[Paper Review] PhD Thesis - Topics in SUSY Phenomenology at the LHC

Roger H. K. Kadala|arXiv (Cornell University)|May 7, 2012

Particle physics theoretical and experimental studies6 references3 citations

TL;DR

This PhD thesis investigates supersymmetry (SUSY) phenomenology at the LHC, focusing on enhanced detection of third-generation squarks via b-tagging and top-tagging in models with inverted squark mass hierarchy. It demonstrates that b-jet tagging significantly improves the LHC's reach for SUSY signals, especially for light top and bottom squarks, and proposes a model-independent method to extract neutralino mass matrix parameters from dilepton mass distributions using Lorentz invariants and χ² fitting, with promising results for mixed higgsino-gaugino scenarios.

ABSTRACT

This dissertation focuses on phenomenological studies for possible signals for supersymmetric events at the Large Hadron Collider (LHC). We have divided our endeavours into three separate projects. First, we consider SUSY models where the gluino production at the LHC should be rich in top and bottom quark jets. Requiring $b$-jets in addition to missing energy $\eslt$ should, therefore, enhance the supersymmetry signal relative to Standard Model backgrounds. We quantify the increase in the supersymmetry reach of the LHC from $b$-tagging in a variety of well-motivated models of supersymmetry. We also explore $top$-tagging at the LHC. Second, we explore the prospects for detecting the direct production of third generation squarks in models with an inverted squark mass hierarchy. This is signalled by $b$-jets + $\eslt$ events harder than in the Standard Model, but softer than those from the production of gluinos and heavier squarks. We find that these events can be readily separated from SM background (for third generation squark masses in the 200-400 GeV range), and the contamination from the much heavier gluinos and squarks although formidable can effectively be suppressed. Third, we attempt to extract model-independent information about neutralino properties from LHC data. assuming only the particle content of the MSSM and that all two-body neutralino decays are kinematically suppressed, with the neutralino inclusive production yielding a sufficient cross section. We show that the Lorentz invariant dilepton mass distribution encodes clear information about the relative sign of the mass eigenvalues of the parent and daughter neutralinos. We show that we can extract most neutralino mass matrix parameters if there is a double mass edge.

Motivation & Objective

• To improve the LHC's sensitivity to supersymmetric signals by leveraging b-jet and top-jet tagging in models with inverted squark mass hierarchy.
• To quantify the enhancement in SUSY discovery reach through b-tagging in well-motivated SUSY models, including mSUGRA and non-universal Higgs mass models.
• To explore the feasibility of directly detecting third-generation squarks in inverted mass hierarchy scenarios with 200–400 GeV masses.
• To extract model-independent information about neutralino properties from dilepton mass spectra, assuming kinematically suppressed two-body decays.
• To develop and validate a method for constraining MSSM neutralino mass matrix parameters using Lorentz-invariant dilepton mass distributions and χ² fitting.

Proposed method

• Simulated SUSY events using MADGRAPH for event generation and PYTHIA for parton showering and detector simulation.
• Applied b-tagging and top-tagging algorithms to isolate signals from third-generation squarks in inverted mass hierarchy models.
• Used Lorentz-invariant dilepton mass distribution as a probe for neutralino mass eigenstate properties, particularly the relative sign of mass eigenvalues.
• Applied Fierz transformations to the decay amplitude to relate the dilepton mass spectrum to neutralino mass matrix elements.
• Performed χ² fitting to dilepton mass distributions to extract constraints on MSSM parameters (M1, M2, µ, tanβ), marginalizing over other parameters.
• Implemented lepton pT cuts (0 GeV and 6 GeV) to suppress SM backgrounds and improve signal sensitivity.

Experimental results

Research questions

• RQ1 To what extent does b-tagging enhance the LHC's reach for SUSY signals in models with inverted squark mass hierarchy?
• RQ2 Can third-generation squarks with masses between 200–400 GeV be directly detected via b-jet + Emiss_T signatures despite SM and heavier SUSY backgrounds?
• RQ3 Can the Lorentz-invariant dilepton mass distribution encode information about the relative sign of neutralino mass eigenvalues in the MSSM?
• RQ4 What constraints can be placed on MSSM neutralino mass matrix parameters (M1, M2, µ, tanβ) using only the dilepton mass spectrum and kinematic endpoints?
• RQ5 How robust are the fitting techniques for neutralino parameters when applied to compressed SUSY spectra with suppressed two-body decays?

Key findings

• B-tagging significantly enhances the LHC's reach for SUSY signals in models with inverted squark mass hierarchy, particularly for light top and bottom squarks.
• For third-generation squarks in the 200–400 GeV mass range, events with b-jets and missing transverse energy can be effectively separated from SM backgrounds.
• Contamination from heavier gluinos and squarks can be suppressed through optimized analysis cuts, enabling clean signal extraction.
• The Lorentz-invariant dilepton mass distribution encodes the relative sign of the parent and daughter neutralino mass eigenvalues, enabling model-independent constraints.
• In the favorable case of a double mass edge with mixed higgsino-gaugino neutralinos, the χ² fitting method successfully recovers MSSM parameters: M1 = −95 to −58 GeV, M2 = 360 to 475 GeV, µ = 87 to 127 GeV, with tanβ remaining unconstrained.
• The Z-boson exchange can mimic slepton exchange in dilepton spectra, introducing a degeneracy that may require additional data for resolution.