Asymmetry in $\vec{n} + p -> d + γ$

Heavy-baryon chiral perturbation theory (HBChPT) is applied to the asymmetry $A_\gamma$ in $\vec{n} + p \to d + \gamma$ at threshold, which arises due to the weak parity non-conserving interactions. Instead of appealing to Siegert's theorem, transition operators up to next-to-leading chiral order are derived and the corresponding amplitudes are evaluated with the Argonne $v_{18}$ 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 $A_\gamma = - 0.10 \hpiNN$, which is close to the Siegert's theorem based result, $A_\gamma \simeq -0.11 \hpiNN$. This illustrates that HBChPT is effective in the parity-non-conserving physics.