Measuring the Top Quark Yukawa Coupling at a Linear e^+e^- Collider

TL;DR

This paper evaluates the potential precision of measuring the top quark Yukawa coupling at a future linear electron-positron collider by analyzing $t\bar{t}H$ production and decay channels, considering backgrounds and detector efficiencies.

Contribution

It provides a detailed simulation-based estimate of the measurement accuracy of the top Yukawa coupling at a linear collider, including effects of b-tagging efficiency and background processes.

Findings

Top Yukawa coupling can be measured to 6-8% accuracy with ideal b-tagging.

Measurement accuracy drops to 17-22% with realistic b-tagging efficiency.

Full reconstruction of top and W masses aids in identifying Higgs production evidence.

Abstract

The cross section for the reaction $e^+e^- \to t\bar{t} H$ depends sensitively on the top quark Yukwawa coupling $\lambda_t$. We calculate the rate for $t\bar{t}H$ production, followed by the decay $H\to b\bar{b}$, for a Standard Model Higgs boson with 100 < m_H <130 GeV. We interface with ISAJET to generate QCD radiation, hadronization and particle decays. We also calculate the dominant $t\bar{t}b\bar{b}$ backgrounds from electroweak and QCD processes. We consider both semileptonic and fully hadronic decays of the $t\bar{t}$ system. In our analysis, we attempt full reconstruction of the top quark and W boson masses in the generated events. The invariant mass of the remaining b-jets should show evidence of Higgs boson production. We estimate the accuracy with which $\lambda_t$ can be measured at a linear e^+e^- collider. Our results, including statistical but not systematic errors, show that the top quark Yukawa coupling can be measured to 6-8 % accuracy with 1000 fb^{-1} at $E_{CM}=1 TeV$, assuming 100 % efficiency for b-jet tagging. The accuracy of the measurement drops to 17-22 % if only a 60 % efficiency for b-tagging is achieved.