  We perform a detailed study of top-quark partner production in the ttbar plus
large missing energy final-state at the LHC, presenting results for both scalar
and fermionic top-quark partners in the semi-leptonic and dileptonic decay
modes of the top quarks. We compare the results of several simulation tools:
leading-order matrix elements, next-to-leading order matrix elements,
leading-order plus parton shower simulations, and merged samples that contain
the signal process with an additional hard jet radiated. We find that
predictions from leading-order plus parton shower simulations can significantly
deviate from NLO QCD or LO merged samples and do not correctly model the
kinematics of the ttbar + ETmiss signature. They are therefore not a good
framework for modeling this new physics signature. On the other hand, the
acceptances obtained with a merged sample of the leading-order process together
with the radiation of an additional hard jet are in agreement with the NLO
predictions. We also demonstrate that the scale variation of the inclusive
cross section, plus that of the acceptance, does not accurately reflect the
uncertainty of the cross section after cuts, which is typically larger. We show
the importance of including higher-order QCD corrections when using kinematic
distributions to determine the spin of the top-quark partner.
