The International Linear Collider as a Kaluza-Klein Factory

TL;DR

This work analyzes how a future International Linear Collider (ILC) can function as a Kaluza-Klein factory within the minimal Universal Extra Dimension framework, focusing on KK-number violating interactions that permit single production of $n=2$ states and on the gamma-gamma mode to study Higgs and KK excitations. The authors show that $\gamma_2$ and $Z_2$ appear as narrow $s$-channel resonances with measurable cross sections, enabling precise measurements of the KK spectrum and discrimination between UED and supersymmetry, provided $R^{-1}$ is within the ILC's reach. They also examine how loop-induced Higgs production and decays are modified by KK excitations, notably a $\sim$25–30% suppression in $\gamma\gamma\to h$ for $R^{-1}=300$ GeV and an enhancement of $h\to gg$, and discuss access to excited Higgs states via $W_0W_1$ fusion and $\gamma\gamma$ Higgs pair production. Overall, the paper argues that ILC (and potentially CLIC) can rigorously probe minimal UED parameters $R^{-1}$ and $\Lambda$, test KK-parity phenomenology, and probe the Higgs sector, with a strong emphasis on precise resonance studies and soft-$\tau$ detection in excited states.

Abstract

In the minimal Universal Extra Dimension model, single production of n = 2 gauge bosons provides a unique discriminating feature from supersymmetry. We discuss how the proposed International Linear Collider can act as a n = 2 factory, much in the same vein as LEP. We also touch upon the potential of the gamma-gamma mode of the collider to study the production and the decay of an intermediate mass Higgs boson and its KK excitations.

Figures (3)

• Figure 1: $X_{Vf}$, the KK number violating couplings, as a function of $\Lambda R$, for $R^{-1}=300$ GeV (the values are independent of $R$). From top to bottom, the curves are for $X_{ZL}$, $X_{\gamma e}$, $X_{\gamma L}$, $X_{ZQ}$, $X_{\gamma d}$, $X_{\gamma u}$, and $X_{\gamma Q}$ respectively. For their definitions, see text.
• Figure 2: Decay widths of $Z_2$ (upper pair) and $\gamma_2$ (lower pair) as a function of $\Lambda R$, for $R^{-1}=450$ GeV and 300 GeV (upper and lower curves in a pair).
• Figure 3: The relative enhancement of $h\to gg$ decay width for $R^{-1}=300$, 400, 500, and 600 GeV (top to bottom). The SM expectation is the blue line at the bottom.