Neutron electric dipole moment in a supersymmetric model with singlet quarks

The neutron electric dipole moment is investigated in a supersymmetric electroweak model admitting ${\rm SU(2)}_W$ singlet and $Q = -1/3$, say $D$-type, quarks. Significant $CP$ violating phases appear in the coupling between the singlet $D$-type quarks and the ordinary $d$-type quarks. Then, through the $d$-$D$ mixing effects on the squark mass terms the gluino one-loop diagrams provide naturally the contributions $\sim 10^{-26} e~{\rm cm}$ comparable to the current experimental bound on the neutron electric dipole moment. The $CP$ violation in the $d$-$D$ coupling would also be relevant for the electroweak baryogenesis.