New Signal for Universal Extra Dimensions

TL;DR

This paper investigates universal extra dimensions (UED) with gravity-mediated KK-number violating decays, which allow the lightest KK particle, $\gamma^*$, to decay to a photon and a KK graviton, producing a distinctive two-photon plus missing energy collider signature. The authors analyze the one-loop mass spectrum in a fat-brane setup, outline the conditions under which gravity-mediated decays dominate or coexist with mass-splitting decays, and compute collider cross sections for the Tevatron and LHC under representative parameters (e.g., $\Lambda R = 20$, $M_D = 5$ TeV, $N=2,6$). They derive discovery and exclusion reach for the compactification scale $1/R$, finding Tevatron Run I excludes $1/R \lesssim 380$ GeV, Run II extends to about $490$–$540$ GeV, and the LHC can probe up to roughly $3$ TeV, with the exact reach depending on the gravity-decay width regime. The work highlights the importance of the relative decay widths and provides a clear, testable collider signature to probe UED in current and future hadron colliders. The results have practical significance for planning searches in the $\gamma\gamma + {\not E_T}$ channel and understanding how gravity in extra dimensions can shape observable collider phenomena.

Abstract

In the universal extra dimensions (UED) scenario, the tree level masses of the first level Kaluza-Klein (KK) excitations of Standard Model particles are essentially degenerate. Radiative corrections will, however, lift this degeneracy, allowing the first level excitations to decay to the lightest KK particle (LKP), which is the gamma^*. KK number conservation implies that the LKP is stable. Then, since the SM particles radiated during these decays are rather soft, the observation of KK excitations production and decay in collider experiments will be quite difficult. We propose to add to this model KK number violating interactions mediated by gravity, which allow the gamma^* to decay to a photon and a KK graviton. For a variety a models and a large range of parameters, these decays will occur within the detector. Thus, pair production of KK excitations will give rise to a striking collider signal, consisting of two hard photons plus large missing energy (due to escaping gravitons). We evaluate the cross-section for these signals at the Tevatron and LHC, and derive the reach of these colliders in the search for universal extra dimensions.

Figures (4)

• Figure 1: Cross-sections for $\gamma \gamma X {\not E_T}$ signal coming from universal extra dimensions at Tevatron Run I (solid line) and Run II (dashed line). The kinematic cuts applied are described in the text.
• Figure 2: Cross-sections for $\gamma \gamma X {\not E_T}$ signal coming from universal extra dimensions at LHC for $N=6$ (solid line) and $N=2$ (dashed line). The kinematic cuts applied are described in the text.
• Figure 3: Decay widths for the first level KK excitations of gauge bosons (left) and fermions (right). The solid lines correspond to gravity mediated decays, with $N=2$ (1) and $N=6$ (2). The dashed lines correspond to decays allowed by mass splittings: $g^* \rightarrow q \bar{q}^{\bullet} , q \bar{q}^{\circ} + \hbox{h.c.}$ (a), $W^* \rightarrow l \bar{\nu}^{\bullet}, \nu \bar{l}^{\bullet} + \hbox{h.c.}$ (b), $Z^* \rightarrow l \bar{l}^{\bullet} + \hbox{h.c.}$ (c) (left), and $q^{\bullet} \rightarrow q' W^* , q Z^*$ (a), $q^{\circ} \rightarrow q \gamma^*$ (b), $l^{\bullet} \rightarrow l \gamma^*$ (c) (right).
• Figure 4: Branching ratios to final states: $\gamma \gamma$ (solid line), jet + $\gamma$ (dotted line) and lepton ($e$ or $\mu$) + $\gamma$ (dashed line) at the Tevatron Run II (left) for $N=2$, and LHC (right) for $N=6$.