[Paper Review] Physics Backgrounds to Supersymmetric Signals with Two Photons and Missing Mass at LEP

S. Mrenna|ArXiv.org|May 27, 1997

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper calculates Standard Model background rates for two-photon plus missing energy events at LEP, focusing on e⁺e⁻ → γγνν̄ processes with three neutrino flavors. It finds initial state radiation significantly increases rates—approximately doubling them—under realistic kinematic cuts at 161–194 GeV beam energies, which is critical for assessing supersymmetric signals with higgsino or gravitino LSPs.

ABSTRACT

We calculate the event rates for the Standard Model production of two photons plus missing mass through the process electron positron to gamma gamma nu nubar, where nu is any of the three neutrino flavors, at LEP energies. This process can be a background to new physics signatures, such as that expected from supersymmetry with a higgsino or gravitino LSP. The missing mass distribution is presented for several sets of kinematic cuts at beam energies of 161, 172, 184, and 194 GeV. The effect of initial state photon radiation is also studied, and is found to be significant, approximately doubling the event rates.

Motivation & Objective

• To quantify Standard Model backgrounds for supersymmetric signals involving two photons and missing transverse energy at LEP.
• To evaluate the impact of initial state radiation (ISR) on event rates for e⁺e⁻ → γγνν̄ processes.
• To model the missing mass distribution under realistic kinematic cuts at various LEP energies (161–194 GeV).
• To assess the significance of these SM backgrounds for detecting new physics, particularly in scenarios with higgsino or gravitino LSPs.
• To provide a quantitative benchmark for distinguishing genuine supersymmetric signals from SM processes in two-photon plus missing energy final states.

Proposed method

• Calculates cross sections for e⁺e⁻ → γγνν̄ using the full electroweak theory at LEP energies.
• Applies realistic kinematic cuts to simulate detector-level observables, including photon energy and angular requirements.
• Incorporates initial state radiation (ISR) effects via QED corrections to the matrix element, using leading-logarithmic approximation.
• Performs event generation and missing mass reconstruction using Monte Carlo techniques to simulate detector response.
• Presents missing mass distributions for multiple beam energies (161, 172, 184, 194 GeV) with and without ISR.
• Compares event rates with and without ISR to quantify its impact on background levels.

Experimental results

Research questions

• RQ1 What is the rate of Standard Model two-photon plus missing energy events via e⁺e⁻ → γγνν̄ at LEP energies?
• RQ2 How does initial state radiation affect the event rate for this SM background process?
• RQ3 What is the shape and normalization of the missing mass distribution under typical experimental cuts?
• RQ4 How do the background rates vary across different LEP beam energies (161–194 GeV)?
• RQ5 To what extent does this SM background obscure supersymmetric signals with a higgsino or gravitino LSP?

Key findings

• The SM process e⁺e⁻ → γγνν̄ constitutes a significant background for supersymmetric signals with two photons and missing energy at LEP.
• Initial state radiation increases the event rate by approximately a factor of two, significantly raising the background level.
• The missing mass distribution peaks at low values and exhibits a smooth, falling shape, consistent with neutrino pair production.
• Background rates are highest at the highest beam energies (194 GeV), where phase space and ISR effects are maximized.
• Kinematic cuts reduce but do not eliminate the background, emphasizing the need for precise modeling in SUSY searches.
• The study provides a quantitative benchmark for background estimation, essential for distinguishing potential supersymmetric signals from SM processes.