CP-Violating Yukawa Couplings in the Skyrme Model and the Neutron Electric Dipole Moment

Abstract

We argue that the large-$\Nc$ behaviour of the Yukawa couplings in the Skyrme model involves issues more subtle than the vanishing of linear fluctuations needed for classical stability of the skyrmion. The chiral fluctuations about the skyrmion must be quantized in order to reach a conclusion. An improved quantization procedure allows us to confront this question directly. The pion-nucleon coupling constants $\gcup$ (CP conserving) and $\bgcup$ (CP violating) are calculated in the large-$\Nc$, three-flavour Skyrme model by direct evaluation of the leading matrix elements appearing in the LSZ reduction formula. We find that $\gcup \sim \Nc^{{3 \over 2}}$, but that, at most, $\bgcup \sim m^2_\pi \Nc^{-\shalf}$. These results show that the leading contribution to the neutron electric dipole moment in large-$\Nc$ Skyrme model is the direct one ($\Dn \sim \Nc m^2_\pi$), rather than the pion loop contribution.