  In this Thesis we have investigated some effects appearing in top quark and
Higgs boson decays with flavor changing neutral currents (FCNC) in the
framework of generic Two Higgs Doblet Models (2HDM) and the Minimal
Supersymmetric Standard Model (MSSM). We have applied these two extensions to
the SM to see whether they can produce new FCNC effects. Introductory chapters
deal with the 2HDM and the MSSM. In the next chapters we have computed the
following FCNC decays: (1) the branching ratios (B) of the top quark to Higgs
bosons and charm quark in the 2HDM; (2) B and number of events at the Large
Hadron Collider (LHC) of the Higgs bosons to top and charm quarks in the 2HDM;
(3) B of the Higgs bosons to bottom and strange quarks in the MSSM; (4) cross
section and number of events at the LHC of the Higgs bosons to FCNC final
states involving the heavy quarks like the top and bottom quark in the MSSM
(Higgs bosons = h^0, H^0, A^0). We also studied the experimental signatures
that would allow discover of the nature of these Higgs bosons in the LHC. In
this study we have applied the severe restrictions from observed low-energy
FCNC processes like b -> s\gamma. Our general conclusion is that the physics of
the processes with flavor changing neutral currents can be very important in
seeing the physics beyond the Standard Model and to disentangle the nature of
the most adequate model. Experiments at the LHC can be crucial to unravel signs
of FCNC physics beyond the SM.
