Phenomenological survey of free fermionic heterotic–string models

A glossary overview of the phenomenological studies of quasi–realistic free fermionic heterotic string models is presented. I elaborate on the correspondence of these models with Z2 × Z2 orbifolds. Introduction The standard model of particle physics passes all experimental observations with flying colors. The gauge charges of the standard model matter states are strongly suggestive of the embedding of the standard model in larger grand unified groups. This is particularly striking in the context of SO(10) grand unification in which each of the standard model matter generation is embedded in a single 16 spinorial representation of SO(10). We recall that the standard model gauge charges were experimentally discovered and therefore are experimental observables. To account for these charges in the framework of the standard model requires 3× 3× 6 = 54 distinct parameters, taking into account the three group factors, the three generations and the six multiplets (including the right–handed neutrino) within each generation. The embedding of the standard model reduces this number to one, being the number of spinorial representations needed to accommodate the three generations of the standard model, namely three. The evidence for the realisation of grand unification structures in nature is therefore striking indeed. The standard model and grand unification in themselves cannot, however, be the end of the story. The first apparent question that pops to mind is how three generations came to be and not two, four or five. Next, there are the various mass and flavor mixing parameters of the standard model. The origin of these parameters is not explained in the context of the standard model, nor in grand unified theories.