Testing the Nature of Kaluza-Klein Excitations at Future Lepton Colliders

TL;DR

This paper evaluates how future lepton colliders can identify and distinguish Kaluza-Klein excitations of gauge fields, especially when their masses are beyond the reach of the LHC, providing insights into extra-dimensional models.

Contribution

It analyzes the potential of lepton colliders to resolve ambiguities in KK excitation signatures and explores detection prospects for multiple extra dimensions.

Findings

Lepton colliders can distinguish KK excitations from other Z' signals.

Second KK excitations are generally too heavy for LHC detection.

Multiple extra dimensions likely produce KK states beyond LHC reach.

Abstract

With one extra dimension, current high precision electroweak data constrain the masses of the first Kaluza-Klein excitations of the Standard Model gauge fields to lie above $\simeq 4$ TeV. States with masses not much larger than this should be observable at the LHC. However, even for first excitation masses close to this lower bound, the second set of excitations will be too heavy to be produced thus eliminating the possibility of realizing the cleanest signature for KK scenarios. Previous studies of heavy $Z'$ and $W'$ production in this mass range at the LHC have demonstrated that very little information can be obtained about their couplings to the conventional fermions given the limited available statistics and imply that the LHC cannot distinguish an ordinary $Z'$ from the degenerate pair of the first KK excitations of the $\gamma$ and $Z$. In this paper we discuss the capability of lepton colliders with center of mass energies significantly below the excitation mass to resolve this ambiguity. In addition, we examine how direct measurements obtained on and near the top of the first excitation peak at lepton colliders can confirm these results. For more than one extra dimension we demonstrate that it is likely that the first KK excitation is too massive to be produced at the LHC.