Scalar Dark Matter Effects in Higgs and Top Quark Decays

We study possible observational effects of scalar dark matter, the darkon D, in Higgs h and top quark t decay processes, h->DD and t->cDD in the minimal Standard Model (SM) and its two Higgs doublet model (THDM) extension supplemented with a SM singlet darkon scalar field D. We find that the darkon D can have a mass in the range of sub-GeV to several tens of GeV, interesting for LHC and ILC colliders, to produce the required dark matter relic density. In the SM with a darkon, t->cDD only occurs at loop level giving a very small rate, while the rate for Higgs decay h->DD can be large. In THDM III with a darkon, where tree level flavor changing neutral current (FCNC) interaction exists, a sizable rate for t->cDD is also possible.