[Paper Review] Determining the Mass of Supersymmetric Scalars at the CLIC Multi-TeV e+e- Collider

M. Battaglia, M. Gruwé|ArXiv.org|Dec 10, 2002

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper investigates the precision measurement of supersymmetric scalar masses—specifically smuons ($\tilde{\mu}_L$) and scalar tops ($\tilde{t}_2$)—at the CLIC multi-TeV $e^+e^-$ collider using two methods: muon energy spectrum analysis and threshold scans. It demonstrates that with 1 ab$^{-1}$ luminosity, smuon masses can be determined with 1.3–3.0% accuracy, improving significantly with beam polarization, and extends the energy spectrum technique to scalar top masses with a relative accuracy of ±7.5% for $\tilde{t}_2$.

ABSTRACT

The determination of the smuon mass at the CLIC multi-TeV e+e- linear collider has been studied for two CMSSM benchmarks. Results are given for both the analysis of the muon energy spectrum and the threshold scan method. The effects of detector resolution, beam-beam interactions and accelerator- induced backgrounds are discussed. The energy spectrum technique is also applied to the t -> t ~g process to determine the scalar top mass, in scenarios with the gluino lighter than the squarks.

Motivation & Objective

• To evaluate the precision of supersymmetric scalar mass measurements at the CLIC $e^+e^-$ collider under realistic conditions.
• To compare the performance of two mass determination techniques: muon energy spectrum analysis and threshold scan.
• To extend the energy spectrum method to scalar top quarks ($\tilde{t}_2$) in scenarios where $\tilde{t}_2 \to t\tilde{g}$ is kinematically allowed.
• To assess the impact of detector resolution, beam-beam effects, and accelerator-induced backgrounds on mass reconstruction accuracy.
• To quantify improvements from beam polarization in threshold scan measurements.

Proposed method

• The muon energy spectrum method reconstructs the smuon mass by fitting the endpoint of the muon energy distribution in $e^+e^- \to \tilde{\mu}_L\tilde{\mu}_L \to \mu^+\chi_1^0\mu^-\chi_1^0$ events.
• The threshold scan method measures the $e^+e^- \to \tilde{\mu}_L\tilde{\mu}_L$ cross-section rise near the kinematic threshold, using two energy points optimized for mass and width sensitivity.
• A multi-dimensional discriminant based on $M_{\mu\mu}$, $M_{\text{recoil}}$, $E_{\text{miss}}$, $\mu\mu$ acolinearity, $|\cos\theta_{\text{thrust}}|$, $E_t$, and $E_{\text{hem}}$ suppresses background from $W^+W^-$ and $\chi_1\chi_2$ final states.
• The scalar top mass is extracted from the endpoint of the top quark energy spectrum in $e^+e^- \to \tilde{t}_2\tilde{t}_2 \to t\tilde{g}t\tilde{g}$, with contributions from other decay channels included in the fit.
• Simulations use Pythia 6.2 with full Geant-3 detector simulation, including beamstrahlung effects and overlayed backgrounds such as $\gamma\gamma \to \text{hadrons}$ and parallel muons.
• A $\chi^2$ fit to the muon energy spectrum is used to extract $M_{\tilde{\mu}}$, with both $M_{\tilde{\mu}}$ and $M_{\chi_1^0}$ left free in a two-parameter fit to assess systematic uncertainties.

Experimental results

Research questions

• RQ1 Can the smuon mass be reconstructed with sub-3% accuracy using the muon energy spectrum at CLIC with $\sqrt{s} = 3$ TeV?
• RQ2 How does beam polarization improve the precision of threshold scan measurements for smuon masses at CLIC?
• RQ3 To what extent can the scalar top mass be determined from the top quark energy spectrum when $\tilde{t}_2 \to t\tilde{g}$ is the dominant decay mode?
• RQ4 How do beam-beam effects and accelerator-induced backgrounds affect the accuracy of sfermion mass measurements at CLIC?
• RQ5 What is the achievable relative accuracy in measuring $M_{\tilde{t}_2}$ using the energy spectrum method, given competing decay channels and detector resolution?

Key findings

• For benchmark point H, the smuon mass $M_{\tilde{\mu}_L}$ is measured with a relative accuracy of ±1.3% using the muon energy spectrum method at $\sqrt{s} = 3$ TeV and 1 ab$^{-1}$ luminosity.
• For benchmark point E, the muon energy spectrum method achieves a relative accuracy of ±3.0% for $M_{\tilde{\mu}_L}$, with a measured value of $1145 \pm 25$ GeV.
• Threshold scans at $\sqrt{s} = 3.8$–4.2 TeV with unpolarized beams yield a mass uncertainty of ±29 GeV for $M_{\tilde{\mu}_L}$ in benchmark point E, improving to ±17 GeV with 80/60% beam polarization.
• The energy spectrum method is successfully extended to scalar top quarks in benchmark point E, achieving a relative mass accuracy of ±7.5% for $M_{\tilde{t}_2}$ with 3 ab$^{-1}$ luminosity.
• The two-parameter $\chi^2$ fit to the muon energy spectrum yields $M_{\tilde{\mu}_L} = 1145 \pm 25$ GeV and $M_{\chi_1^0} = 652 \pm 22$ GeV, demonstrating consistency with nominal values.
• Background suppression using a multi-dimensional discriminant effectively isolates the $\tilde{\mu}_L\tilde{\mu}_L$ signal, preserving signal efficiency across the muon energy range.