Top Quark Mass Measurement Using the Template Method in the Lepton + Jets Channel at CDF II

TL;DR

This paper reports a high-precision measurement of the top quark mass in tt̄ events in the lepton+jets channel using the CDF II detector, leveraging a template method with an in situ jet energy scale calibration anchored to the W→jj resonance. By partitioning the data into jet-tagging categories and performing a simultaneous fit to reconstructed mt and mjj templates, the analysis achieves M_top = 173.5^{+3.9}_{-3.8} GeV/c^2 (stat.+JES) with JES constrained to a precise value; a cross-check with a traditional M_top-only fit yields consistent results. The work demonstrates a robust methodology for reducing JES-related systematics and provides a framework for future Run II analyses with larger data sets. The findings contribute to precise electroweak tests and Higgs sector constraints within the standard model.

Abstract

This article presents a measurement of the top quark mass using the CDF II detector at Fermilab. Colliding beams of protons and anti-protons at Fermilab's Tevatron (sqrt{s}=1.96 TeV) produce top/anti-top pairs, which decay to W^+W^-bbbar; events are selected where one W decays to hadrons, and one W decays to either e or mu plus a neutrino. The data sample corresponds to an integrated luminosity of approximately 318 pb^-1. A total of 165 ttbar events are separated into four subsamples based on jet transverse energy thresholds and the number of b jets identified by reconstructing a displaced vertex. In each event, the reconstructed top quark invariant mass is determined by minimizing a chi-squared for the overconstrained kinematic system. At the same time, the mass of the hadronically decaying W boson is measured in the same event sample. The observed W boson mass provides an in situ improvement in the determination of the hadronic jet energy scale, JES. A simultaneous likelihood fit of the reconstructed top quark masses and the W boson invariant masses in the data sample to distributions from simulated signal and background events gives a top quark mass of 173.5 +3.7/-3.6 (stat.+JES) +/- 1.3 (other syst.) GeV/c^2, or 173.5 +3.9/-3.8 GeV/c^2.

Figures (32)

• Figure 1: An elevation view of the CDF Run II detector. From the collision region outwards, CDF consists of a silicon strip detector, a tracking drift chamber, an electromagnetic calorimeter, a hadronic calorimeter, and muon chambers.
• Figure 2: The efficiency of the secondary vertex $b$-tagging algorithm is shown as a function of jet $E_{T}\xspace$ for $b$ jets in the central region of the detector ($|\eta|<1$), where the tracking efficiency is high. The shaded band gives the $\pm1~\sigma\xspace$ range for $b$-tagging efficiency. The curve is measured using a combination of data and Monte Carlo simulated samples.
• Figure 3: The $t\bar{t}$-specific corrections are shown for $W$ jets (left) and $b$ jets (right) as a function of jet $p_{T}$ for several values of $|\eta|$. On the top is the correction factor, and on the bottom is the fractional resolution passed to the fitter. The histograms give the distributions of jet $p_{T}$ (arbitrarily normalized) from a signal Monte Carlo sample with generated top quark mass of $178~\mathrm{GeV}/c^{2}$.
• Figure 4: Results of the dijet balancing procedure are shown for data and simulated dijet events with $p_{T}^{\text{jet}}>20~\mathrm{GeV}$. Probe jets from throughout the detector are compared with a reference jet in the central region; the ratio of the $p_{T}$ of the jets is plotted as a function of the probe jet $\eta$. The simulation models well the detector response as a function of $\eta$.
• Figure 5: The systematic uncertainties on jet energy are shown for jets in the central calorimeter ($0.2<|\eta|<0.6$). For non-central jets, the total uncertainty has a different contribution from the eta-dependent uncertainty. In this plot the corrected jet transverse momentum $p_{T}\xspace^{\text{corr}}$ is the process-independent estimate of the parton $p_{T}\xspace$. At low $p_{T}\xspace^{\text{corr}}$, the main contribution to the systematic is from the uncertainty on the fraction of jet energy lost outside the cone, while at high $p_{T}\xspace^{\text{corr}}$ it is from the linearity corrections to obtain an absolute jet energy scale.