[Paper Review] "Pair production of scalar top quarks in e+e- collisions at ILC."

A. Bartl, W. Majerotto+3 more|arXiv (Cornell University)|Apr 14, 2008

Particle physics theoretical and experimental studies6 references3 citations

TL;DR

This paper investigates top squark pair production in $e^+e^-$ collisions at the International Linear Collider (ILC), focusing on the decay chain $\tilde{t}_1 \to b\tilde{\chi}_1^\pm \to b\tilde{\chi}_1^0 W^\pm$, with $W \to \mu\nu$. Using PYTHIA6.4 simulations and optimized kinematic cuts, the authors achieve a high signal-to-background ratio (1806 signal vs. 12 background events at $\sqrt{s} = 500$ GeV, $L = 1000~\text{fb}^{-1}$), enabling precise top squark mass reconstruction with sub-GeV accuracy when the lightest neutralino mass is known.

ABSTRACT

We study the pair production of scalar top quarks in e+e- collisions with the subsequent decay of the top squarks into b-quarks and charginos. We simulate this process using PYTHIA6.4 for beam energies 2E_beam = 350, 400, 500, 800, 1000 GeV. Proposing a set of criteria we obtain a good separation of the signal stop events from top quark pair production which is the main background. The number of stop production events obtained with the proposed cuts for different energies is calculated for an integrated luminosity of 1000 1/fb. We propose a method to reconstruct the mass of the top squark, provided the mass of the lightest neutralino is known, and estimate the error of the mass determination for the case sqrt{s} = 500 GeV.

Motivation & Objective

• To investigate the feasibility of detecting scalar top quark pair production in $e^+e^-$ collisions at the ILC.
• To distinguish top squark signal events from the dominant top quark pair production background using kinematic and invariant mass variables.
• To develop a method for reconstructing the scalar top quark mass with high precision, assuming knowledge of the lightest neutralino mass.
• To evaluate signal and background event rates across multiple center-of-mass energies ($\sqrt{s} = 350$ to $1000$ GeV) for an integrated luminosity of $1000~\text{fb}^{-1}$.
• To assess the robustness of the analysis for higher stop masses, such as $M_{\tilde{t}_1} = 200$ GeV, and its implications for mass measurement accuracy.

Proposed method

• Monte Carlo event generation using PYTHIA6.4 for signal ($e^+e^- \to \tilde{t}_1\tilde{\bar{t}}_1$) and background ($e^+e^- \to t\bar{t}$) processes at $\sqrt{s} = 350, 400, 500, 800, 1000$ GeV.
• Incorporation of beamstrahlung effects via CIRCE1 to model realistic beam energy spectra in $e^+e^-$ collisions.
• Application of three optimized kinematic cuts (14)–(16) to suppress top quark pair background while preserving signal events.
• Reconstruction of the scalar top quark mass using the invariant mass distribution of the $b$-jet and the two non-$b$ jets from $W$ decay ($M_{\text{inv}}(b, \text{JETS}_{W^*})$).
• Use of global variables such as missing energy, total visible energy, scalar sum of transverse momenta, and invariant masses of jet systems to enhance signal-background separation.
• Assumption of known lightest neutralino mass ($\tilde{\chi}_1^0$) to enable precise top squark mass determination from the peak position of the $M_{\text{inv}}(b, \text{JETS}_{W^*})$ distribution.

Experimental results

Research questions

• RQ1 Can the signal of scalar top quark pair production in $e^+e^-$ collisions be effectively separated from the dominant top quark pair production background at the ILC?
• RQ2 What kinematic cuts maximize the signal-to-background ratio for stop pair production at $\sqrt{s} = 500$ GeV with $1000~\text{fb}^{-1}$ luminosity?
• RQ3 To what extent can the scalar top quark mass be reconstructed from the invariant mass distribution of the $b$-jet and $W$-decay jets, given knowledge of the lightest neutralino mass?
• RQ4 How does the signal efficiency and background suppression vary with increasing stop quark mass, e.g., from 167.9 GeV to 200 GeV?
• RQ5 What is the expected number of observable signal and background events across different ILC center-of-mass energies?

Key findings

• At $\sqrt{s} = 500$ GeV and $L = 1000~\text{fb}^{-1}$, the proposed cuts yield 1806 signal stop events and only 12 background top events, resulting in a signal-to-background ratio of approximately 150:1.
• The invariant mass distribution $M_{\text{inv}}(b, \text{JETS}_{W^*})$ for $M_{\tilde{t}_1} = 167.9$ GeV exhibits a clear peak near 110 GeV, enabling precise mass reconstruction with high statistical significance.
• For a stop mass of 200 GeV, the number of signal events drops to 509 at $\sqrt{s} = 500$ GeV, but the peak in the $M_{\text{inv}}(b, \text{JETS}_{W^*})$ distribution remains resolvable, allowing for mass determination with reduced accuracy.
• The method of mass reconstruction based on $M_{\text{inv}}(b, \text{JETS}_{W^*})$ achieves sub-GeV precision in mass determination when the lightest neutralino mass is known, as demonstrated for $M_{\tilde{t}_1} = 167.9$ GeV.
• The signal-to-background ratio remains favorable across the energy range $400 \leq \sqrt{s} \leq 800$ GeV, suggesting that these energy regions are optimal for high-precision top squark mass measurements.
• The $e^+e^-$ channel, combined with the previously studied $\gamma\gamma$ channel, is shown to be highly suitable for detailed study of stop pair production at the ILC.