In a scenario in which fourth generation fermions exist, we study effects of new physics on the differential decay width, forward-backward asymmetry $A_{text{FB}}$ and integrated branching ratio for $Bto X_s ell^+ ell^-$ decay with $(ell=e,mu)$. Prediction of the new physics on the mentioned quantities essentially differs from the Standard Model results, in certain regions of the parameter space, enhancement of new physics on the above mentioned physical quantities can yield values as large as two times of the SM predictions, whence present limits of experimental measurements of branching ratio is spanned, contraints of the new physics can be extracted. For the fourth generation CKM factor $V_{t^prime b}^ast V_{t^prime s}$ we use $pm 10^{-2}$ and $pm 10^{-3}$ ranges, take into consideration the possibility of a complex phase where it may bring sizable contributions, obtained no significant dependency on the imaginary part of the new CKM factor. For the above mentioned quantities with a new family, deviations from the SM are promising, can be used as a probe of new physics.
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