Constraints on the $\pi NN$ Coupling Constant from the $NN$ System

The sensitivity of the deuteron and of $pp$ scattering to the $\pi NN$ and $\rho NN$ coupling constants is investigated systematically. We find that the deuteron can be described about equally well with either {\it large $\pi$ and $\rho$} or {\it small $\pi$ and $\rho$} coupling constants. However, $pp$ scattering clearly {\it requires} the strong $\rho$, but favors the weak $\pi$ (particularly, in $^3P_0$ at low energies). This apparent contradiction between bound-state and scattering can be resolved by either assuming charge-dependent $\pi NN$ coupling constants or by adding a heavy pion to the NN model. In both cases, the neutral-pion coupling constant is small ($g^2_{\pi^0}/4\pi= 13.5$).