Models for the weak parity-violating vector meson exchange potential and the circular polarization in n + p → d + γ

Abstract

We propose three models which lead to a p.v. nucleon-nucleon interaction mediated by charged and neutral vector mesons. Besides the ad hoc hypothesis of octet dominance, we consider two quark models (the Bose quark and colour Fermi quark model), which give a dynamical explanation of the ΔI = 1 2 rule in strangeness changing hyperon decays. They lead to p.v. potentials with different isospin dependence. We also derive the weak NN̄ ρ° coupling from a SU(2) × U(1) gauge theory with neutral currents. The circular polarization P γ of the γ-quanta in the capture of thermal neutrons on protons is calculated for these different models. The Reid hard-core (HC) and soft-core (SC) potentials have been chosen to take into account the strong interaction. Then the naively factorized Cabbibo current-current interaction with charged rho exchange only gives P γ HC = −2.7 × 10 −8 and P γ SC = −2.1 × 10 −8. The strong octet dominance and Bose quark model lead to a vanishing circular polarization | P γ | ≈ (1–4) × 10 −9. The colour Fermi quark model enhances the circular polarization and gives P γ ≈ −5 × 10 −8.