[Paper Review] Understanding SUSY limits from LEP

A. Lipniacka|ArXiv.org|Oct 25, 2002

Particle physics theoretical and experimental studies4 references3 citations

TL;DR

This paper analyzes LEP constraints on the Minimal Supersymmetric Standard Model (MSSM) within two benchmark scenarios: gravity-mediated MSSM and minimal SUGRA (mSUGRA). It demonstrates that LEP sets stringent indirect limits on sparticle masses—particularly the lightest neutralino (LSP) and Higgs boson—through Higgs and chargino searches, with results highly dependent on model parameters like tanβ, A0, and top quark mass.

ABSTRACT

LEP results have constrained heavily the Minimal Supersymmetric Standard Model, while providing hints for light Higgs boson and for ``SUSY-assisted'' gauge couling unification. In this paper the results obtained at LEP within two scenarios, the gravity-mediated MSSM framework and the minimal SUGRA scenario are presented. Model-dependence and coverage of LEP results is discussed.

Motivation & Objective

• To evaluate the impact of LEP experimental results on the parameter space of the Minimal Supersymmetric Standard Model (MSSM).
• To assess model-dependent constraints on sparticle masses, particularly the lightest neutralino (LSP), in the context of R-parity-conserving SUSY.
• To investigate how indirect limits from Higgs and chargino searches extend beyond direct kinematic reach in constrained MSSM (CMSSM) and mSUGRA frameworks.
• To quantify the dependence of exclusion limits on key parameters such as tanβ, A0, and top quark mass in mSUGRA.
• To clarify the interplay between Higgs searches, stau LSP regions, and slepton/squark mass limits in the mSUGRA scenario.

Proposed method

• Uses the constrained MSSM (CMSSM) with non-universal Higgs parameters and the minimal SUGRA (mSUGRA) framework as benchmark models.
• Applies LEP experimental results from Higgs, chargino, slepton, and neutralino searches to derive exclusion regions in the m1/2–m0 parameter space.
• Applies radiative corrections to the Higgs mass, dependent on top quark mass and stop masses, to determine m_h^0 limits.
• Evaluates the role of A0 in shifting the stau LSP region and altering Higgs exclusion bounds, particularly for large negative A0.
• Translates exclusion regions into direct limits on sparticle masses, such as mχ̃0₁, mχ̃±₁, mẽR, and mH±, using 95% confidence level criteria.
• Compares results across different values of tanβ and top quark mass to assess sensitivity and robustness of limits.

Experimental results

Research questions

• RQ1 How do LEP results constrain the mass of the lightest neutralino (LSP) in the mSUGRA scenario, and how does this depend on A0 and tanβ?
• RQ2 To what extent do Higgs searches at LEP set indirect limits on sparticle masses beyond the direct kinematic reach of the collider?
• RQ3 How does the value of A0 influence the exclusion regions for the stau LSP and the Higgs boson mass in mSUGRA?
• RQ4 What is the role of the top quark mass in determining the sensitivity of LEP to MSSM parameter space in the mSUGRA model?
• RQ5 How do slepton and squark mass limits in mSUGRA depend on tanβ and A0, especially in regions where the stau becomes the LSP?

Key findings

• LEP sets a direct lower limit on the lightest Higgs boson mass of m_h^0 > 114.4 GeV/c² for tanβ < 6 or m_A > 120 GeV/c².
• In the mSUGRA scenario with A₀ = 0, the lightest neutralino mass limit is constrained to mχ̃0₁ > 50 GeV/c² due to chargino and neutralino searches.
• For large negative A₀ values, Higgs searches become less effective, and the neutralino mass limit degrades to ~50 GeV/c², set by chargino searches.
• The right-handed selectron mass limit in mSUGRA (A₀ = 0) is indirectly constrained by Higgs searches at low tanβ and by stau LSP conditions at high tanβ.
• The stau LSP region expands with increasingly negative A₀, reducing the Higgs exclusion region and shifting the dominant constraint to chargino and neutralino searches.
• The limit on the lightest neutralino mass degrades to the chargino search limit when A₀ is large and negative, particularly for tanβ > 15.