Decayt→cγin models withSUL(3)×UX(1)gauge symmetry

Abstract

The one-loop level mediated $t\ensuremath{\rightarrow}c\ensuremath{\gamma}$ decay is analyzed in the framework of 331 models, which are based on the $S{U}_{L}(3)\ifmmode\times\else\texttimes\fi{}{U}_{X}(1)$ 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 ${Z}^{\ensuremath{'}}$ and a neutral scalar boson $\ensuremath{\phi}$. These models also predict new charged gauge and scalar bosons, together with three new quarks, which can be exotic (with electric charges of $\ensuremath{-}4/3e$ and $5/3e$) or standard-model-like. Apart from the contribution of the $W$ boson, the $t\ensuremath{\rightarrow}c\ensuremath{\gamma}$ 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 $t\ensuremath{\rightarrow}c\ensuremath{\gamma}$ 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 ${10}^{\ensuremath{-}6}$, with the dominant contributions arising from the charged gauge bosons and a relatively light neutral scalar boson with flavor-changing couplings, whereas the ${Z}^{\ensuremath{'}}$ contribution would be of the order of ${10}^{\ensuremath{-}9}$ for ${m}_{{Z}^{\ensuremath{'}}}>2\text{ }\text{ }\mathrm{TeV}$. 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 $t\ensuremath{\rightarrow}c\ensuremath{\gamma}$ branching ratio would receive the dominant contribution from the neutral scalar boson, which could be of the order of ${10}^{\ensuremath{-}7}$ for a Higgs mass of a few hundreds of GeVs.