Measurement of the top quark mass in the lepton+jets final state with the matrix element method

TL;DR

This study measures the top quark mass in the lepton+jets channel using the Matrix Element method with an in-situ jet energy scale determination, based on 370 pb-1 of D0 Run II data. By combining event-by-event probabilities for tt̄ production and W+jets background and incorporating detector transfer functions, the analysis delivers a precise m_top value while controlling the dominant JES systematic. The topological and b-tagging analyses yield consistent results, with m_top around 169–170 GeV and JES close to the reference scale, underscoring the robustness of the in-situ calibration. The work demonstrates a powerful approach to reducing JES-related systematics in top mass measurements, contributing to precision tests of the Standard Model.

Abstract

We present a measurement of the top quark mass with the Matrix Element method in the lepton+jets final state. As the energy scale for calorimeter jets represents the dominant source of systematic uncertainty, the Matrix Element likelihood is extended by an additional parameter, which is defined as a global multiplicative factor applied to the standard energy scale. The top quark mass is obtained from a fit that yields the combined statistical and systematic jet energy scale uncertainty. Using a data set of 370 pb-1 taken with the D0 experiment at Run II of the Fermilab Tevatron Collider, the mass of the top quark is measured using topological information to be: mtop(topo) = 169.2 +5.0-7.4 (stat.+JES) +1.5-1.4 (syst.) GeV, and when information about identified $b$ jets is included: mtop(b-tag) = 170.3 +4.1-4.5 (stat.+JES) +1.2-1.8 (syst.) GeV. The measurements yield a jet energy scale consistent with the reference scale.

Figures (18)

• Figure 1: Distribution of the topological likelihood for the $370\,\rm pb^{-1}\xspace$ D0 Run II data sample. The distribution for $e$+ jets events is shown in plot (a) and for $\mu$+ jets events in plot (b). The points with error bars indicate the data, and the fitted fractions of $t\bar{t}$ events (open area), $W$+ jets events (diagonally hatched), and QCD multijet events (horizontally hatched area) are superimposed.
• Figure 2: Monte Carlo study of the effect of charm-jet tagging on the signal to background probability ratio in the $b$-tagging analysis, for $t\bar{t}$ events generated with $m_{\rm top}\xspace=175\,\mathrm{Ge V}\xspace$ that contain two $b$-tagged jets. The $P_{\rm sig}$ values are calculated for the assumption $m_{\rm top}\xspace=175\,\mathrm{Ge V}\xspace$. (a) Only the two jet-parton assignments in which tagged jets are assigned to $b$ quarks are considered. (b) All weighted jet parton-assignments enter the probability calculation. In both plots, the hatched histogram corresponds to those cases where the two $b$-tagged jets are correctly assigned to $b$ quarks, which happens $84\%$ of the time in the double tag sample.
• Figure 3: Jet transfer functions for light quark jets, $0.0<\left|\eta\right|<0.5$, for parton energies $E_{p}=30\,{\rm GeV}$ (solid), $60\,{\rm GeV}$ (dashed), and $90\,{\rm GeV}$ (dash-dotted curve). The parametrization corresponds to the reference jet energy scale, $JES=1.0$.
• Figure 4: Observed $t\bar{t}$ cross section computed with the leading-order matrix element for (a) $e$+ jets and (b) $\mu$+ jets events as a function of the top quark mass $m_{\rm top}$ for different choices of the $JES$ scale factor: $JES=1.12$ (dash-dotted), $JES=1.0$ (solid), and $JES=0.88$ (dotted lines).
• Figure 5: Distributions of $\log_{10}(P_{\rm sig}\xspace/P_{\rm bkg}\xspace)$ for $t\bar{t}$ events with $m_{\rm top}\xspace=175\,\mathrm{Ge V}\xspace$ (solid) and $W$+ jets events (dashed lines) for (a) $e$+ jets events and (b) $\mu$+ jets events. The $P_{\rm sig}$ values are calculated for the assumption $m_{\rm top}\xspace=175\,\mathrm{Ge V}\xspace$. The distributions for signal and background events are normalized individually. The distributions for those $t\bar{t}$ events that fail the requirement of jets matched to partons are shown separately (dash-dotted lines).