[Paper Review] Probing mSUGRA Models at Linear Colliders

T. Kamon, R. Arnowitt+2 more|ArXiv.org|Feb 27, 2003

Particle physics theoretical and experimental studies3 citations

TL;DR

This study investigates the feasibility of probing minimal supersymmetry (mSUGRA) models in the co-annihilation region at a 500-GeV linear collider using $ au$-lepton plus missing energy signatures. Using polarized $e^+e^-$ collisions and optimized event selection, it demonstrates that an active mask covering down to $2^ ext{--}$ is essential to suppress $e^+e^- o e^+e^- au^+ au^-$ backgrounds, enabling $>5\sigma$ discovery sensitivity for $ ilde{\chi}_2^0\tilde{\chi}_1^0$ and $\tilde{\tau}_1^+\tilde{\tau}_1^-$ signals at $500\ \text{fb}^{-1}$ luminosity.

ABSTRACT

A feasibility study of a 500-GeV linear collider is presented for mSUGRA models in co-annihilation region. We find an active mask is critical to suppress $e^+ e^- o e^+ e^- τ^+ τ^-$ events to probe the models.

Motivation & Objective

• To assess the discovery potential of a 500-GeV linear collider for mSUGRA models in the co-annihilation region where $\tilde{\tau}_1$ and $\tilde{\chi}_1^0$ are quasi-degenerate.
• To evaluate the impact of $e^+e^- o e^+e^- au^+\tau^-$ backgrounds on the detection of $\tilde{\chi}_2^0\tilde{\chi}_1^0$ and $\tilde{\tau}_1^+\tilde{\tau}_1^-$ signals.
• To determine the necessity and effectiveness of an active mask in suppressing misidentified $\tau$-like jets from $e^+e^-$ and $\gamma\gamma$ processes.
• To optimize event selection cuts for distinguishing SUSY signals from Standard Model backgrounds, including weak and two-photon processes.
• To quantify discovery sensitivity for three mSUGRA benchmark points with $\tan\beta = 40$, $A_0 = 0$, and $\mu > 0$.

Proposed method

• Simulated $e^+e^-$ collisions at $\sqrt{s} = 500\ \text{GeV}$ using ISAJET for SUSY processes ($\tilde{\chi}_2^0\tilde{\chi}_1^0$, $\tilde{\tau}_1^+\tilde{\tau}_1^-$) and WPHACT for SM backgrounds ($\nu\bar{\nu}\tau^+\tau^-$, $e^+e^-\tau^+\tau^-$).
• Employed electron-beam polarization ($\mathcal{P}(e^-) = \pm 0.9$) to enhance signal sensitivity: left-handed for $\tilde{\chi}_2^0\tilde{\chi}_1^0$, right-handed for $\tilde{\tau}_1^+\tilde{\tau}_1^-$.
• Applied a multi-stage event selection: $N_{\text{jet}} = 2$, $\tau_h$ jet identification (1 or 3 tracks, $M_{\text{jet}} < 2\ \text{GeV}/c^2$, $E_{\text{jet}} > 3\ \text{GeV}$), opposite charge jets, $-q_\cdot\cos\theta < 0.75$, $\tilde{p}_T^\text{miss} > 5, 10, 20\ \text{GeV}/c$, accoplanarity $> 40^\circ$, and vetoing electromagnetic clusters and electrons.
• Used LCDRoot software package for simulation and analysis of events across three mSUGRA benchmark points with $m_{1/2} = 360\ \text{GeV}$, $m_0 = 210, 215, 225\ \text{GeV}$, and $\Delta M = 5.7, 10.6, 20.2\ \text{GeV}$.
• Evaluated background suppression by comparing results with and without an active mask covering down to $2^\circ$ and $1^\circ$ in the forward region.
• Calculated statistical significance using $>5\sigma$ as discovery threshold, comparing signal over background for each benchmark point and polarization setting.

Experimental results

Research questions

• RQ1 Can a 500-GeV linear collider achieve $>5\sigma$ discovery sensitivity for $\tilde{\chi}_2^0\tilde{\chi}_1^0$ and $\tilde{\tau}_1^+\tilde{\tau}_1^-$ in mSUGRA models with $\tan\beta = 40$?
• RQ2 How critical is the use of an active mask in suppressing $e^+e^- o e^+e^-\tau^+\tau^-$ events that mimic the $\tau_h + \text{missing energy}$ signature?
• RQ3 What is the impact of beam polarization on enhancing signal-to-background ratios for $\tilde{\chi}_2^0\tilde{\chi}_1^0$ and $\tilde{\tau}_1^+\tilde{\tau}_1^-$ production?
• RQ4 How do different $\tilde{\tau}_1$-$\tilde{\chi}_1^0$ mass splittings ($\Delta M = 5.7, 10.6, 20.2\ \text{GeV}$) affect detectability?
• RQ5 To what extent do misidentified quark jets from $e^+e^- o e^+e^-q\bar{q}$ processes contaminate the $\tau_h$ signal, and how can they be mitigated?

Key findings

• An active mask covering down to $2^\circ$ is essential to suppress $e^+e^- o e^+e^-\tau^+\tau^-$ backgrounds, enabling $>5\sigma$ discovery sensitivity for $\tilde{\chi}_2^0\tilde{\chi}_1^0$ and $\tilde{\tau}_1^+\tilde{\tau}_1^-$ signals.
• For $\tilde{\chi}_2^0\tilde{\chi}_1^0$ production with $\mathcal{P}(e^-) = +0.9$, the number of signal events exceeds 1,000 at $\tilde{p}_T^\text{miss} > 5\ \text{GeV}/c$ for benchmark A3, with $>5\sigma$ significance over SM backgrounds.
• For $\tilde{\tau}_1^+\tilde{\tau}_1^-$ with $\mathcal{P}(e^-) = -0.9$, $\tilde{\chi}_2^0\tilde{\chi}_1^0$ signal is discovered at $>5\sigma$ for all three mSUGRA points, with up to 2,510 events for A3 at $\tilde{p}_T^\text{miss} > 5\ \text{GeV}/c$.
• Without an active mask, the $\gamma\gamma$-initiated $e^+e^-\tau^+\tau^-$ background dominates, with over 11,000 events expected for $\tilde{p}_T^\text{miss} > 5\ \text{GeV}/c$, making discovery impossible.
• With a $1^\circ$ mask, the $\gamma\gamma$ background is reduced to 38 events, significantly improving signal visibility and enabling $>5\sigma$ discovery for $\tilde{\tau}_1^+\tilde{\tau}_1^-$ at $\tilde{p}_T^\text{miss} > 5\ \text{GeV}/c$.
• The SM weak process ($\nu\bar{\nu}\tau^+\tau^-$) contributes 2,320–2,486 events at $\tilde{p}_T^\text{miss} > 10\ \text{GeV}/c$, but remains subdominant compared to $\gamma\gamma$-initiated backgrounds without masking.