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Aspects of nonsupersymmetric string phenomenology

We consider a big class of chiral four-dimensional string models which bear [Formula: see text] gauge symmetry. Supersymmetry (SUSY) is spontaneously broken via the Scherk–Schwarz mechanism associated with two large internal dimensions. We study two central issues in nonsupersymmetric string phenomenology: the suppression of the cosmological constant and the decompactification problem. We identify a subclass of chiral nonsupersymmetric models with fermion–boson degeneracy leading to exponentially suppressed cosmological constant. In addition, we propose a novel solution to the decompactification problem related to the cancellation of the linear growth of the gauge part of the thresholds with a similar growth of the universal part. We show that the proposed mechanism resolves the decompactification problem related to the non-Abelian gauge couplings in approximately 40% of the models under consideration.