Electric dipole moment of neutron in the Kobayashi-Maskawa model with four generations of quarks

We show that the existence of a possible fourth heavy generation of quarks gives rise to a significant enhancement to the neutron electric dipole moment in comparison with the Standard Model prediction. The smaller degree of suppression in this case is linked to the presence of the operators of dimension $\leq$ 6 which enter into the effective Lagrangian with coefficients proportional to the square of the top quark mass. Numerically, the enhancement is mainly associated with chromoelectric dipole moment of the s quark which appears at three loop level, of the order $\al_s\al_w^2m_sm_t^2/m^4_w$ from the CP-odd combination of mixing angles between second, third and fourth generations. Its value is calculated explicitly in the limit of large masses of the fourth generation of quarks. The corresponding contribution to the electric dipole moment of the neutron is $5\cdot10^{-30} e\cdot cm$ in the most optimistic scenarios about the values of the Kobayashi-Maskawa matrix elements. The additive renormalization of $\theta$-term in this model is estimated as $10^{-13}$.