[Paper Review] Contrasting Supersymmetry and Universal Extra Dimensions at the CLIC Multi-TeV e+e- Collider

M. Battaglia, Datta, AseshKrishna+3 more|arXiv (Cornell University)|Feb 3, 2005

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper compares Universal Extra Dimensions (UED) and supersymmetry (SUSY) at the CLIC e⁺e⁻ collider, demonstrating that angular distributions of final-state muons, radiative return photon spectra, and threshold scans enable unambiguous discrimination between the two models. With 1 ab⁻¹ luminosity, mass measurements achieve sub-0.1% accuracy, and the sharp ISR photon peak in UED provides a unique signature absent in SUSY.

ABSTRACT

Universal extra dimensions and supersymmetry have rather similar experimental signatures at hadron colliders. The proper interpretation of an LHC discovery in either case may therefore require further data from a lepton collider. In this paper we identify methods for discriminating between the two scenarios at the linear collider. We study the processes of Kaluza-Klein muon pair production in universal extra dimensions in parallel to smuon pair production in supersymmetry, accounting for the effects of detector resolution, beam-beam interactions and accelerator induced backgrounds. We find that the angular distributions of the final state muons, the energy spectrum of the radiative return photon and the total cross-section measurement are powerful discriminators between the two models. Accurate determination of the particle masses can be obtained both by a study of the momentum spectrum of the final state leptons and by a scan of the particle pair production thresholds. We also calculate the production rates of various Kaluza-Klein particles and discuss the associated signatures.

Motivation & Objective

• To identify robust experimental signatures that distinguish Universal Extra Dimensions (UED) from supersymmetry (SUSY) at a multi-TeV e⁺e⁻ collider like CLIC.
• To assess the feasibility of measuring particle masses with high precision using kinematic distributions and threshold scans in both UED and SUSY scenarios.
• To evaluate the impact of detector effects—such as beamstrahlung, resolution, and acceptance—on discrimination power.
• To explore the unique radiative return signature in UED, specifically the sharp peak in ISR photon energy due to Z₂ resonance, as a distinctive discriminator from SUSY.
• To compare production rates and final-state signatures of Kaluza-Klein particles across UED and SUSY, focusing on muon and electron pair production.

Proposed method

• Comparative analysis of dominant Feynman diagrams for muon pair production in UED (via KK-number violating vertex) and SUSY (via s-channel Z/γ exchange).
• Simulation of final-state kinematics including initial-state radiation (ISR), beamstrahlung, detector resolution, and acceptance cuts.
• Use of angular distributions (dσ/dcosθ) of final-state muons to distinguish spin-1/2 superpartners (SUSY) from spin-1/2 KK partners (UED).
• Analysis of radiative return photon energy spectra to identify the characteristic sharp peak in UED due to Z₂ resonance, absent in SUSY.
• Threshold scan strategy: measuring total cross-section as a function of center-of-mass energy to determine resonance masses with high precision.
• Cross-check of mass measurements via momentum spectrum analysis and comparison with theoretical predictions under both models.

Experimental results

Research questions

• RQ1 Can angular distributions of final-state muons at CLIC distinguish between spin-1/2 superpartners in SUSY and spin-1/2 KK partners in UED?
• RQ2 Does the radiative return photon spectrum in UED exhibit a unique, resolvable peak due to Z₂ resonance that is absent in SUSY?
• RQ3 To what extent can threshold scans at CLIC enable precise mass determination of new particles in both UED and SUSY models?
• RQ4 How do beamstrahlung, detector resolution, and acceptance affect the discrimination power between UED and SUSY at CLIC?
• RQ5 What are the relative production rates of level-1 and level-2 KK particles in UED, and how do they compare to SUSY counterparts?

Key findings

• The angular distribution of final-state muons provides a strong discriminator: UED shows a flatter distribution due to vector-like couplings, while SUSY exhibits a forward-backward asymmetry due to chiral couplings.
• The radiative return photon spectrum in UED features a sharp peak at high photon energy due to resonant Z₂ exchange, a signature absent in SUSY, enabling unambiguous identification.
• With 1 ab⁻¹ of integrated luminosity, the mass of the KK muon (μ₁) can be determined with a precision better than 0.1% using threshold scans and momentum spectrum analysis.
• The total cross-section measurement at threshold provides a robust cross-check on the spin and quantum numbers of the new particles, confirming the model hypothesis.
• Production rates for level-1 KK leptons and quarks in UED are comparable to those in SUSY for similar mass scales, but higher KK modes are kinematically suppressed and difficult to resolve.
• The presence of a Z₂ resonance in UED leads to a distinctive, resolvable peak in the ISR photon spectrum, observable even after detector simulation with Eγ > 10 GeV and 1° < θγ < 179°.