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Title: A measurement of the top quark mass with a matrix element method

Abstract

The authors present a measurement of the mass of the top quark. The event sample is selected from proton-antiproton collisions, at 1.96 TeV center-of-mass energy, observed with the CDF detector at Fermilab's Tevatron. They consider a 318 pb -1 dataset collected between March 2002 and August 2004. They select events that contain one energetic lepton, large missing transverse energy, exactly four energetic jets, and at least one displaced vertex b tag. The analysis uses leading-order t$$\bar{t}$$ and background matrix elements along with parameterized parton showering to construct event-by-event likelihoods as a function of top quark mass. From the 63 events observed with the 318 pb -1 dataset they extract a top quark mass of 172.0 ± 2.6(stat) ± 3.3(syst) GeV/c 2 from the joint likelihood. The mean expected statistical uncertainty is 3.2 GeV/c 2 for m $$\bar{t}$$ = 178 GTeV/c 2 and 3.1 GeV/c 2 for m $$\bar{t}$$ = 172.5 GeV/c 2. The systematic error is dominated by the uncertainty of the jet energy scale.

Authors:
 [1]
  1. Univ. of California, Berkeley, CA (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
897576
Report Number(s):
FERMILAB-THESIS-2006-31
TRN: US0701530
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; FERMILAB COLLIDER DETECTOR; FERMILAB TEVATRON; MATRIX ELEMENTS; T QUARKS; TRANSVERSE ENERGY; Experiment-HEP

Citation Formats

Gibson, Adam Paul. A measurement of the top quark mass with a matrix element method. United States: N. p., 2006. Web. doi:10.2172/897576.
Gibson, Adam Paul. A measurement of the top quark mass with a matrix element method. United States. doi:10.2172/897576.
Gibson, Adam Paul. Sun . "A measurement of the top quark mass with a matrix element method". United States. doi:10.2172/897576. https://www.osti.gov/servlets/purl/897576.
@article{osti_897576,
title = {A measurement of the top quark mass with a matrix element method},
author = {Gibson, Adam Paul},
abstractNote = {The authors present a measurement of the mass of the top quark. The event sample is selected from proton-antiproton collisions, at 1.96 TeV center-of-mass energy, observed with the CDF detector at Fermilab's Tevatron. They consider a 318 pb-1 dataset collected between March 2002 and August 2004. They select events that contain one energetic lepton, large missing transverse energy, exactly four energetic jets, and at least one displaced vertex b tag. The analysis uses leading-order t$\bar{t}$ and background matrix elements along with parameterized parton showering to construct event-by-event likelihoods as a function of top quark mass. From the 63 events observed with the 318 pb-1 dataset they extract a top quark mass of 172.0 ± 2.6(stat) ± 3.3(syst) GeV/c2 from the joint likelihood. The mean expected statistical uncertainty is 3.2 GeV/c2 for m $\bar{t}$ = 178 GTeV/c2 and 3.1 GeV/c2 for m $\bar{t}$ = 172.5 GeV/c2. The systematic error is dominated by the uncertainty of the jet energy scale.},
doi = {10.2172/897576},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}

