Study of FCNC mediated Z boson effect in the semileptonic rare baryonic   decays $\Lambda_b \to \Lambda l^+ l^- $
  We study the effect of the FCNC mediated $Z$ boson in the rare semileptonic baryonic decays $\Lambda_b \to \Lambda l^+ l^-$. We consider the model where the standard model fermion sector is extended by an extra vector like down quark, as a consequence of which it allows CP violating Z mediated flavor changing neutral current at the tree level. We find that due to this non-universal $Zbs $ coupling, branching ratios of the rare semileptonic $\Lambda_b$ decays are enhanced reasonably from their corresponding standard model values and the zero point of the forward backward asymmetry for $\Lambda_b \to \Lambda \mu^+ \mu^-$ is shifted to the left. 
A.K. Giri, R. Mohanta