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.