Grand unification at intermediate mass scales through extra dimensions

Abstract

One of the drawbacks of conventional grand unification scenarios has been that the unification scale is too high to permit direct exploration. In this paper, we show that the unification scale can be significantly lowered (perhaps even to the TeV scale) through the appearance of extra spacetime dimensions. Such extra dimensions are a natural consequence of string theories with largeradius compactifications. We show that extra spacetime dimensions naturally lead to gauge coupling unification at intermediate mass scales, and moreover may provide a natural mechanism for explaining the fermion mass hierarchy by permitting the fermion masses to evolve with a power-law dependence on the mass scale. We also show that proton-decay constraints may be satisfied in our scenario due to the higher-dimensional cancellation of proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experimental collider signatures; and embeddings into string theory. The latter also enables us to develop several novel methods of explaining the fermion mass hierarchy via D-branes. Our results therefore suggest a new approach towards understanding the physics of grand unification as well as the phenomenology of large-radius string compact-

Power law scaling in universal extra dimension scenarios

“…Higher dimensional theories, with radii of compactification around an inverse TeV, have been investigated from the perspective of high energy experiments, phenomenology, string theory, cosmology, and astrophysics. Such TeV scale extra-dimensional scenarios could lead to a new mechanism of supersymmetry breaking [1], address the issue of fermion mass hierarchy from a different angle [2], provide a cosmologically viable dark matter candidate [3], interpret the Higgs as a quark composite leading to a successful electroweak symmetry breaking without the necessity of a fundamental Yukawa interaction [4], and, as mentioned before and what constitutes the central issue of our present study, lower the unification scale down to a few TeV [5,6]. Our concern here is a specific framework, called the Universal Extra Dimension (UED) scenario, where there is a single flat extra dimension, compactified on an S 1 /Z 2 orbifold, which is accessed by all the SM particles [7].…”

Search for physics beyond the standard model in final states with a lepton and missing transverse energy in proton-proton collisions ats=8 TeV

“…Another extra dimensions model, the TeV −1 model, has been proposed [11][12][13][14], in which only the fermions are confined to ordinary three-dimensional space, with the SM gauge bosons and the Higgs field propagating in compactified extra dimensions. Under the assumption of a single extra dimension, the model is specified by one parameter R ¼ 1=M C , the size of the compactified dimension, with M C being the corresponding compactification scale.…”

“…Additionally, events are allowed to include an arbitrary number of jets, as they may originate from initial state radiation. Interpretations of the observations are made in the context of various theoretical models: the sequential standard model (SSM) with a W 0 -boson [1], a helicitynonconserving contact interaction model (HNC-CI) [2], a DM model with a DM particle recoiling against a W-boson [3][4][5], namely the sequential standard model with same-sign couplings (SSMS) and the sequential standard model with opposite-sign couplings (SSMO) [6][7][8], split universal extra dimensions (split-UED) [9,10], and a TeV −1 model [11][12][13][14], the latter two predicting an additional spatial dimension.…”

On gauge couplings and thresholds in Type I Gepner models and otherwise

“…One-loop thresholds for the gauge couplings have been computed [66]- [71] and some steps beyond one-loop have been made [72]. More recently it has been argued that large extra dimensions naturally emerge in this approach [73]- [75]. In these cases, predictions for processes with missing energy at near future colliders [76]- [79] have been put forward.…”