[Paper Review] Feasibility of slepton precision measurements at a muon collider

A. Freitas|arXiv (Cornell University)|Jul 19, 2011

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper investigates the feasibility of precision measurements of sleptons—supersymmetric partners of leptons—at a future muon collider, focusing on challenges from detector blind spots due to beam pipe shielding. It demonstrates that with optimized cuts and dual-energy running, slepton masses and spin can be measured with percent-level precision when the slepton-neutralino mass difference exceeds ~100 GeV, but measurements become infeasible for smaller mass differences due to overwhelming two-photon backgrounds.

ABSTRACT

Detectors at a high-energy muon collider must be protected from the decay products of beam muons by installing shielding material around the beam pipe. In this article, the impact of these blind detector regions on new-physics signatures with invisible final-state particles is shown by studying the production of sleptons, the superpartners of leptons. Special attention is given to large backgrounds from two-photon collisions. It is demonstrated how the influence of these backgrounds can be controlled by implementing suitable cuts or running at two different center-of-mass energies, thus permitting precision measurements of the mass and spin of the sleptons. However, these methods become ineffective for small mass differences between the sleptons and their decay products, and it will be difficult to analyze the slepton signal at a muon collider in this case.

Motivation & Objective

• To assess the impact of detector blind cones—caused by shielding around the beam pipe—on precision measurements of sleptons at a muon collider.
• To evaluate the dominance of two-photon collisions as a background source in missing-energy signatures involving sleptons.
• To determine whether slepton mass and spin can be measured with high precision despite limited angular coverage and background contamination.
• To investigate the dependence of measurement feasibility on the slepton-neutralino mass difference and shielding cone size.
• To explore whether dual-energy running can enhance mass determination accuracy for sleptons and neutralinos.

Proposed method

• Simulates slepton pair production via μ⁺μ⁻ collisions at √s = 3 TeV, assuming R-parity-conserving decay to final states with two charged leptons and two invisible neutralinos.
• Applies a set of kinematic and angular selection cuts (eqs. 4–10) to suppress Standard Model backgrounds, including those from two-photon processes.
• Introduces an additional cut (eq. 11) on the lepton energy spectrum (200–1000 GeV) to enhance separation between scalar and fermionic slepton signals.
• Uses a binned χ² test on the muon energy distribution to statistically distinguish scalar (slepton) from fermionic (sneutrino) final states.
• Performs Monte Carlo simulations to model backgrounds and signal distributions, with luminosity set to 1000 fb⁻¹.
• Compares results for two extreme shielding cone angles: 6° (optimistic) and 20° (conservative), to assess robustness of findings.

Experimental results

Research questions

• RQ1 To what extent do beam pipe shielding cones compromise the detection of missing-energy signals in slepton production at a muon collider?
• RQ2 How significant is the two-photon background in mimicking missing energy in slepton events, and can it be effectively suppressed?
• RQ3 Can slepton mass and spin be determined with high precision when the slepton-neutralino mass difference is large (>100 GeV)?
• RQ4 What is the impact of shielding cone size (6° vs. 20°) on the precision of slepton mass and spin measurements?
• RQ5 Under what conditions does the two-photon background render slepton signal measurement impossible, particularly for small mass differences?

Key findings

• For slepton-neutralino mass differences above ~100 GeV, the two-photon background can be effectively suppressed using optimized kinematic cuts, enabling clear identification of the slepton signal.
• With 1000 fb⁻¹ of integrated luminosity, the scalar and fermionic slepton hypotheses can be distinguished with a statistical significance of 7.7σ for a 6° shielding cone and 6.0σ for a 20° cone.
• Dual-energy running at √s = 3 TeV and a second center-of-mass energy allows for precision mass measurements of both sleptons and neutralinos at the percent level.
• When the slepton-neutralino mass difference is small, the signal becomes indistinguishable from the two-photon background, rendering mass and spin measurements infeasible.
• The size of the shielding cone has only a mild influence on measurement performance, with similar significance levels achieved across both 6° and 20° configurations.
• The cut (11) on the lepton energy spectrum (200–1000 GeV) is critical for suppressing γγ backgrounds and enabling spin discrimination, but it also removes signal events for small Δm, limiting its utility in low-mass-difference scenarios.