[Paper Review] General Features of Supersymmetric Signals at the ILC: Solving the LHC Inverse Problem

Carola F. Berger, James S. Gainer+3 more|University of North Texas Digital Library (University of North Texas)|Dec 17, 2007

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper investigates whether the International Linear Collider (ILC) at √s = 500 GeV can resolve the LHC Inverse Problem in the Minimal Supersymmetric Standard Model (MSSM), where 162 pairs of MSSM parameter points produce indistinguishable LHC signatures. Using full SM background simulations, detector effects, and beam polarization, the study finds that 57 out of 162 model pairs are distinguishable at 5σ, demonstrating the ILC’s potential to uniquely map SUSY parameters despite LHC degeneracies.

ABSTRACT

We present the first detailed, large-scale study of the Minimal Supersymmetric Standard Model (MSSM) at a $\sqrt s=500$ GeV International Linear Collider, including full Standard Model backgrounds and detector simulation. We investigate 242 points in the MSSM parameter space, which we term models, that have been shown by Arkani-Hamed et al to be difficult to study at the LHC. In fact, these points in MSSM parameter space correspond to 162 pairs of models which give indistinguishable signatures at the LHC, giving rise to the so-called LHC Inverse Problem. We first determine whether the production of the various SUSY particles is visible above the Standard Model background for each of these parameter space points, and then make a detailed comparison of their various signatures. Assuming an integrated luminosity of 500 fb$^{-1}$, we find that only 82 out of 242 models lead to visible signatures of some kind with a significance $\geq 5$ and that only 57(63) out of the 162 model pairs are distinguishable at $5(3)σ$. Our analysis includes PYTHIA and CompHEP SUSY signal generation, full matrix element SM backgrounds for all $2 o 2, 2 o 4$, and $2 o 6$ processes, ISR and beamstrahlung generated via WHIZARD/GuineaPig, and employs the fast SiD detector simulation org.lcsim.

Motivation & Objective

• To determine whether the ILC can resolve the LHC Inverse Problem, where multiple MSSM parameter points produce identical LHC signatures.
• To assess the observability of SUSY particles at the ILC across a broad, realistic sample of 242 MSSM parameter points.
• To evaluate the effectiveness of beam polarization and detector simulation in distinguishing degenerate MSSM models.
• To investigate the limitations of the ILC in resolving the neutralino and sneutrino sectors despite success with charged sparticles.

Proposed method

• Simulated 242 MSSM parameter points from Arkani-Hamed et al., including 162 degenerate pairs, using full SUSY signal generation via PYTHIA and CompHEP.
• Incorporated full matrix element Standard Model backgrounds for all 2→2, 2→4, and 2→6 final states using WHIZARD/O’MEGA and T. Barklow’s calculations.
• Applied realistic beam energy spectra with ISR and beamstrahlung effects via WHIZARD/GuineaPig.
• Used the org.lcsim fast SiD detector simulation to model detector response and reconstruction efficiency.
• Employed a universal cut-based analysis across multiple SUSY channels: charged slepton, sneutrino, chargino, LSP, and neutralino pairs.
• Applied log-likelihood techniques to assess significance of signals over SM backgrounds, with 500 fb⁻¹ luminosity and 80% electron beam polarization.

Experimental results

Research questions

• RQ1 Can the ILC distinguish between 162 pairs of MSSM models that are indistinguishable at the LHC due to identical signatures?
• RQ2 What fraction of the 242 studied MSSM models produce observable SUSY signals above SM backgrounds at the ILC with 500 fb⁻¹?
• RQ3 How effective is beam polarization in enhancing signal visibility and resolving model degeneracies?
• RQ4 What are the primary limitations of the ILC in resolving the neutralino and sneutrino sectors compared to the charged sparticle sector?

Key findings

• Only 82 out of 242 MSSM models produced visible SUSY signals at the ILC with significance ≥5σ, indicating limited kinematic reach for many models.
• Of the 162 degenerate model pairs from the LHC Inverse Problem, 57 pairs were distinguishable at 5σ and 63 pairs at 3σ, demonstrating significant resolution power.
• Charged SUSY partners were observable in 85 models, but only 78 of these were visible above SM backgrounds after full analysis, highlighting the role of background suppression.
• The neutralino and sneutrino sectors remained a major challenge, with only 4 additional models (beyond those with charged partners) being observable, indicating poor sensitivity to neutral states.
• Beam polarization was essential for reducing SM backgrounds and enabling model discrimination, especially in cases with suppressed cross sections.
• The study confirms that the ILC can uniquely map SUSY parameters for many models, particularly those with accessible charged sparticles, but neutral sector resolution remains a key limitation.