Asymmetry in n→+p→d+γ

Heavy-baryon chiral perturbation theory (HBChPT) is applied to the asymmetry Aγ in n→+p→d+γ at threshold, which arises due to the weak parity non-conserving interactions. Instead of appealing to Siegertʹs theorem, transition operators up to next-to-leading chiral order are derived and the corresponding amplitudes are evaluated with the Argonne v18 wavefunctions. In addition to the impulse contribution, both parity-conserving and parity-non-conserving two-body one-pion-exchange diagrams appear up to this order. Our prediction for the asymmetry is Aγ=−0.10hπNN(1), which is close to the Siegertʹs theorem based result, Aγ≃−0.11hπNN(1). This illustrates that HBChPT is effectively applied to the parity-non-conserving physics.