Tev-scale Guts

The possibility of extra large spacetime dimensions has recently received considerable attention . One of the first serious investigations of the phenomenological properties of theories with extra large dimensions was made in Ref. , where the lightness of the supersymmetry-breaking scale was related to the largeness of a string-theoretic compactification radius. More recently, in Ref., extra large dimensions figured prominently in a radical new proposal for avoiding the gauge hierarchy problem by lowering the Planck scale to the TeV scale. The extra dimensions required to achieve this are in the (sub-)millimeter range, and imply a profound change in Newton’s gravitational force law at such distances. Likewise, in Refs. , it was shown how extra large dimensions could also be used to lower the fundamental string scale to the TeV scale. The idea of taking the string scale in the TeV range originates in Ref. , and makes use of special features of open-string theory first pointed out in Ref. . Besides the Planck and string scales, there also exists one additional high fundamental scale in physics: the GUT scale. Indeed, while the Planck and (perturbative heterotic) string scales are related directly to each other, the GUT scale stands independently. In this talk, therefore, we will concentrate on our complementary proposal 3,6 of lowering the fundamental GUT scale to the