[Paper Review] Phenomenological constraints on universal extra dimensions at LHC and electroweak precision test

Takuya Kakuda, Kenji Nishiwaki+3 more|arXiv (Cornell University)|Apr 23, 2013

Particle physics theoretical and experimental studies11 references6 citations

TL;DR

This paper constrains Kaluza-Klein (KK) scales in universal extra dimension (UED) models using LHC Higgs signal strength data and electroweak precision measurements via the S and T parameters. It finds that the minimal 5D UED model is excluded below 650 GeV (LHC) and 700 GeV (S,T analysis), while 6D UED models on various compactified manifolds face KK scale bounds between 850–1350 GeV (LHC) and 900–1500 GeV (S,T), with the latter being slightly more stringent.

ABSTRACT

We show two types of bounds on five- and six-demensional universal extra dimension (UED) models from the latest results of the Higgs search at the LHC and of the electroweak precision data for the S and T parameters. The UED models on which we put lower bounds are the minimal UED model in five dimensions and the six dimensional ones. The highest possible ultraviolet cutoff scale for each UED model is evaluated from the vacuum stability of the Higgs potential by solving the renormalization group equation of the Higgs self coupling. The bounds on the KK scale in the minimal UED model is 650 GeV from the LHC results and 700 GeV from the S, T analysis at the 95% confidence level, while those in the several 6D UED models are from 850 GeV to 1350 GeV (Higgs search) and from 900 GeV to 1500 GeV (S,T analysis).

Motivation & Objective

• To derive phenomenological bounds on Kaluza-Klein (KK) scales in universal extra dimension (UED) models using the latest LHC Higgs signal strength data.
• To evaluate constraints from electroweak precision measurements via the S and T parameters in 5D and 6D UED models.
• To determine the highest possible ultraviolet (UV) cutoff scale for each UED model by ensuring vacuum stability of the Higgs potential through renormalization group equation (RGE) analysis.
• To compare the relative strength of constraints from Higgs signal searches versus electroweak precision data across multiple 6D UED compactifications.
• To assess the viability of various 6D UED models on manifolds including T²/Z₂, T²/Z₄, S², RP², and the projective sphere.

Proposed method

• The Higgs signal strength at the LHC is computed using the ratio of UED to SM cross sections for Higgs production via gluon fusion and decay into γγ, ZZ, and WW final states.
• The loop-induced Higgs decay and production amplitudes are modified by KK excitations of top quarks and gauge bosons, with contributions calculated via effective loop functions J^SM and J^KK.
• The S and T parameters are computed from loop contributions of KK particles, including corrections from Higgs mass calibration and threshold effects near the UV cutoff scale.
• The UV cutoff scale is determined by solving the renormalization group equation for the Higgs self-coupling to ensure vacuum stability.
• Constraints are derived by comparing predicted S and T values with global fits to experimental data (S = 0.05 ± 0.09, T = 0.08 ± 0.07) at 95% confidence level.
• The analysis includes 5D minimal UED and multiple 6D UED models on T²/Z₂, T²/(Z₂×Z′₂), T²/Z₄, S², S²/Z₂, RP², and the projective sphere.

Experimental results

Research questions

• RQ1 What is the lower bound on the KK scale in the minimal 5D UED model from LHC Higgs signal strength measurements at 95% confidence level?
• RQ2 How do the S and T parameter constraints from electroweak precision data compare with those from LHC Higgs signal searches in 6D UED models?
• RQ3 What are the model-dependent KK scale bounds for 6D UED models compactified on T²/Z₂, T²/Z₄, S², RP², and the projective sphere?
• RQ4 How do contributions from KK top quarks, gauge bosons, and Higgs bosons affect the S and T parameters in UED models?
• RQ5 Which set of constraints—Higgs signal strengths or electroweak precision data—provides the more stringent bound on the KK scale in UED models?

Key findings

• The minimal 5D UED model is excluded below 650 GeV at 95% confidence level based on LHC Higgs signal strength data.
• The same model is excluded below 700 GeV at 95% confidence level when constrained by S and T parameter measurements.
• For 6D UED models on T²/Z₂, T²/(Z₂×Z′₂), T²/Z₄, S², S²/Z₂, RP², and the projective sphere, KK scale bounds range from 850 to 1350 GeV from Higgs signal searches.
• The S and T parameter analysis yields slightly stronger bounds, with KK scales excluded below 900–1500 GeV across the 6D UED models.
• The bounds from electroweak precision data are slightly more stringent than those from Higgs signal searches in current data, though future LHC data may reverse this trend.
• The highest UV cutoff scale for each UED model is determined by vacuum stability of the Higgs potential via RGE analysis of the Higgs self-coupling.