Rare decays of the top quark in the standard and two-Higgs-doublet models
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Abstract
The decays $t\ensuremath{\rightarrow}\mathrm{cV}$, where $V=\ensuremath{\gamma}, g, or Z$, and $t\ensuremath{\rightarrow}\mathrm{cH}$ are calculated, keeping all masses, in the standard model (SM) for all possible values of the top-quark mass that are consistent with present data. We find that the branching fractions for these processes are small, the largest being $B(t\ensuremath{\rightarrow}cH)\ensuremath{\sim}{10}^{\ensuremath{-}7}$ and $B(t\ensuremath{\rightarrow}cg)\ensuremath{\sim}{10}^{\ensuremath{-}10}$. Our calculations are then extended to include the possible contributions of charged Higgs bosons to the transitions $t\ensuremath{\rightarrow}\mathrm{cV}$ in the context of general two-Higgs-doublet models. These new contributions can enhance the SM branching fractions by as much as 3-4 orders of magnitude, e.g., $B(t\ensuremath{\rightarrow}cg)\ensuremath{\sim}{10}^{\ensuremath{-}7}\ensuremath{-}{10}^{\ensuremath{-}6}$, for various values of the parameters. General expressions for flavor-changing neutral-current decays of heavy quarks including all masses and momenta, in the SM and in the two-Higgs-doublet models, are also given.
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