Flavor changing neutral current decays $t\to cX$ ($X=\gamma ,g,Z,H$) and $t\to c\bar{\ell }\ell$ ($\ell =\mu ,\tau$) via scalar leptoquarks

Abstract

The flavor changing neutral current decays $t\to cX$ ($X=\gamma ,g,Z,H$) and $t\to c\bar{\ell }\ell$ ($\ell =\mu ,\tau$) are studied in a renormalizable scalar leptoquark (LQ) model with no proton decay, where a scalar $SU(2)$ doublet with hypercharge $Y=7/6$ is added to the standard model, yielding a non-chiral LQ ${\Omega }_{5/3}$. Analytical results for the one-loop (tree-level) contributions of a scalar LQ to the $f_i\to f_{j}X$ ($f_i\to f_j{\bar{f}}_m{f}_l$) decays, with $f_a=q_a,{\ell }_a$, are presented. We consider the scenario where ${\Omega }_{5/3}$ couples to the fermions of the second and third families, with its right- and left-handed couplings obeying ${\lambda }_R^{\ell u_i}/{\lambda }_L^{\ell u_i}=O(\epsilon )$, where $\epsilon$ parametrizes the relative size between these couplings. The allowed parameter space is then found via the current constraints on the muon $(g-2)$, the $\tau \to \mu \gamma$ decay, the LHC Higgs boson data, and the direct LQ searches at the LHC. For $m_{{\Omega }_{5/3}}=1$ TeV and $\epsilon =10^{-3}$, we find that the $t\to cX$ branching ratios are of similar size and can be as large as $10^{-8}$ in a tiny area of the parameter space, whereas $\mathrm{Br}(t\to c\bar{\tau }\tau )$ [$\mathrm{Br}(t\to c\bar{\mu }\mu )$] can be up to $10^{-6}$ ($10^{-7}$).

24 pags, 10 figures, 1 table