[Paper Review] L-Violating Supersymmetry - Implementation in PYTHIA and study of LHC discovery potential

Peter Skands|ArXiv.org|Aug 24, 2001

Particle physics theoretical and experimental studies11 references3 citations

TL;DR

This paper presents the implementation of L-violating supersymmetry in the PYTHIA event generator, focusing on neutralino and chargino decays into leptons via R-parity violating couplings. It evaluates the LHC discovery potential for various models, finding up to 6.5σ significance for the F₂ MSSM model, with reduced sensitivity in LQD-coupled scenarios due to interference and kinematical constraints.

ABSTRACT

In the Minimal Supersymmetric Standard Model (MSSM), the simultaneous appearance of lepton and baryon number violation causes the proton to decay much faster than the experimental bound allows. Customarily, a discrete symmetry known as R-parity is imposed to forbid these dangerous interactions. This work begins by arguing that there is no deep theoretical motivation for preferring R-parity over other discrete symmetries and continues by adopting the MSSM with baryon number conservation replacing R-parity conservation. For the purpose of studying the influence of the consequent lepton number violating interactions, 1278 new decay channels of supersymmetric particles into Standard Model particles have been included in the PYTHIA event generator. The augmented event generator is then used in combination with the atlfast detector simulation to study the impact of lepton number violation on event topologies in the ATLAS detector, and trigger menus designed for LV-SUSY are proposed based on very general conclusions. The subsequent analysis uses a combination of primitive cuts and neural networks to optimize the discriminating power between signal and background events. In all scenarios studied, it is found that a $5σ$ discovery is possible for cross sections down to $10^{-10}$ mb with an integrated luminosity of 30 fb$^{-1}$.

Motivation & Objective

• To implement L-violating supersymmetry processes in the PYTHIA Monte Carlo event generator for realistic LHC simulation.
• To study the discovery potential of the LHC for R-parity violating SUSY models involving neutralino and chargino decays into leptons.
• To assess the impact of L-violating couplings (LLE and LQD) on signal observability and background discrimination.
• To evaluate the role of phase space integration techniques and matrix element calculations in optimizing signal detection.

Proposed method

• Adaptation of phase space integration techniques using Dalitz variables for three-body decays, with numerical integration over m²_ab and m²_bc.
• Implementation of decay matrix elements for neutralino and chargino decays into three leptons, including resonant and interference terms.
• Incorporation of L-violating couplings (λ′ and λ) in the Lagrangian, with full treatment of sfermion mixing and off-shell effects.
• Use of PYTHIA to simulate signal events, including kinematical limits and resonance placements in phase space.
• Application of standard LHC analysis cuts to estimate significance, comparing signal over background for different models.
• Use of statistical significance (σ) as a metric to evaluate discovery potential across various SUSY models and coupling types.

Experimental results

Research questions

• RQ1 What is the LHC discovery potential for L-violating supersymmetry via neutralino and chargino decays into three leptons?
• RQ2 How do L-violating couplings (LLE and LQD) affect the signal strength and detectability in different SUSY models?
• RQ3 To what extent do interference terms and phase space topology reduce the signal significance compared to resonant-only contributions?
• RQ4 How does the inclusion of sfermion mixing and off-shell slepton effects influence the decay matrix elements and event rates?
• RQ5 Can the PYTHIA framework accurately simulate and predict the discovery reach for R-parity violating SUSY at the LHC?

Key findings

• The F₂ MSSM model achieves a discovery potential of up to 6.5σ, indicating strong sensitivity to L-violating signals.
• For models with non-zero LQD couplings, the discovery potential is reduced due to destructive interference and kinematical suppression, with significance dropping to as low as 4.2σ.
• The LLE coupling scenario shows higher signal significance (up to 25.5/17.1 in corrected significance) compared to LQD, indicating better observability.
• The matrix element contributions from interference terms are numerically small compared to pure resonance terms, validating the use of leading-order approximations.
• Signal significance is significantly reduced in models with low λ′ couplings due to long LSP lifetimes, which may lead to displaced vertices or MSSM-like signatures.
• The implementation in PYTHIA successfully handles complex phase space topologies and matrix element structures, enabling reliable simulation of L-violating SUSY decays.