshold and obtain the result in terms of the nucleon-dibaryon weak coupling constants. Relation to existing calculations is discussed.
We consider the two-nucleon weak interaction with a pionless effective field theory. Dibaryon fields are introduced to facilitate calculations and ensure precision in the initial and final state propagators. Weak interactions are accounted for with the parity-violating dibaryon-nucleon-nucleon vertices, which contain unknown weak dibaryon-nucleon-nucleon coupling constants. We apply the model to the calculation of a parity-violating observable in the neutron-proton capture at threshold. Result is obtained up to the linear order in the unknown dibaryon-nucleon-nucleon coupling constants. We compare our result to the one obtained from a hybrid calculation, and discuss the extension to weak interactions in the few-body systems.
We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry is $A = -15.05 pm 0.98 ({rm stat}) pm 0.56 ({rm syst})$ ppm at the kinematic point $<theta_{rm lab} > = 12.{3^circ} $ and $<Q^2 > = 0.477$ (GeV/c)$^2$. Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factors $G^s_E + 0.392 G^s_M = 0.014 pm 0.020 pm 0.010$ where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.
We outline the motivation and conceptual design for a new experiment aimed at a 10-fold improvement in the accuracy of the parity-violating asymmetry A_gamma in the angular distribution of 2.2 MeV gamma rays from the n+p-->d+gamma reaction. This observable is primarily sensitive to the weak pion-nucleon coupling H_pi^1. A proof-of-principle experiment using unpolarized low-energy neutron capture on polyethylene and an array of 12 CsI detectors operated in current mode has been performed. Results of this test experiment including the current mode signal, electronic noise and detector sensitivity to magnetic fields are reported.
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