  In this work, we study the $SU(2)_L$ singlet complex scalar extended KSVZ
model that, in addition to providing a natural solution to the strong-CP
problem by including a global Peccei-Quinn symmetry, also furnishes two
components of dark matter that satisfy observer relic density without
fine-tuning of model parameters. Furthermore, this model provides a rich
phenomenology by introducing a vector-like quark whose presence can be sensed
in collider experiments and dark matter production mechanisms. We explore the
possibility of democratic Yukawa interaction of the vector-like quark with all
up-type quarks and scalar dark matter candidate. We also employ next-to-leading
order NLO-QCD correction for VLQ pair production to study a unique search at
the LHC, generating a pair of boosted tops with sizeable missing transverse
momentum. Multivariate analysis with jet substructure variables has a strong
ability to explore a significant parameter space of this model at the 14 TeV
LHC.
