$Z_2\times Z_2$ orbifold compactification -- the origin of realistic free fermionic models

Abstract

All the realistic free fermionic models utilize a set of basis vectors, the NAHE set, that correspond to $Z_2\times Z_2$ orbifold compactification with nontrivial background fields. I argue that the realistic features of free fermionic models, like the number of generations and the fermion mass spectrum are due to the underlying $Z_2\times Z_2$ orbifold compactification.

Comments: 4 pages, latex, To appear in proceedings of the Beyond the Standard Model IV Conference, Lake Tahoe, CA, Dec 13-18, 1994. IASSNS preprint number is corrected.