Relativistic effects on parity-violating phenomena in the low-energy np → dγ process

The photon circular polarisation Pγ and the asymmetry Αγ in the process n + p → d + γ are investigated in a Lorentz and gauge-invariant formalism in which the deuteron and initial np-distorted wavefunctions involve negative-energy components. In the low-energy limit, the formalism gives the conventional non-relativistic amplitude plus the relativistic correction terms associated with the negative-energy components of the deuteron and initial np-distorted wavefunctions. A large relativistic contribution to Pγ, but a smaller one to Αγ, are suggested by estimates from certain models of the weak and strong interactions. The large relativistic contribution to Pγ is not suppressed by terms required by gauge invariance. (author)