Loop Induced Flavor Changing Neutral Decays of the Top Quark in a General Two-Higgs-Doublet.pdf

a r X i v : h e p - p h / 0 0 1 1 0 9 1 v 2 1 3 F e b 2 0 0 1 UAB-FT-491 KA-TP-22-2000 hep-ph/0011091 Loop Induced Flavor Changing Neutral Decays of the Top Quark in a General Two-Higgs-Doublet Model Santi Be?jara, Jaume Guaschb, Joan Sola?a aGrup de F??sica Teo?rica and Institut de F??sica d’Altes Energies, Universitat Auto?noma de Barcelona, E-08193, Bellaterra, Barcelona, Catalonia, Spain bInstitut fu?r Theoretische Physik, Universita?t Karlsruhe, Kaiserstra?e 12, D-76128 Karlsruhe, Germany. ABSTRACT Decays of the top quark induced by flavor changing neutral currents (FCNC) are known to be extremely rare events within the Standard Model. This is so not only for the decay modes into gauge bosons, but most notably in the case of the Higgs channels, e.g. t → HSM + c, with a branching fraction of 10?13 at most. Therefore, detection of FCNC top quark decays in a future high-energy, and high-luminosity, machine like the LHC or the LC would be an indisputable signal of new physics. In this paper we show that within the simplest extension of the SM, namely the general two-Higgs-doublet model, the FCNC top quark decays into Higgs bosons, t → (h0, H0, A0) + c, can be the most favored FCNC modes – comparable or even more efficient than the gluon channel t → g + c. In both cases the optimal results are obtained for Type II models. However, only the Higgs channels can have rates reaching the detectable level (10?5), with a maximum of order 10?4 which is compatible with the charged Higgs bounds from radiative B-meson decays. We compare with the previous results obtained in the Higgs sector of the MSSM. 1 Introduction In the near and middle future, with the upgrades of the Tevatron (Run II, TeV33), the advent of the LHC, and the construction of an e+e? linear collider (LC), new results on top quark physics [1], and possibly also on Higgs physics, will be obtained that may be extremely helpful to complement the precious information already collected at LEP 100 and 200 from