  We investigate the flavor-changing interactions in an extension of the
standard model with singlet quarks and singlet Higgs, which are induced by the
mixing between the ordinary quarks and the singlet quarks ($ q $-$ Q $ mixing).
We consider the effects of the gauge and scalar interactions in the $ \Delta F
= 2 $ mixings of $ K^0 $, $ B_d $, $ B_s $ and $ D^0 $ mesons to show the
currently allowed range of the $ q $-$ Q $ mixing. Then, we investigate the new
physics around the electroweak scale to the TeV scale, which is accessible to
the Large Hadron Collider. Especially, the scalar coupling mediated by the
singlet Higgs may provide distinct signatures for the decays of the singlet
quarks and Higgs particles, which should be compared with the conventionally
expected ones via the gauge and standard Higgs couplings. Observations of the
singlet quarks and Higgs particles will present us important insights on the $
q $-$ Q $ mixing and Higgs mixing.