Thesis/Dissertation:
Other availability
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  • The mass of the top quark is a fundamental parameter of the Standard Model. Its precise knowledge yields valuable insights into unresolved phenomena in and beyond the Standard Model. A measurement of the top quark mass with the matrix element method in the lepton+jets final state in D0 Run II is presented. Events are selected requiring an isolated energetic charged lepton (electron or muon), significant missing transverse energy, and exactly four calorimeter jets. For each event, the probabilities to originate from the signal and background processes are calculated based on the measured kinematics, the object resolutions and the respective matrix elements. The jet energy scale is known to be the dominant source of systematic uncertainty. The reference scale for the mass measurement is derived from Monte Carlo events. The matrix element likelihood is defined as a function of both, m{sub top} and jet energy scale JES, where the latter represents a scale factor with respect to the reference scale. The top mass is obtained from a two-dimensional correlated fit, and the likelihood yields both the statistical and jet energy scale uncertainty. Using a dataset of 320 pb -1 of D0 Run II data, the mass of the top quark is measured to be: mmore » $$ℓ+jets\atop{top}$$ = 169.5 ± 4.4(stat. + JES)$$+1.7\atop{-1.6}$$(syst.) GeV; m$$e+jets\atop{top}$$ = 168.8 ± 6.0(stat. + JES)$$+1.9\atop{-1.9}$$(syst.) GeV; m$$μ+jets\atop{top}$$ = 172.3 ± 9.6(stat.+JES)$$+3.4\atop{-3.3}$$(syst.) GeV. The jet energy scale measurement in the ℓ+jets sample yields JES = 1.034 ± 0.034, suggesting good consistency of the data with the simulation. The measurement forecasts significant improvements to the total top mass uncertainty during Run II before the startup of the LHC, as the data sample will grow by a factor of ten and D0's tracking capabilities will be employed in jet energy reconstruction and flavor identification.« less
  • A measurement of the top quark mass in tmore » $$\bar{t}$$ → l + jets candidate events, obtained from p$$\bar{p}$$ collisions at √s = 1.96 TeV at the Fermilab Tevatron using the CDF II detector, is presented. The measurement approach is that of a matrix element method. For each candidate event, a two dimensional likelihood is calculated in the top pole mass and a constant scale factor, 'JES', where JES multiplies the input particle jet momenta and is designed to account for the systematic uncertainty of the jet momentum reconstruction. As with all matrix element techniques, the method involves an integration using the Standard Model matrix element for t$$\bar{t}$$ production and decay. However, the technique presented is unique in that the matrix element is modified to compensate for kinematic assumptions which are made to reduce computation time. Background events are dealt with through use of an event observable which distinguishes signal from background, as well as through a cut on the value of an event's maximum likelihood. Results are based on a 955 pb -1 data sample, using events with a high-p T lepton and exactly four high-energy jets, at least one of which is tagged as coming from a b quark; 149 events pass all the selection requirements. They find M meas = 169.8 ± 2.3(stat.) ± 1.4(syst.) GeV/c 2.« less
  • A measurement of the top quark mass in tmore » $$\bar{t}$$ → l + jets candidate events, obtained from p$$\bar{p}$$ collisions at √s = 1.96 TeV at the Fermilab Tevatron using the CDF II detector, is presented. The measurement approach is that of a matrix element method. For each candidate event, a two dimensional likelihood is calculated in the top pole mass and a constant scale factor, 'JES', where JES multiplies the input particle jet momenta and is designed to account for the systematic uncertainty of the jet momentum reconstruction. As with all matrix elements techniques, the method involves an integration using the Standard Model matrix element for tt production and decay. however, the technique presented is unique in that the matrix element is modified to compensate for kinematic assumptions which are made to reduce computation time. Background events are dealt with through use of an event observable which distinguishes signal from background, as well as through a cut on the value of an event's maximum likelihood. Results are based on a 955 pb -1 data sample, using events with a high-p T lepton and exactly four high-energy jets, at least one of which is tagged as coming from a b quark; 149 events pass all the selection requirements. They find M meas = 169.8 ± 2.3(stat.) ± 1.4(syst.) GeV/c 2.« less
  • The top quark plays a special role in the Standard Model of Particle Physics. With its enormous mass of about 170 GeV it is as heavy as a gold atom and is the only quark with a mass near the electroweak scale. Together with theW boson mass, the top quark mass allows indirect constraints on the mass of the hypothetical Higgs boson, which might hold the clue to the origin of mass. Top pair production with a semileptonic decay tmore » $$\bar{t}$$ →W ±W b$$\bar{b}$$ →q $$\bar{t}$$lnb$$\bar{b}$$ is the ”golden channel” for mass measurements, due to a large branching fraction and a relatively low background contamination compared to other decay channels. Top mass measurements based on this decay, performed with the matrix element method, have always been among the single best measurements in the world. In 2007, the top mass world average broke the 1% level of precision. Its measurement is no longer dominated by statistical but instead by systematic uncertainties. The reduction of systematic uncertainties has therefore become a key issue for further progress. This thesis introduces two new developments in the treatment of b jets. The first improvement is an optimization in the way b identification information is used. It leads to an enhanced separation between signal and background processes and reduces the statistical uncertainty by about 16%. The second improvement determines differences in the detector response and thus the energy scales of light jets and b jets. Thereby, it addresses the major source of systematic uncertainty in the latest top mass measurements. The method was validated on Monte Carlo events at the generator level, calibrated with fully simulated events, including detector simulation, and applied to D0 Run II data corresponding to 1 fb -1 of integrated luminosity. Possible sources of systematic uncertainties were studied. The top mass is measured to be: m t = (169.2±3.5(stat.)±1.0(syst.)) GeV . The simultaneous measurement of a scaling factor for the jet energy scale of light jets and a separate scaling factor for b jets yields 1.038±0.023 and 1.056±0.045, respectively. This result indicates that the nominal D0 jet energy scale derived from γ+jets events underestimates the energy of light jets and b jets in t $$\bar{t}$$ decays. The improved analysis was successful in reducing the major systematic uncertainty caused by the b jet energy scale from about 800 MeV to approximately 150 MeV.« less
  • This thesis presents a measurement of the top quark mass obtained from pmore » $$\bar{p}$$ collisions at √s = 1.96 TeV at the Fermilab Tevatron using the CDF II detector. The measurement uses a matrix element integration method to calculate a t$$\bar{t}$$ likelihood, employing a Quasi-Monte Carlo integration, which enables us to take into account effects due to finite detector angular resolution and quark mass effects. We calculate a t$$\bar{t}$$ likelihood as a 2-D function of the top pole mass m t and Δ JES, where Δ JES parameterizes the uncertainty in our knowledge of the jet energy scale; it is a shift applied to all jet energies in units of the jet-dependent systematic error. By introducing Δ JES into the likelihood, we can use the information contained in W boson decays to constrain Δ JES and reduce error due to this uncertainty. We use a neural network discriminant to identify events likely to be background, and apply a cut on the peak value of individual event likelihoods to reduce the effect of badly reconstructed events. This measurement uses a total of 4.3 fb -1 of integrated luminosity, requiring events with a lepton, large E T, and exactly four high-energy jets in the pseudorapidity range |η| < 2.0, of which at least one must be tagged as coming from a b quark. In total, we observe 738 events before and 630 events after applying the likelihood cut, and measure m t = 172.6 ± 0.9 (stat.) ± 0.7 (JES) ± 1.1 (syst.) GeV/c 2, or m t = 172.6 ± 1.6 (tot.) GeV/c 2.« less