Exotic Quark Production in ep Collisions

We investigate the single production and decay of charge -1/3, weak isosinglet vectorlike exotic $D$ quarks in string inspired $E_6$ theories at future ep colliders; THERA with $\sqrt{s}$=1 TeV, L=40 $p{b}^{-1}$ and CERN Large Hadron Electron Collider (LHeC) with $\sqrt{s}$=1.4 TeV, $L={10}^4$ $p{b}^{-1}$. We found that an analysis of the decay modes of $D$ should probe the mass ranges of 100-450 GeV and 100-1200 GeV at the center of mass energies, 1 and 1.4 TeV, respectively.