A Mechanism for Kaluza-Klein Number Violation in UED and Implications for LHC

TL;DR

This paper explores how Kaluza-Klein number violation in Universal Extra Dimensions affects fermion mixing, leading to observable single production of KK excitations at the LHC, with implications for collider phenomenology and electroweak precision tests.

Contribution

It introduces a mechanism for KK number violation in UED models with a non-universal Higgs, enabling single KK fermion production and new decay channels at the LHC.

Findings

Large cross sections for KK quark production at the LHC

Distinctive final states with top quarks and gauge bosons

Potential to probe compactification scales up to several TeV

Abstract

If the Higgs does not propagate into an otherwise universal extra dimension, then tree-level Kaluza-Klein number is not conserved in the Yukawa interactions. This leads to mixing between zero-mode fermions and their associated Kaluza-Klein excitations. The effect can be quite large for the top quark, since its mass (171.4 plus or minus 2.1 GeV) is approximately one-half the current Tevatron mass bound (about 350-400 GeV) for Kaluza-Klein excitations of quarks with one universal extra dimension. In contrast to the SM, the bi-unitary transformation that diagonalizes the mass matrix does not diagonalize the effective 4D Yukawa coupling matrix because of the presence of the 5D kinetic terms. This leads to violation of tree-level Kaluza-Klein number conservation in the gauge sector due to a unique feature of the 5D theory. This allows Kaluza-Klein excitations of the fermions to be produced singly, and opens up new decay channels in the gauge sector. We compute the cross section for the production of the lowest-lying Kaluza-Klein excitations of the top and bottom quarks in the non-universal Higgs model at the Large Hadron Collider, and consider their subsequent decays. The effect is quite large: For example, pair production of Kaluza-Klein excitations of the bottom quark will lead to a pair of top quarks plus a pair of charged weak gauge bosons. Since the production is via the strong interaction, such final states stand out well against the Standard Model background, and are observable at the LHC for a wide range of compactification scales. Finally, we discuss the effects that such a model has on the T- parameter, and propose a model with maximal mixing where the compactification scale may be as large as a universal extra dimension.