CP asymmetries of B to phi K_S and B to eta' K_S in SUSY GUT Model with Non-universal Sfermion Masses

We analyze CP asymmetries of B \to \phi K_S and B \to \eta' K_S in a supersymmetric grand unified theory in which only the third generation sfermions contained in 10(Q, U^c, E^c) of SU(5) can have a different mass from the others. One of the advantages of this nonuniversal mass model is that the first two generation sfermion masses can be large whereas both (left and right handed) stops are light so as to stabilize the weak scale. Therefore, we studied a minimal supersymmetric standard model parameter region in which a fine tuning in Higgs sector is relaxed owing to light masses of stops, gluino and higgsinos. In such a parameter region, the chargino contribution is as important as the gluino one. We show that the CP asymmetries of B \to \phi K_S and B \to \eta' K_S can deviate from their standard model predicted values by O(0.1) because of constructive interference between gluino and chargino contributions.