Navigating into the realm of
Non-Supersymmetric String Theories

Abstract

For more than two decades, remarkable progress has been made in the construction of supersymmetric Standard Model-like theories from the heterotic string theory. In particular, considerable effort has been invested in studying the abundant phenomenological features of heterotic strings exhibiting N = 1 spacetime supersymmetry. At the same time, their non-supersymmetric counterparts have received little attention on the grounds that strings which do not exhibit spacetime supersymmetry admit large one-loop dilaton tadpoles, and are therefore unstable. Nonetheless, in this epoch of data acquisition from high-luminosity experiments, the observational absence of supersymmetry is striking. Consequently, non-supersymmetric theories receive a profound interest in the particle physics community. In this thesis,
a class of non-supersymmetric, tachyon-free, four-dimensional string models is constructed via a string generalisation of Scherk-Schwarz compactification. Such models demonstrate greatly enhanced finiteness and stability properties, and exhibit some general features on their mass spectra, the behaviour of the one-loop cosmological constant and their interpolation properties. Special attention is paid to how from an exponentially suppressed one-loop cosmological constant, and therefore from an almost vanishing dilaton tadpole, finiteness and
stability ensue. The existence of such models is characterised by prominent phenomenological features which involve their natural energy scales, particle-charge
assignments, and the magnitudes of the associated Yukawa couplings and scalar masses. A radical result is the existence of Standard Model-like theories emerging
as the low energy limit of non-supersymmetric strings; there are no light superpartners and supersymmetry is absent at all energy scales.