Parity violation in the np → dγ capture: a field-theoretical approach and relativistic effects

A field-theoretical approach to the problem of the parity violation in the radiative neutron capture by proton is developed enabling one to study the role of the relativistic effects in the observables for this process. The deuteron and the scatteting state are described by the Bethe-Salpeter functions satisfying certain symmetry relations in view of the generalized Pauli principle. In the case of photon circular polarization the isovector relativistic contributions violating the Siegert theorem, which are proportional to the photon energy, do not produce a considerable effect on the existing discrepance between the observed magnitude of the effect and the theoretical estimates, though they are significant in models with neutral currents, where the parity violating weak πNN coupling constant is large. It is shown that the electroweak effect similar to that found for the np → dγ capture is present also in the case of the bremsstrahlung in the nucleon-nucleon scattering (in terms approximately ω0 in the photon energy)