The parity-violating nucleon-nucleon force in the 1/N_c expansion
  Several experimental investigations have observed parity violation in nuclear systems-a consequence of the weak force between quarks. We apply the $1/N_c$ expansion of QCD to the P-violating T-conserving component of the nucleon-nucleon (NN) potential. We show there are two leading-order operators, both of which affect $\vec{p}p$ scattering at order $N_c$. We find an additional four operators at $O(N_c^0 \sin^2 \theta_W)$ and six at $O(1/N_c)$. Pion exchange in the PV NN force is suppressed by $1/N_c$ and $\sin^2 \theta_W$, providing a quantitative explanation for its non-observation up to this time. The large-$N_c$ hierarchy of other PV NN force mechanisms is consistent with estimates of the couplings in phenomenological models. The PV observed in $\vec{p}p$ scattering data is compatible with natural values for the strong and weak coupling constants: there is no evidence of fine tuning. 
Daniel R. Phillips, Daris Samart, and Carlos Schat
nucl-th hep-ph nucl-ex