String Phenomenology Concepts Sample Clauses

String Phenomenology Concepts. ‌ String phenomenology aims to explain the physics of the universe in the context of string theory. As argued in the preceding section, it assumes that string theory gives a precise description of the universe at energy scales up to the Xxxxxx scale, and intends to unify models of lower-energy physics, such as SUSY, GUTs, or models of cosmological inflation [54, 55, 57], into a single description in string theory. A defining feature of model building in string phenomenology is the fact that (super-)string theory is well-defined exactly in ten spacetime dimensions, whereas there are only four observed dimensions of spacetime. This seems to be a problem at first sight, but there is a way out: six of the nine spatial dimensions required by string theory might parametrize a compact manifold whose finite size is too small to be observable at currently measurable energy scales. This idea of compactification was first proposed in the 1920s [58, 59], and has been reanimated from 1983 onwards [49, 60–62]. Not only does compactification solve the apparent disparity of spacetime dimensions, it also turns out that the geometry of the compactification manifold determines much of the physics in the remaining four non-compact dimensions. In fact, the reformulation of physical questions into geometric constraints on the compactification is both a driving force of and a main challenge for string phenomenology. Apart from the compactification manifold, a second choice to be made in any string phenomenology model is the type of low-energy string theory used as framework, with prominent choices being type IIA/IIB, and E8×E8 heterotic string theory. In type IIA/IIB string theory models, reviewed e.g. in [63], the non-abelian gauge degrees of freedom are localized on branes, which trace out a subspace of the compactification manifold. This locality enables a modular approach to model building, where one can study the gauge physics largely independent of global properties of the compactification. On the other hand, phenomenological models in heterotic string theory (see e.g. [64] for a review) allow for the presence of exceptional gauge groups. Such gauge groups have their own desirable consequences, for instance the easy implementation of a large top Yukawa coupling in GUT models. There is a framework that unifies the advantages of type IIA/IIB and heterotic com- pactifications, at the expense of leaving the regime of perturbative string theory. This framework is called F-theor...