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Bounds on Kaluza-Klein excitations of the SM vector bosons from electroweak tests

A. Strumia

TL;DR

The paper studies bounds on the compactification scale $M$ in a minimal SM extension to 5 dimensions where SM gauge bosons propagate in one extra dimension. It uses electroweak precision observables, especially Z-pole asymmetries, to constrain $M$ for two Higgs scenarios (bulk vs brane) and with possible universal corrections. The main results are lower bounds $M>3.5$ TeV for a bulk Higgs and $M>4.3$ TeV for a brane Higgs at 95% CL, while a universal-corrections scenario can accommodate a heavier Higgs up to 500 GeV with $M>3.4$ TeV; the dominant constraint arises from the effective weak mixing angle $s_W^2$ in Z decays. The findings suggest that KK signals at the Tevatron are unlikely, but the LHC could probe KK scales up to roughly 6–7 TeV, and disentangling such signals from compositeness remains challenging. The results are in agreement with a contemporaneous independent analysis.

Abstract

Within a minimal extension of the SM in 4+1 dimensions, we study how Kaluza Klein excitations of the SM gauge bosons affect the electroweak precision observables. Asymmetries in Z decays provide the dominant bound on the compactification scale M of the extra dimension. If the higgs is so light that will be discovered at LEP2, we find the following 95% CL bounds: M > 3.5 TeV (if the higgs lives in the extra dimension) and M > 4.3 TeV (if the higgs is confined to our 4 dimensions). In the second case Kaluza Klein modes give "universal"corrections and a good fit of precision data can be obtained with an heavier higgs (up to 500 GeV) and with a smaller M > 3.4 TeV.

Bounds on Kaluza-Klein excitations of the SM vector bosons from electroweak tests

TL;DR

The paper studies bounds on the compactification scale in a minimal SM extension to 5 dimensions where SM gauge bosons propagate in one extra dimension. It uses electroweak precision observables, especially Z-pole asymmetries, to constrain for two Higgs scenarios (bulk vs brane) and with possible universal corrections. The main results are lower bounds TeV for a bulk Higgs and TeV for a brane Higgs at 95% CL, while a universal-corrections scenario can accommodate a heavier Higgs up to 500 GeV with TeV; the dominant constraint arises from the effective weak mixing angle in Z decays. The findings suggest that KK signals at the Tevatron are unlikely, but the LHC could probe KK scales up to roughly 6–7 TeV, and disentangling such signals from compositeness remains challenging. The results are in agreement with a contemporaneous independent analysis.

Abstract

Within a minimal extension of the SM in 4+1 dimensions, we study how Kaluza Klein excitations of the SM gauge bosons affect the electroweak precision observables. Asymmetries in Z decays provide the dominant bound on the compactification scale M of the extra dimension. If the higgs is so light that will be discovered at LEP2, we find the following 95% CL bounds: M > 3.5 TeV (if the higgs lives in the extra dimension) and M > 4.3 TeV (if the higgs is confined to our 4 dimensions). In the second case Kaluza Klein modes give "universal"corrections and a good fit of precision data can be obtained with an heavier higgs (up to 500 GeV) and with a smaller M > 3.4 TeV.

Paper Structure

This paper contains 7 sections, 14 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. The model
  3. Results
  4. Conclusion
  5. Note added
  6. Acknowledgments
  7. NRO operators

Figures (2)

  • Figure 1: Bounds on the compactification scale $M$ and on the higgs mass in three extensions of the SM to five dimensions: (a) with a higgs doublet confined to 4 dimensions; (b) with a higgs doublet that can propagate to the extra dimension; (c) a combination of (a) and (b). The level curves correspond to $\Delta \chi^2=\{1,2.3,3.8,6,9.2\}$.
  • Figure 2: $95\%\,\hbox{\rm C.L.}$ lower bounds on the compactification scale $M$ from electroweak data for $m_h=100\,{\rm GeV}$ as function of $\sin\beta$ ($\sin\beta=1$ corresponds to a higgs confined to 4 dimensions). The continuous line shows the bound from the global fit, while dashed lines show the bounds from single measurements.