Okay; In Chiral Perturbation theory one gets the following first order Lagrangian: $tr[\partial_{\mu}U\partial^{\mu}U^{\dagger}]+ \bar{N}(i\partial_0 - \frac{g_a}{2f}\tau \cdot (\sigma \cdot \nabla)\pi - \frac{1}{4f}\tau \cdot(\pi \times \partial_0 \pi) )N -\frac{1}{2}C_S \bar{N}N\bar{N}N - \frac{1}{2}C_T(\bar{N}\sigma N)(\bar{N}\sigma N) $.
I can see that there are two contact terms giving a potential of the form $ V= C_S +C_T \sigma_1 \cdot \sigma_2 $. But, it is argued I should also have a one pion exchange term, equal to $ \simeg \tau_1 \cdot \tau_2 \frac{ \sigma_1 \cdot q \sigma_2 \c…