Testing models with non – minimal Higgs sector through the decay t → q + WZ

Testing models with non–minimal Higgs sector through the decay t → q + W Z Abstract We study the contribution of charged Higgs boson to the rare decay of the top quark t → q + W Z (q = d, s, b) in models with Higgs sectors that includes doublets and triplets. Higgs doublets are needed to couple charged Higgs with quarks, whereas the Higgs triplets are required to generate the non-standard vertex HW Z at tree–level. It is found that within a model that respects the custodial SU (2) c symmetry and avoids FCNC by imposing discrete symmetries , the decay mode t → b + W Z can reach a B.R. of order 10 −2 , whereas the decay modes t → (d, s) + W Z, can reach a similar B.R. in models where FCNC are suppressed by flavour symmetries. 1 Introduction.-The mass of the top quark, which is larger than any other fermion mass in the Standard Model (SM) and almost as large as its scale of electroweak symmetry breaking (EWSB), can not be explained within the SM [1]. This has originated speculations about the possible relationship between the top quark and the nature of the mechanism responsible for EWSB. Several models have been proposed, where such large mass can be accomodated or plays a significant role. In the supersymmetric (SUSY) extensions of the SM [2], the large value of the top mass can drive the radiative breaking of the electroweak symmetry; furthermore within the context of SUSY-GUT's the fermions of the third–family can be accomodated in scheemes where their masses arise from a single Yukawa term [3]. On the other hand, in some top–condensate (TC) models [4] it is postulated that new strong interactions bind the heavy top into a composite Higgs scenario. From a more phenomenological point of view, it is also intriguing to notice that the top decay seems to be dominated by the SM mode (t → bW), not only within the SM but also in theories beyond it, which makes the top decay width almost insensitive to the presence of new physics; unless the scale of new physics is lighter than the top mass itself, such that new states can appear in its decays. This is the case, for instance, in the so–called THDM–III [5], where the flavour changing mode t → c + h can be important for a …