Flavor changing neutral current decays $t\to c X$ ($X=γ,\,g,\, Z,\, H$) and $t\to c\bar \ell\ell $ ($\ell=μ,\,τ$) via scalar leptoquarks
The flavor changing neutral current decays $t\to c X$ ($X=γ,\,g,\, Z,\, H$) and $t\to c\bar \ell\ell $ ($\ell=μ,\,τ$) 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 $Ω_{5/3}$. 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 $Ω_{5/3}$ couples to the fermions of the second and third families, with its right- and left-handed couplings obeying $λ_R^{\ell u_i}/λ_L^{\ell u_i}=O(ε)$, where $ε$ parametrizes the relative size between these couplings. The allowed parameter space is then found via the current constraints on the muon $(g-2)$, the $τ\to μγ$ decay, the LHC Higgs boson data, and the direct LQ searches at the LHC. For $m_{Ω_{5/3}}=1$ TeV and $ε=10^{-3}$, we find that the $t\to c X$ branching ratios are of similar size and can be as large as $10^{-8}$ in a tiny area of the parameter space, whereas ${\rm Br}(t\to c\bar ττ)$ [${\rm Br}(t\to c\bar μμ)$] can be up to $10^{-6}$ ($10^{-7}$).