CP asymmetry in the decays B→(Xs,Xd)μ+μ- with four generations

We estimate the CP asymmetry A(CP)(q(2)) in the decays B -> X-s mu(+)mu(-) and B -> X-d mu(+)mu(-) in the standard model (SM) with an additional fourth generation. We use a parametrization that allows us to explore the complete parameter space of the 4x4 quark mixing matrix, and constrain these parameters from the current data on B decays. We find that the enhancement in A(CP)(q(2)) depends strongly on the mass of the t('), the up-type quark in the fourth generation. For m(t)(') around 400 GeV, the CP asymmetry in the high-q(2) region (q(2)> 14.4 GeV2) can be enhanced by more than an order of magnitude for B -> X-s mu(+)mu(-) and up to a factor of 6 for B -> X-d mu(+)mu(-). There is no enhancement in the low-q(2) region (1 < q(2)< 6 GeV2). With increasing m(t)('), A(CP)(q(2)) in the high-q(2) (low-q(2)) region first decreases (increases) and then saturates at a value a few times the SM prediction. In the high-q(2) region of B -> X-s mu(+)mu(-), this saturation value may be up to 25 times the SM expectation.