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MSSM Higgs : Window into Susy GUTs

The Minimal Supersymmetric SO(10) GUT has developed into a fully realistic theory in which not only are the gauge couplings unified but the known fermion spectrum and mixing matrices could fit accurately using the latitude introduced by inclusion of quantum corrections to the GUT-effective MSSM-SM matching conditions. The fits yield predictions about the nature of the sparticle spectrum on the basis of the required threshold corrections. This indicated a necessarily large value for $A_0$ in 2008 : well before Higgs discovery at 126 GeV made it a commonplace assumption. GUT scale threshold corrections to the normalization of the emergent effective MSSM Higgs ameliorate the long standing Susy GUT puzzle of fast dimension five operator mediated proton decay. Numerical investigation indicates that B-violation rates below or near the current experimental upper limits are feasible in fully realistic models. Our results imply that UV completion models with large numbers of fields, like Kaluza-Klein models or String Theory, must be able to compute threshold corrections to be considered quantitative theories and not just fables. Required improvements in the fitting procedure are discussed. A generalization of the NMSGUT by gauging the flavour symmetry of the kinetic terms,while retaining renormalizability and the successful MSGUT symmetry breaking patterns, may allow dynamical generation of the observed Yukawa structure of the MSSM via the spontaneous breaking of the full gauge symmetry down to the MSSM at the unification scale. Focus on the emergence of the MSSM Higgs from the multiple Higgs doublets in the GUT thus provides a crucial window to view the energetically remote UV dynamics specified in fully calculable and realistic MSGUTs.