Asymmetry in $\vec{n}+p->d+\gamma$

Abstract

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 $A_{\gamma }=-0.10\text{hpiNN}$, which is close to the Siegert's theorem based result, $A_{\gamma }\simeq -0.11\text{hpiNN}$. This illustrates that HBChPT is effective in the parity-non-conserving physics.

Comments: Discussions are extensively refined. To appear in Phys. Lett. B