Dark Matter and Collider Phenomenology of Universal Extra Dimensions

TL;DR

This paper surveys Universal Extra Dimensions (UED), where all Standard Model fields propagate in flat, compactified extra dimensions, yielding a KK tower with near-degenerate masses. A key feature is KK parity, ensuring loop-level contributions to SM observables, pair production of KK states, and a stable LKP that can serve as dark matter; the most studied LKP is the KK photon B^{(1)}. The authors detail the KK spectrum, electroweak and collider constraints, and the rich dark matter phenomenology, including relic density, direct/indirect detection, gamma rays, and antimatter signals, while outlining collider signatures at the LHC and ILC and strategies to distinguish UED from SUSY. They also explore beyond-one-dimension scenarios, KK graviton considerations, and benchmark models, emphasizing the importance of multi-messenger probes to validate or refute UED as the correct description of TeV-scale physics.

Abstract

We review the phenomenology of models with flat, compactified extra dimensions where all of the Standard Model fields are allowed to propagate in the bulk, known as Universal Extra Dimensions (UED). UED make for an interesting TeV-scale physics scenario, featuring a tower of Kaluza-Klein (KK) states approximately degenerate in mass at the scale set by the inverse size of the compactification radius. KK parity, the four-dimensional remnant of momentum conservation in the extra dimensions, implies two basic consequences: (1) contributions to Standard Model observables arise only at loop level, and KK states can only be pair-produced at colliders, and (2) the lightest KK particle (LKP) is stable, providing a suitable particle dark matter candidate. After a theoretical overview on extra dimensional models, and on UED in particular, we introduce the model particle spectrum and the constraints from precision electroweak tests and current colliders data. We then give a detailed overview of the LKP dark matter phenomenology, including the LKP relic abundance, and direct and indirect searches. We then discuss the physics of UED at colliders, with particular emphasis on the signatures predicted for the Large Hadron Collider and at a future Linear Collider, as well as on the problem of discriminating between UED and other TeV-scale new physics scenarios, particularly supersymmetry. We propose a set of reference benchmark models, representative of different viable UED realizations. Finally, we collect in the Appendix all the relevant UED Feynman rules, the scattering cross sections for annihilation and coannihilation processes in the early universe and the production cross section for strongly interacting KK states at hadron colliders.

Figures (44)

• Figure 1: The spectrum of first level Kaluza-Klein states, including the effects of radiative corrections and boundary terms. A compactification radius of $R^{-1}=500$ GeV, Higgs mass of 120 GeV and cutoff of $\Lambda = 20\,R^{-1}$ have been used. From Ref. Cheng:2002ab.
• Figure 2: Left: The magnitude of the contributions to the parameter $\hat{T}$ from each of the first three KK levels (dashed) for $m_{KK}=400$ GeV as a function of Higgs mass, as well as the sum over the first 10 KK modes (solid). The magnitude of the Higgs-dependent correction (the second term in the expression for $\hat{T}$ in Eq. (\ref{['thatterms']})) is shown as the dotted line (first from above). Right: The 95% (dashed) and 99% (dotted) confidence limit exclusion regions, as a function of Higgs mass and mass $m_{KK}=1/R$. From Ref. Flacke:2005hb.
• Figure 3: The production cross section of KK quarks and gluons at the Tevatron Run I (upper panel) and Run II (lower panel). The solid black curve represents the total production cross section, while the other lines show the separate contributions from KK quark pairs (red dotted line) KK quark-gluon (green dashed line) and KK gluon pairs (blue dot-dashed line). The KK top production cross section is indicated by the orange double-dot-dashed line. Adapted from Ref. Macesanu:2002db.
• Figure 4: The $n=1$ KK decay chain, showing the dominant (solid) and sub-dominant (dotted) transitions and the resulting decay products. Adapted from Ref. Cheng:2002ab.
• Figure 5: The Feynman diagram leading to the multi-lepton plus 2 jets plus missing energy signature in KK SU(2) double associated production.