Extra spacetime dimensions and unification

Abstract

We study the effects of extra spacetime dimensions at intermediate mass scales, as expected in string theories with large-radius compactifications, and focus on the gauge and Yukawa couplings within the Minimal Supersymmetric Standard Model. We find that extra spacetime dimensions naturally lead to the appearance of grand unified theories at scales substantially below the usual GUT scale. Furthermore, we show that extra spacetime dimensions provide a natural mechanism for explaining the fermion mass hierarchy by permitting the Yukawa couplings to receive power-law corrections. We also discuss how proton-decay constraints may be addressed in this scenario, and suggest that proton-decay amplitudes may be exactly cancelled to all orders in perturbation theory as a result of new Kaluza-Klein selection rules corresponding to the extra

Power law scaling in universal extra dimension scenarios

“…The beta function contributions are the same for each of the ΛR KK levels, which, in effect, can be summed. After this, the scale dependence is not logarithmic any more, it shows power law behaviour, as illustrated by Dienes et al in [6]. This illustration shows that if ΛR ≫ 1, then to a very good accuracy the calculation basically boils down to computing the number of KK states up to the cutoff scale.…”

“…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].…”

Neutrinoless double-beta decay and physics beyond the standard model

“…Theories with large compact extra dimensions of TeV size [64][65][66][67][68][69][70][71][72] have enriched dramatically the perspectives of the search for physics beyond the Standard Model. Among the possible higher-dimensional realizations, sterile neutrinos propagating in such extra dimensions [73][74][75][76] may provide interesting alternatives for generating the observed light neutrino masses.…”

Top quark Kaluza-Klein mode mixing in the Randall-Sundrum bulk standard model andB→Xsγ

“…Apart from the gauge hierarchy problem, TeV scale extra dimensions accessible to the SM fields have drawn a lot of interest due to various motivations such as gauge coupling unification [3], new mechanisms for supersymmetry breaking [4], the explanation of fermion mass hierarchies [5], and the presence of the Higgs doublet as a composite of top quarks [6].…”