Chargino Contributions in $B \to \phi K_S$ Asymmetry

CP asymmetry in $B \to \phi K_S$ decay is studied in a special context of supersymmetry theories, in which the charginos play an important role. We find that in addition to the gluino, chargino can also make large contributions to CP asymmetry in $B \to \phi K_S$ decay. After considering the constraints from $B \to J/psi K_S$ decay, we study three special scenarios: (a). Large mixing on left-handed charm and top squarks (LL mixing); (b). Large mixing on right-handed charm and top squarks (RR mixing); (c). Large mixing on left-handed charm and top squarks plus right-handed charm and top squarks (LL + RR mixing). We show quantitatively that because of large squark mixing within second and third generations, an $\mathcal O$(1) effect on CP violation in $B \to \phi K_S$ is possible.