  The associated production of $Z$ boson and a pair of new quarks at the Large
Hadron Collider (LHC) is studied. The cross sections for both sequential
fermions and vector-like fermions are presented. It is found that for
sequential fermions the cross sections can reach $1\sim 10^2$ fb for heavy
quark mass $m_Q$ from 1000 GeV to 200 GeV.
  For vector-like quarks, the cross sections are suppressed by mixing parameter
$\sin\theta_L$. Focusing on process $pp \to b'b'$, we investigate the
possibility of detecting the $6l+2j$ signal. For a $b'$ with light mass and a
large branching ratio of $b'\to bZ$, it is found that only several signal
events (parton level) can be produced with 1000 fb$^{-1}$ integrated
luminosity. Although the signal events are rare, all the final states are
produced centrally and multi lepton final states are clear at hadron collider,
which could be easily detected.

High Energy Physics - Phenomenology