Probing New Physics in Rare B and Z Degrees Decays

I analyze flavor changing neutral current (FCNC) phenomena in B meson and Z boson decays. I study the rare processes b to sgamma, b to sg, b to sl^{+ }l^{-} and b to snu|nu in left-right symmetric (LRS) extentions of the standard model of the electroweak interactions. The branching ratios of these processes depend upon the mass M_{R} of the right-handed gauge boson W_{R } and the mixing xi between W_{R} and W_ {L}. Taking M_{R } > 1.6 TeV and xi < 4 times 10^{-3}, I conclude that the predictions of LRS models do not significantly differ from those of the standard model. However, the above bounds on M _{R} and xi are plagued by a major theoretical uncertainties. I show that, making less severe assumptions, it is possible to relax these limits. This leads to an interesting enhancement of rare B decay process in LRS models. In the second part, after discussing a new source of FCNC in supersymmetric models, I analyze the hadronic and leptonic flavor-changing decays of Z^ circ in N = 1 supergravity models. I show that the constraint from b to sgamma implies that gluino mediated Z to b| s can have a branching ratio at most a few units times 10^{-8}. As for the leptonic decay Z to taumu ^{-} I draw the same conclusion considering the constraint coming from Z to mugamma.