Higgs mediated flavor violating top quark decays t→uiH, uiγ, uiγγ, and the process γγ→tc in effective theories

The rare top quark couplings tuiγ and tuiγγ (ui=u, c) induced at the one-loop level by a flavor violating tuiH vertex are studied within the context of an effective Yukawa sector that incorporates SUL(2)xUY(1)-invariant operators of up to dimension six. Data on the recently observed D0-D0 mixing are employed to constrain the tuiH vertex, which is then used to predict the t→uiH, t→uiγ, and t→uiγγ decays, as well as the γγ→tui+tui reaction in the context of the ILC. It is found that the t→cH and t→cγγ decays can reach sizable branching ratios as high as 5x10-3 and 10-4, respectively. As for the t→cγ decay, it can have a branching ratio of 5x10-8 that is about 6 orders of magnitude larger than the standard model prediction, which, however, is still very small to be detected. As for tc production, it is found that, due to the presence of a resonant effect in the convoluted cross section σ(e+e-→γγ→tc+tc), about (0.5-2.7)x103 tc events may be produced at the ILC for a value of the Higgs mass near to the top mass.