[Paper Review] Measurement of the top Yukawa Coupling at a 1 TeV International Linear Collider using the SiD detector

P. Roloff, J. Strube|arXiv (Cornell University)|Jul 29, 2013

Particle physics theoretical and experimental studies19 references4 citations

TL;DR

This study presents a precision measurement of the top Yukawa coupling at the 1 TeV International Linear Collider using the SiD detector, employing full geant4-based simulation to analyze $e^+e^- \to t\bar{t}H$ events with Higgs decay to $b\bar{b}$. A combined statistical uncertainty of 4.5% is achieved for 1 ab$^{-1}$ luminosity using optimized event selection in six- and eight-jet final states with beam polarization, demonstrating the ILC's capability for high-precision top quark coupling measurements.

ABSTRACT

One of the detector benchmark processes investigated for the SiD Detailed Baseline Design (DBD) is given by: e+e- -> ttH, where H is the Standard Model Higgs boson of mass 125 GeV. The study is carried out at a centre-of-mass energy of 1 TeV and assuming an integrated luminosity of 1 ab-1. The physics aim is a direct measurement of the top Yukawa coupling at the ILC. Higgs boson decays to beauty quark-antiquark pairs are reconstructed. The investigated final states contain eight jets or six jets, one charged lepton and missing energy. Additionally, four of the jets in signal events are caused by beauty quark decays. The analysis is based on a full simulation of the SiD detector using GEANT4. Beam-related backgrounds from gammagamma -> hadrons interactions and incoherent e+e- pairs are considered. This study addresses various aspects of the detector performance: jet clustering in complex hadronic final states, flavour-tagging and the identification of high energy leptons.

Motivation & Objective

• To measure the top Yukawa coupling with high precision at the 1 TeV International Linear Collider (ILC), a key test of the Standard Model prediction that coupling scales with fermion mass.
• To evaluate the physics potential of the SiD detector for reconstructing complex final states involving top quarks, Higgs bosons, and bottom quarks.
• To assess the impact of beam-induced backgrounds—specifically incoherent $e^+e^-$ pairs and $\gamma\gamma \to$ hadrons—on event reconstruction and analysis performance.
• To optimize event selection strategies using jet clustering, lepton identification, and flavor tagging to distinguish signal from dominant backgrounds.

Proposed method

• Full geant4-based simulation of the SiD detector (sidloi3 model) is used to model particle interactions, tracking, and calorimetry for $e^+e^- \to t\bar{t}H$ processes at $\sqrt{s} = 1$ TeV.
• Monte Carlo samples are generated using WHIZARD for signal and PYTHIA 6 for parton showering and hadronization, with detector response simulated via SLC and SLC-Pandora reconstruction frameworks.
• Backgrounds from incoherent $e^+e^-$ pairs and $\gamma\gamma \to$ hadrons are overlayed at the hit level using a CLIC-developed procedure to model beam-related occupancy and pile-up.
• Multivariate analysis with Boosted Decision Trees (BDTs) is applied to discriminate signal from background, trained separately on six-jet and eight-jet final states with distinct lepton and jet multiplicity preselection.
• Flavor tagging is performed using the LCFIPlus package to identify jets from bottom quarks, leveraging the high $p_T$ and long decay length of $b$-hadrons.
• Beam polarization is used in two configurations: $P(e^-) = -80\%, P(e^+) = +20\%$ and vice versa, to enhance signal sensitivity and reduce background contamination.

Experimental results

Research questions

• RQ1 Can the top Yukawa coupling be measured with sub-5% statistical uncertainty at the 1 TeV ILC using the SiD detector?
• RQ2 How do beam-induced backgrounds from $\gamma\gamma \to$ hadrons and incoherent $e^+e^-$ pairs affect the reconstruction of $t\bar{t}H$ events with $H \to b\bar{b}$?
• RQ3 What is the optimal combination of jet multiplicity, lepton identification, and flavor tagging for maximizing signal significance in $t\bar{t}H$ events?
• RQ4 To what extent does beam polarization improve the sensitivity to the top Yukawa coupling in the $t\bar{t}H$ process?
• RQ5 How do detector-level effects such as jet clustering and lepton isolation impact the final measurement precision?

Key findings

• A statistical uncertainty of 4.5% on the top Yukawa coupling is achieved for an integrated luminosity of 1 ab⁻¹, split equally between two beam polarization configurations.
• The signal yield is 191.6 events for the six-jet final state ($H \to b\bar{b}$) and 299.4 events for the eight-jet final state, with significant background suppression via multivariate analysis.
• Beam-induced backgrounds do not significantly degrade tracking performance, but hadronic activity from $\gamma\gamma \to$ hadrons impacts calorimeter energy reconstruction.
• Flavor tagging efficiency is critical, as four of the jets in signal events originate from $b$-quark decays, and their identification improves signal-to-background discrimination.
• The analysis achieves high signal purity: only 1.6 signal events remain in the eight-jet final state after selection, indicating effective background rejection.
• The study confirms the feasibility of direct top Yukawa coupling measurement at the ILC with precision competitive with LHC results, leveraging clean $e^+e^-$ initial states and full detector simulation.