Wilson line breaking and gauge coupling unification .

We estimate the effect of threshold corrections coming from Wilson lines to gauge coupling unification in the weakly coupled heterotic string with orbifold compactification. By expressing the corrections in terms of an effective field theory calculation we are able to estimate the minimal threshold corrections in a realistic model without constructing the full string theory. Using this we show that the effect of the gauge boson Kaluza Klein excitations is systematically to reduce the string prediction for the unification scale. In the case of discrete Wilson lines the effect of the Wilson lines on both the gauge and matter sectors is fixed. We show that the combined effect of the gauge and matter threshold corrections of the Kaluza Klein excitations of the MSSM states can readily bring both the prediction for the unification scale and for the strong coupling constant into good agreement with experiment.