I. Cortés-Maldonado, G. Hernández-Tomé, G. Tavares-Velasco
The one-loop level mediated $t-> c\gamma$ decay is analyzed in the framework of 331 models, which are based on the SU_L(3)XU_X(1) gauge symmetry and require that the quark families transform differently in order to cancel anomalies, thereby inducing 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 charge -4/3e and 5/3e) or standard model like. Apart from the contribution of the W boson, the $t-> c\gamma$ decay receives contributions induced by the Z' gauge boson and the neutral scalar boson, which are generic for 331 models. On the other hand, in the so-called minimal 331 model, there are 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 dilogarithms and other transcendental functions. 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 the lightest neutral scalar boson, whereas the Z' contribution would be of the order of 10^{-9} for m_{Z'}>2 TeV. Thus, in 331 models without exotic quarks, the $t-> c\gamma$ branching ratio would receive the main 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.
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http://arxiv.org/abs/1305.2606
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