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Weakly-coupled heterotic string is known to have problems of dilaton/moduli stabilization, supersymmetry breaking (by hidden-sector gaugino condensation), gauge coupling unification, QCD axion, as well as cosmological problems involving dilaton/moduli and axion. The author studies these problems by adopting the point of view that they arise mostly due to limited calculational power, little knowledge of the full vacuum structure, and an inappropriate treatment of gaugino condensation. It turns out that these problems can be solved or are much less severe after a more consistent and complete treatment. There are two kinds of non-perturbative effects in the construction of string effective field theory: the field-theoretical non-perturbative effects of gaugino condensation (with an important constraint ignored in the past) and the stringy nonperturbative effects conjectured by S. Shenker, which are best described using the linear multiplet formalism. Stringy non-perturbative corrections to the Kaehler potential are invoked to stabilize the dilaton at a value compatible with a weak coupling regime. Modular invariance is ensured through the Green-Schwarz counterterm and string threshold corrections which, together with hidden matter condensation, lead to moduli stabilization at the self-dual point where the vev`s of moduli`s F components vanish. In the vacuum, supersymmetry is broken at a realistic scale with vanishing cosmological constant. As for soft supersymmetry breaking, this model always leads to a dilaton-dominated scenario. For the strong CP problem, the model-independent axion has the right properties to be the QCD axion. Furthermore, there is a natural hierarchy between the dilaton/moduli mass and the gravitino mass, which could solve both the cosmological moduli problem and the cosmological problem of the model-independent axion.

Supersymmetry breaking in superstring theory by Gaugino condensation and its phenomenology