The problems of unification-mismatch and low $\alpha_3$: A solution with   light vector-like matter
  The commonly accepted notion of a weak unified coupling $\alpha_X \approx 0.04$, based on the assumption of the MSSM--spectrum, is questioned. It is suggested that the four--dimensional unified string coupling should very likely have an intermediate value $(\sim 0.2-0.3$, say) so that it may be large enough to stabilize the dilaton but not so large as to disturb the coupling--unification relations. Bearing this in mind, as well as the smallness of the MSSM unification scale $M_X$ compared to the string scale, the consequences of a previously suggested extension of the MSSM spectrum are explored. The extension contains two vector--like families of quarks and leptons with relatively light masses of order 1 TeV, having the quantum numbers of ${\bf 16}+{\bf \overline{16}}$ of $SO(10)$. It is observed that such an extension provides certain unique advantages. These include: (a) removing the stated mismatch between MSSM and string unifications with regard to $\alpha_X$ and to some extent $M_X$ as well, (b) achieving coupling unification with a relatively low value of $\alpha_3(m_Z)$, in accord with its world average value, and (c) following earlier works, providing a simple explanation of the observed inter--family mass--hierarchy. The extension provides scope for exciting new discoveries, beyond those of SUSY and Higgs particles, at future colliders, including the LHC and the NLC. 
K.S. Babu (Princeton, Institute for Advanced Study) and Jogesh C. Pati   (University of Maryland)