Search for t→ch at e+e− linear colliders

We study the rare top-quark decay t→ch, where h is a generic Higgs boson, at a linear collider. If kinematically accessible, all models contain this decay at some level due to quark flavor mixing. Some models, such as Model III of the two-Higgs doublet model, have a tree-level top–charm–Higgs coupling, and the branching ratio is close to 0.5%. Others, such as the MSSM, have a coupling induced at one-loop, and can have a branching ratio in the range of 10−5–5×10−4. We find that a linear collider of s=500 GeV and a luminosity of 500 fb−1 will begin to be sensitive to this range of the coupling.