Unification and extra space-time dimensions

We analyse the phenomenological implications of a particular class of supersymmetric models with additional space-time dimensions below the unification scale. Assuming the unification of the gauge couplings and using a two-loop calculation below the scale of the additional space-time dimensions, we show that the Value of alpha(3)(M-z) is further increased from the two-loop Minimal Supersymmetric Standard Model prediction. We consider whether decompactification threshold effects could bring alpha(3)(M-z) into agreement with experiment and discuss how accurately the threshold effects must be known in this case. (C) 1998 Elsevier Science B.V. All rights reserved.