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) approximate to 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 (similar to 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 vectorlike families of quarks and leptons with relatively light masses of order 1 TeV, having the quantum numbers of 16 + <(16)over bar> 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)(mz), 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.