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

TL;DR

This paper analyzes the rare top quark decay $t-> c \gamma$ within 331 models featuring an extended gauge symmetry, exploring contributions from new gauge bosons, scalars, and exotic quarks, and providing numerical estimates of branching ratios.

Contribution

It presents the most general analytical expression for the $t-> c \\gamma$ amplitude in 331 models and evaluates the decay's branching ratio considering current experimental bounds.

Findings

Branching ratio could reach ~10^{-6} in minimal 331 models.

Charged gauge bosons and light neutral scalars dominate contributions.

Branching ratio could be ~10^{-7} in models without exotic quarks.

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.