In this book, we emphasized the chiral nature of the SM. The orbifold construction has been discussed toward obtaining three family models from the E8 ×E′8 heterotic string. But there are other methods also for obtaining 4D string models. Among these, we review very briefly on the fermionic construction and the interesting brane setup. Since obtaining three families is one of the most important objectives going beyond 4D, we discuss these other constructions up to the point of introducing the possibility for three families. We do not attempt to review the whole ideas and realistic model buildings.
KeywordsGauge Group Open String Heterotic String Supersymmetry Condition Orbifold Action
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