Decay $t-> cγ$ in models with SU_L(3)XU_X(1) gauge symmetry

Abstract: The one-loop level mediated $t-> c \gamma$ decay is analyzed in the framework of 331 models, which induce three-level flavor-changing neutral currents mediated by an extra neutral gauge boson, Z', and a neutral scalar boson, \phi. These models also predict new charged gauge and scalar bosons, together with three new quarks, which can be exotic (with electric charges of -4/3e and 5/3e) or standard model like. Apart from the contribution of the W boson, the $t\to c\gamma$ decay receives contributions induced by the extra gauge boson and the neutral scalar boson, which are generic for 331 models. In the so-called minimal 331 model, there are additional contributions from the new charged gauge and scalar bosons accompanied by the exotic quarks. We present analytical results for the most general $t-> c\gamma$ amplitude in terms of transcendental functions. For the numerical analysis we focus on the minimal 331 model: the current bounds on the model parameters are examined and a particular scenario is discussed in which the corresponding branching ratio could be of the order of 10^{-6}, with the dominant contributions arising from the charged gauge bosons and a relatively light neutral scalar boson with flavor-changing couplings, whereas the Z' contribution would be of the order of 10^{-9} for m_{Z'}>2 TeV. However, a further suppression could be expected due to a potential suppression of the values of the flavor-changing coupling constants. Under the same assumptions, in 331 models without exotic quarks, the $t-> c\gamma$ branching ratio would receive the dominant contribution from the neutral scalar boson, which could be of the order of 10^{-7} for a Higgs mass of a few hundreds of GeVs.