[Paper Review] Top Physics at the LHC

C. Weiser|ArXiv.org|Jun 10, 2005

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper outlines the top quark physics programme at the LHC, focusing on precision measurements of the top quark mass, production properties, and couplings using $t\overline{t}$ and single top events. It demonstrates that with 10 fb$^{-1}$ of data, a top quark mass precision of ~1–2 GeV is achievable, and that $t\overline{t}$ events enable calibration of jet energy scales and b-tagging, while also offering a pathway to discover a light Higgs boson via associated $ttH$ production.

ABSTRACT

Top quark physics will be a prominent topic in Standard Model physics at the LHC. The enormous amount of top quarks expected to be produced will allow to perform a wide range of precision measurements. An overview of the planned top physics programme of the ATLAS and CMS experiments at the LHC is given.

Motivation & Objective

• To establish a comprehensive top quark physics programme at the LHC using ATLAS and CMS experiments.
• To achieve a top quark mass measurement with a precision of ~1–2 GeV using $t\overline{t}$ events.
• To determine the CKM matrix element $|V_{tb}|$ with high precision through single top production.
• To use $t\overline{t}$ events for detector calibration of jet energy scales and b-tagging efficiencies.
• To explore the potential of $ttH$ production for discovering a light Higgs boson and measuring the top-Higgs Yukawa coupling.

Proposed method

• Use semi-leptonic $t\overline{t}$ decays ($t\overline{t} \to b\overline{b}l\nu q\overline{q}$) with isolated leptons, missing transverse energy, and at least four jets for top mass reconstruction.
• Apply constraints from the W boson mass to reduce sensitivity to light jet energy scale uncertainties in top mass measurements.
• Utilize b-jet tagging to identify top quark decay products and suppress combinatorial backgrounds in reconstruction.
• Measure the top quark charge by analyzing photon transverse momentum spectra in $t\overline{t}\gamma$ events or using jet charge estimators in semi-leptonic decays.
• Study W boson helicity fractions in top quark decays via the angle $\Theta^{*}_{l}$ between the lepton and W boson in the top quark rest frame.
• Use fully reconstructed $t\overline{t}$ systems to identify b-jets from Higgs boson decay in $ttH$ events for Higgs coupling measurements.

Experimental results

Research questions

• RQ1 Can the top quark mass be measured with a precision of ~1–2 GeV at the LHC using $t\overline{t}$ events?
• RQ2 Can the top quark charge be unambiguously determined using $t\overline{t}\gamma$ events or jet charge estimators?
• RQ3 What is the sensitivity of $t\overline{t}$ events to new physics via resonant $t\overline{t}$ mass states?
• RQ4 Can $ttH$ production be used to discover a light Higgs boson and measure the top-Higgs Yukawa coupling?
• RQ5 How can $t\overline{t}$ events be used to calibrate jet energy scales and b-tagging efficiencies in situ?

Key findings

• After 10 fb$^{-1}$ of integrated luminosity, the top quark mass can be measured with a precision of ~1–2 GeV, dominated by systematic uncertainties such as b-jet energy scale.
• The semi-leptonic channel provides the most promising path for top mass measurement due to a favorable branching ratio and efficient triggering.
• The di-leptonic and fully hadronic channels can achieve top mass precisions of ~2–3 GeV, though the latter is experimentally challenging due to high QCD backgrounds.
• The $J/\Psi$-tagged semi-leptonic channel offers a method nearly insensitive to jet energy scale, with a statistical uncertainty of ~1 GeV expected after ~100 fb$^{-1}$.
• The longitudinal W boson helicity fraction $h_W(0)$ can be measured with a combined statistical and systematic uncertainty of ~0.045 after 10 fb$^{-1}$.
• Single top production via W-gluon fusion allows a determination of $|V_{tb}|$ with a statistical uncertainty of ~0.4% after 30 fb$^{-1}$, limited by theoretical and luminosity uncertainties.