22 with S$_{ψφ}$ , as shown in the figure 4. The quark level transition b → s l $^{+}$l − is forbidden at the tree level within the SM and can occur only via one or more loops. Hence it has the potential to test higher order corrections to the SM and also to constrain many of its possible extensions. It gives rise to the inclusive decay B → X$_{s}$ l $^{+}$l − which has been experimentally observed [92, 93] with a branching ratio close to its SM predictions, Br ( B → X$_{s}$ℓ $^{+}$ℓ − )(1 < q 2 < 6 GeV $^{2}$)= (1 . 63 ± 0 . 20) × 10 − 6 and Br ( B → X$_{s}$ℓ $^{+}$ℓ − )( q 2 > 14 . 4 GeV $^{2}$)= (3 . 84 ± 0 . 75) × 10 − 7 [94–96]. Apart from the branching ratio of semi-leptonic decay, there are other observables which are sensitive to new physics contribution to b → s transition. One such observable is forward- backward (FB) asymmetry of leptons in B → X$_{s}$ l $^{+}$l − . The FB asymmetry of leptons in B ( p$_{b}$ ) → X$_{s}$ ( p$_{s}$ ) l $^{+}$( p$_{l}$ + ) l − ( p$_{l}$ − ) is obtained by integrating the double differential branching ratio ( d $^{2}$Br/dzdcosθ ) with respect to the angular variable cosθ [97] where z ≡ q $^{2}$/m2 b ≡ ( p$_{l}$ + + p$_{l}$ − ) $^{2}$/m2 b and θ is the angle between the momentum of the B - meson (or the outgoing s -quark) and that of l + in the center of mass frame of the dileptons l $^{+}$l − . FB asymmetry measures the difference in the right-chiral and left-chiral couplings of the leptonic current. FB asymmetry is driven by the top quark [97] and hence it is sensitive to the fourth generation up type quark t ′ . Within the framework of SM4, the FB asymmetry in B → X$_{s}$ l $^{+}$l − is given by where and D ( z ) is given in eq. (22). The FB asymmetry in B → X$_{s}$ l $^{+}$l − becomes zero for a particular value of the dilepton invariant mass. Within SM, the zero of A$_{F B}$ ( q $^{2}$) appears in the low q 2 region, sufficiently away from the charm resonance region to allow the precise prediction of its position in A$_{F B}$ ( z ) = − 3 ( 1 − 4 t 2 z ) 1 / 2 E ( z ) D ( z ) , (77) E ( z ) = Re( C tot 9 C $^{tot}$∗ 10 ) z + 2Re( C tot 7 C $^{tot}$∗ 10 ) , (78) A$_{F B}$ ( z ) = ∫ 1 0 dcosθ d $^{2}$Br dz dcosθ − ∫ 0 − $_{1}$dcosθ d $^{2}$Br dz dcosθ 1 0 dcosθ d $^{2}$Br dz dcosθ + 0 − $_{1}$dcosθ d $^{2}$Br dz d cos θ , (76) D. FB asymmetry in B → X$_{s}$ l $^{+}$l −