The neutron electric dipole moment is investigated in a supersymmetric
electroweak model admitting 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 ∼10−26ecm 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.Comment: 16 page