[Paper Review] Reconstructing CMSSM parameters at the LHC with $\sqrt{s}=14$ TeV via the golden decay channel

Andrew Fowlie, M. Kazana+1 more|arXiv (Cornell University)|Jun 25, 2011

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper demonstrates that a benchmark point in the CMSSM's heavy stau-coannihilation region—consistent with LHC and cosmological data—could be discovered at the LHC via the golden decay channel (squark → slepton → jjℓℓ̄Eₜₜ) at √s = 14 TeV with 300 fb⁻¹ luminosity. Using Monte Carlo simulations and kinematic endpoint analysis, it shows that sparticle masses can be precisely measured, enabling Bayesian reconstruction of CMSSM parameters with high accuracy, except for the trilinear parameter A₀, which improves with inclusion of Planck and Higgs mass data.

ABSTRACT

We identify a benchmark point in the CMSSM's heavy stau-coannihilation region, which is favored by experiments, and demonstrate that it could be accessible to the LHC at $\sqrt{s}=14$ TeV with 300/fb of integrated luminosity via a golden decay measurement. With Monte-Carlo, we simulate sparticle production and subsequent golden decay at the event level and perform pseudo-measurements of sparticle masses from kinematic endpoints in invariant mass distributions. We find that two lightest neutralino masses and the first and second generation left-handed slepton and squark masses could be rather precisely measured with correlated uncertainties. We investigate whether from such measurements one could determine the CMSSM's Lagrangian parameters by including a likelihood from our pseudo-measurements of sparticle masses in a Bayesian analysis of the CMSSM's parameter space. We find that the CMSSM's parameters can be accurately determined, with the exception of the common trilinear parameter. Experimental measurements of the relic density by Planck and the Higgs boson's mass slightly improve this determination, especially for the common trilinear parameter. Finally, within our benchmark scenario, we show that the neutralino dark matter will be accessible to direct searches in future one tonne detectors.

Motivation & Objective

• To identify a CMSSM benchmark point in the heavy stau-coannihilation region that is consistent with current LHC, Planck, and Higgs boson mass constraints.
• To assess the feasibility of discovering this benchmark point at the LHC via the golden decay channel (squark → slepton → jjℓℓ̄Eₜₜ) at √s = 14 TeV with 300 fb⁻¹ luminosity.
• To simulate sparticle production and decay at the event level, and extract sparticle masses using kinematic endpoint techniques in invariant mass distributions.
• To investigate whether measured sparticle masses can be used to reconstruct the underlying CMSSM Lagrangian parameters via Bayesian inference.
• To evaluate the predictive power of the reconstructed model for direct dark matter detection and rare decay branching ratios.

Proposed method

• Monte Carlo simulation of sparticle production and golden decay chain (q̃ → ẽ → qℓℓ̄Eₜₜ) at the LHC with √s = 14 TeV and 300 fb⁻¹ luminosity.
• Kinematic endpoint analysis of invariant mass distributions to extract masses of the two lightest neutralinos, the first-generation left-handed slepton, and squark.
• Construction of a covariance matrix to quantify correlated uncertainties in the measured sparticle masses.
• Bayesian statistical analysis combining pseudo-measurements of sparticle masses with likelihoods from Planck (relic density) and Higgs boson mass (mₕ ≈ 125 GeV) to infer CMSSM parameter space.
• Prediction of spin-independent WIMP-proton scattering cross section (σ̂SIₚ) for direct detection, and branching ratio for Bₛ → μ⁺μ⁻, under the reconstructed CMSSM parameters.
• Assessment of the sensitivity of the model to future one-tonne direct detection experiments and gamma-ray telescopes like CTA.

Experimental results

Research questions

• RQ1 Can a CMSSM benchmark point in the heavy stau-coannihilation region be discovered at the LHC via the golden decay channel at √s = 14 TeV with 300 fb⁻¹?
• RQ2 To what extent can sparticle masses be reconstructed from kinematic endpoints in the golden decay chain?
• RQ3 Can the CMSSM’s Lagrangian parameters be accurately reconstructed from measured sparticle masses using Bayesian inference?
• RQ4 How do additional constraints from Planck (relic density) and the Higgs boson mass improve the determination of CMSSM parameters, especially A₀?
• RQ5 Are the model’s predictions for direct detection (σ̂SIₚ) and rare decays (Bₛ → μ⁺μ⁻) consistent with current and future experimental sensitivities?

Key findings

• The benchmark CMSSM point in the heavy stau-coannihilation region is accessible at the LHC via the golden decay channel at √s = 14 TeV with 300 fb⁻¹ luminosity.
• Kinematic endpoint analysis allows precise measurement of the two lightest neutralino masses, the first-generation left-handed slepton, and squark masses, with small, correlated uncertainties.
• Bayesian reconstruction of CMSSM parameters from measured sparticle masses yields credible regions that envelope the benchmark values, except for the common trilinear parameter A₀, which is recovered with limited precision.
• Inclusion of Planck and Higgs boson mass likelihoods improves the determination of A₀, reducing bias and uncertainty in its posterior distribution.
• The predicted spin-independent WIMP-proton scattering cross section (σ̂SIₚ) is constrained to within an order of magnitude and is expected to be accessible to upcoming one-tonne direct detection experiments.
• The model predicts the branching ratio for Bₛ → μ⁺μ⁻ within 10% of the SM value, but this prediction is limited by parametric uncertainties in SM nuisance parameters and CMSSM parameter errors.