| Summary: | We study the construction of non-supersymmetric Es® E8 heterotic string compactifications of symmetric orbifolds and tensor products of minimal superconformal theories with central charge c=9.The general formalism and features of both powerful techniques are presented and analyzed meticulously. Using the first method we classify all ~ (N=2,3,4 and 6) orbifolds which break space-time supersymmetry and provide us with a realistic chiral theory.Suprisinglyenough, we find two point groups of order 4 and one point group of order 6.The mechanism we propose to lift tachyons from the twisted sectors consists of a combination of the mass level matching principle with the requirement that the left-sector should be tachyon free. Modular invariance and equivalence relations associated with the shift;vectors of the Ea ® E8 lattice, help us to classify all possible shift;vectors which break the Ea ® E8 gauge group.For each viable shift vector we then detennine the massless spectrum of the symmetric Z6 orbifold since the Z4 case has been previously exhausted.The disentangle of representations from the "observable" and "hidden" sectors, the control of the number of chiral matter states both from untwisted and twisted sectors, as well as the gauge symmetry breaking are achieved by considering the presence of constant gauge-background fields (Wilson-lines).The problem of tachyons is resolved by taking advantage of the same method as the one suggested in the absence of Wilson-lines and a classification of all acceptable Wilson-lines and four-dimensional gauge groups is again carried out.Phenomenological implications of these models are discussed and some interesting features already known in string theory are explored. The second method although more complicated is simplified using orbifold techniques. Again space-time supersymmetry is broken, but now with the insertion of some discrete phases (torsions) in the partition function of the theory.The richness of this method leads to some computational difficulties which put restrictions on our ability to construct all of the models allowed by the theory. Therefore, we focus on a class of the so-called A-type invariants and examine how realistic the extracted models are by constructing their massless spectrum and the gauge group they correspond to.Three generation models do emerge in our analysis but further exploration excludes the possibility of identifying these with the standard model.
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