Universal Extra Dimensions at the e-e- Colliders
Hsin-Chia Cheng
TL;DR
Universal Extra Dimensions with a TeV-scale radius $R$ predict KK partners for all SM fields and a stable LKP due to KK parity, giving missing-energy signatures with soft SM decay products. The MUED framework, with boundary terms vanishing at a cutoff $\Lambda$ and calculable one-loop bulk and boundary corrections, yields a level-1 spectrum with $m_{g_1}>m_{Q_1}>m_{q_1}>m_{W_1}\sim m_{Z_1}>m_{L_1}>m_{ell_1}>m_{gamma_1}$ and a mostly $B_1$-like $\gamma_1$ LKP. Hadron colliders can produce large rates for KK states, but the resulting jets and leptons are soft, making detection challenging, with Run II potentially probing $R^{-1}\gtrsim 300$ GeV and the LHC reaching up to $R^{-1}\sim 1.5$ TeV; the $e^- e^-$ mode provides a clean environment to study KK electrons and helps distinguish MUEDs from SUSY via spectroscopy and decay patterns. The study emphasizes that discriminating extra-dimensional scenarios from SUSY requires combining information from multiple collider modes and precise KK-state measurements.
Abstract
Universal Extra Dimensions (UEDs) with compactification radius near the TeV scale provide interesting phenomenology at future colliders. The collider signals of the first Kaluza-Klein (KK) level are very similar to those of a supersymmetric model with a nearly degenerate superpartner spectrum. The heavier first level KK states cascade decay to the lightest KK particles (LKP), which is neutral and stable because of KK-parity. The signatures involve missing energy and relatively soft jets and leptons which can be difficult for detection. The KK electron signal in $e^- e^-$ collisions is free from the problematic two photon background therefore provides a unique opportunity for a detailed studies of the KK electrons in the Universal Extra Dimension scenario.