[Paper Review] Scalar Top Quark Studies with Various Visible Energies

A. Sopczak, Marcela Carena+4 more|ArXiv.org|May 22, 2006

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper investigates scalar top quark (stop) pair production at the International Linear Collider (ILC) using realistic detector simulations, focusing on low-mass-difference scenarios where visible energy is minimal. It demonstrates that the ILC can precisely measure stop properties down to 5 GeV mass differences, enabling critical tests of dark matter relic density and electroweak baryogenesis via co-annihilation, with precision comparable to WMAP observations.

ABSTRACT

The precision determination of scalar top quark properties will play an important role at a future International Linear Collider (ILC). Recent and ongoing studies are discussed for different experimental topologies in the detector. First results are presented for small mass differences between the scalar top and neutralino masses. This corresponds to a small expected visible energy in the detector. An ILC will be a unique accelerator to explore this scenario. In addition to finding the existence of light stop quarks, the precise measurement of their properties is crucial for testing their impact on the dark matter relic abundance and the mechanism of electroweak baryogenesis. Significant sensitivity for mass differences down to 5 GeV are obtained. The simulation is based on a fast and realistic detector simulation. A vertex detector concept of the Linear Collider Flavor Identification (LCFI)collaboration, which studies pixel detectors for heavy quark flavour identification, is implemented in the simulations for c-quark tagging. The study extends simulations for large mass differences (large visible energy) for which aspects of different detector simulations, the vertex detector design, and different methods for the determination of the scalar top mass are discussed. Based on the detailed simulations we study the uncertainties for the dark matter density predictions and their estimated uncertainties from various sources. In the region of parameters where stop-neutralino co-annihilation leads to a value of the relic density consistent with experimental results, as precisely determined by the Wilkinson Microwave Anisotropy Probe (WMAP), the stop-neutralino mass difference is small and the ILC will be able to explore this region efficiently.

Motivation & Objective

• To assess the ILC's capability to probe light scalar top quarks with small mass differences to the lightest neutralino, a key scenario for dark matter co-annihilation.
• To evaluate the performance of a CCD vertex detector (LCFI concept) in tagging c-quarks from stop decays, reducing background and improving signal sensitivity.
• To determine the scalar top quark mass and mixing angle with high precision using multiple methods, including polarization and threshold scans.
• To compute the predicted dark matter relic density from ILC measurements and compare it to WMAP constraints, assessing cosmological consistency.
• To quantify uncertainties in relic density predictions arising from experimental errors and correlations in stop, chargino, and neutralino parameters.

Proposed method

• Utilizes fast and realistic detector simulation (Simdet) incorporating the LCFI CCD vertex detector concept with five layers at radii 15–60 mm for c-quark tagging.
• Simulates stop pair production via $e^+e^- \to \tilde{t}_1\bar{\tilde{t}}_1$ and decay $\tilde{t}_1 \to c\tilde{\chi}^0_1$, focusing on low visible energy from small $m_{\tilde{t}_1} - m_{\tilde{\chi}^0_1}$ differences.
• Applies four mass determination methods: polarization, threshold scan, kinematic fitting, and cross-section measurement, with polarization yielding highest precision.
• Performs $\chi^2$-based error propagation to combine experimental uncertainties from stop, chargino, and neutralino sectors, estimating cosmological relic density $\Omega_{\rm CDM}h^2$.
• Compares simulated ILC precision to WMAP 1$\sigma$ measurements ($0.104 < \Omega_{\rm CDM}h^2 < 0.121$) across benchmark scenarios.
• Uses SPS-5 detector parameters and evaluates background dependence on inner layer radius, showing c-tagging reduces background by ~3× in $c\tilde{\chi}^0_1\bar{c}\tilde{\chi}^0_1$ channel.

Experimental results

Research questions

• RQ1 Can the ILC achieve sufficient sensitivity to stop quarks with mass differences as small as 5 GeV to the lightest neutralino?
• RQ2 How effective is the LCFI CCD vertex detector concept in tagging c-quarks from stop decays, and what impact does it have on background suppression?
• RQ3 Which method—polarization, threshold scan, or kinematic fitting—provides the most precise stop mass measurement in low-visible-energy scenarios?
• RQ4 To what extent can ILC measurements of stop and electroweak sector parameters predict the dark matter relic density with precision comparable to WMAP?
• RQ5 What are the dominant sources of uncertainty in the predicted $\Omega_{\rm CDM}h^2$ from ILC data, and how do they correlate across parameters?

Key findings

• The ILC achieves significant sensitivity to stop-neutralino mass differences down to 5 GeV, enabling exploration of the co-annihilation region critical for dark matter relic density.
• c-quark tagging using the LCFI CCD vertex detector reduces background in the $c\tilde{\chi}^0_1\bar{c}\tilde{\chi}^0_1$ channel by a factor of approximately 3.
• The polarization method provides the highest precision in stop mass determination, with the measured stop mass determined as $m_{\tilde{t}_1} = 122.5 \pm 1.0$ GeV in the benchmark scenario.
• The scalar top mixing angle is constrained to $\cos\theta_{\tilde{t}} < 0.074$, implying $\sin\theta_{\tilde{t}} > 0.9972$, consistent with a highly mixed state.
• ILC measurements constrain the dark matter relic density to $0.100 < \Omega_{\rm CDM}h^2 < 0.124$ at 1$\sigma$, matching the precision of WMAP's $0.104 < \Omega_{\rm CDM}h^2 < 0.121$.
• The uncertainty in the predicted $\Omega_{\rm CDM}h^2$ is dominated by the precision of the stop quark mass measurement, highlighting its central role in cosmological predictions.