Prediction for $alpha_3(M_z)$ in a string-inspired model.pdf

arXiv:hep-ph/9904293v2 25 May 1999 OUTP-99-21-P Prediction forα 3(M z)in a string-inspired model Dumitru Ghilencea 1 Department of Physics, Theoretical Physics, University ofOxford 1 Keble Road, Oxford OX1 3NP, United Kingdom Abstract We apply the Renormalisation Group Evolution (RGE) to analyse the phenomenological impli- cations of an extended supersymmetric model, for the value of the uni?cation scale and the strong coupling at the electroweak scale. The model we consider is predicted to exist in Calabi-Yau pacti?cations with Wilson line mechanismforE 6symmetry breaking, contains additional mat- ter beyond the MSSM spectrum and avoids the “doublet-triplet” splitting problem in the Higgs sector. The calculation is analytical in two-loop order andincludes the e?ects of the heavy thresh- olds due to the additional matter considered. The value ofα 3(M z) can be brought within the experimental limits without a signi?cant change of the uni?cation scale fromthe MSSMprediction. 1E-mail address: D.******@ 1 Introduction The Minimal Supersymmetric Standard Model (MSSM) currently provides the “standard” framework for the study of the physics beyond the Standard Model. The MSSM model takes into account the constraints from the negative searches so far for an experimental signature for Supersymmetry and gives circumstantial evidence for supersymmetric uni?ed