Gauge coupling unification in realistic freefermionic string models
Abstract
We discuss the unification of gauge couplings within the framework of a wide class of realistic freefermionic string models which have appeared in the literature, including the flipped SU(5), SO(6) × SO(4), and various SU(3) × SU(2) × U(1) models. If the matter spectrum below the string scale is that of the Minimal Supersymmetric Standard Model (MSSM), then string unification is in disagreement with experiment. We therefore examine several effects that may modify the minimal string predictions. First, we develop a systematic procedure for evaluating the oneloop heavy string threshold corrections in freefermionic string models, and we explicitly evaluate these corrections for each of the realistic models. We find that these string threshold corrections are small, and we provide general arguments explaining why such threshold corrections are suppressed in string theory. Thus heavy thresholds cannot resolve the disagreement with experiment. We also study the effect of nonstandard hypercharge normalizations, light SUSY thresholds, and intermediatescale gauge structure, and similarly conclude that these effects cannot resolve the disagreement with lowenergy data. Finally, we examine the effects of additional color triplets and electroweak doublets beyond the MSSM. Although not required in ordinary grand unification scenarios, such states generically appear within the context of certain realistic freefermionic string models. We show that if these states exist at the appropriate thresholds, then the gauge couplings will indeed unify at the string scale. Thus, within these string models, string unification can be in agreement with lowenergy data.
 Publication:

Nuclear Physics B
 Pub Date:
 February 1995
 DOI:
 10.1016/05503213(95)004971
 arXiv:
 arXiv:hepth/9505046
 Bibcode:
 1995NuPhB.457..409D
 Keywords:

 High Energy Physics  Theory;
 High Energy Physics  Phenomenology
 EPrint:
 81 pages, standard LaTeX, 7 figures (Encapsulated PostScript), expanded published version