  This paper explores the effects of finite width in processes of pair
production of a heavy eXtra Quark (XQ) with charge 2/3 and its subsequent decay
into a Dark Matter (DM) candidate -- either scalar or vector -- and Standard
Model (SM) up-type quarks at the Large Hadron Collider (LHC). This dynamics has
been ignored so far in standard experimental searches of heavy quarks decaying
to DM and we assess herein the regions of validity of current approaches, based
on the assumption that the XQ has a narrow width. Further, we discuss the
configurations of masses, widths and couplings where the latter breaks down.
