Gauge coupling unification with extra dimensions and gravitational scale effects

We study gauge coupling unification in the presence of extra dimensions com pactified at a few TeV. Achieving unification requires a large number of ga uge boson Kaluza-Klein excitations lighter than the string scale, such that the higher-dimensional gauge couplings are partial derivative(1) in string scale units. Corrections to the gauge couplings from two or more loops are about 10% or larger, hence string (or M) theory is generally expected to b e strongly coupled in similar to TeV-scale extra-dimensional scenarios. Hig her-dimensional operators induced by quantum gravitational effects can shif t the gauge couplings by a few percent. These effects are sufficiently larg e that even the minimal Standard Model, or the MSSM, allow unification at a scale in the similar to TeV range. The strongly coupled unified theory may induce dynamical electroweak symmetry breaking. (C) 2000 Elsevier Science B.V. All rights reserved.