Updated Measurement of the Top Quark Mass in the Lepton+Jets Channel using the Decay Length Technique

Christopher S. Hill, Joseph R. Incandela

(University of California, Santa Barbara)


Public Result:


Abstract:

We present a novel method to measure the top quark mass using the transverse decay length of b-hadrons from top decays. This technique relies solely on tracking and thus avoids the jet energy scale uncertainty that is common to all other methods. We apply our new method to the lepton+jets sample corresponding to 695 pb-1 and extract a measurement of mt = 180.7+15.5-13.4 (stat.) ± 8.6 (syst.) GeV/c2. While this is not a competitive measurement of the top quark mass by itself, since the decay length technique is uncorrelated with other methods, this result will help to reduce the overall uncertainty on the top mass in combination with other CDF results.


Introduction:

In the absence of a direct observation of a Higgs boson, one of the most important measurements that can be made with Run 2 data is a precise determination of the top quark mass. This is because the top quark mass is the dominant uncertainty in global Standard Model fits to the Higgs mass. A precision measurement of the top quark mass constrains the allowed Higgs mass values within the SM. It will tell us where to look before the Higgs is discovered and test whether it is SM or not after a signal has been established. Recently, much progress has been made in reducing the uncertainty in measurements of the top quarks mass. Unfortunately, all currently employed techniques are limited by the same systematic uncertainty, the calorimeter jet energy scale. We have recently developed and studied a novel method to measure the top quark mass using the transverse decay length of b-hadrons from top decays - see PRD 71:054029 (2005). The method exploits the fact that top quarks at the Tevatron are produced nearly at rest. In the rest frame of the top quark, the boost given to the bottom quark as a consequence of the top's decay can be written simply:

The top quark's mass therefore (to the extent that the threshold approximation holds) is strongly correlated with the boost given to the b-quark and the subsequent b-hadron after fragmentation. Thus, the average lifetime of the b-hadrons resultant from top decays can be used to statistically infer the mass of the top quark. In this analysis, rather than measuring the average lifetime, we simply measure the experimentally more accessible transverse decay length of the b-hadrons. This correlation is illustrated in the following Figure, which plots the generator level transverse decay length of b-hadrons from top quarks with varied mass. We use the sample mean as an estimator of the slope of the exponential distribution, from which we deduce the top quark mass.

This technique relies on tracking to precisely determine the decay length. It does not use any calorimeter information and thus avoids any jet energy scale uncertainty.


Brief Description of the Analysis:

This analysis is performed on 695 pb-1 data collected by CDF during Run 2A of the Tevatron. Lepton+Jets top quark candidate events are selected by requiring an isolated high ET lepton (e or mu) accompanied by significant missing energy and three or more high ET jets. One or more of these jets must be tagged by the reconstruction of a displaced secondary vertex. The 2D distance bewteen the primary vertex and the secondary vertex is the transverse decay length, which we denote L2D:

This quantity is histogrammed for all tags in selected events and the mean of this distribution is computed. The measured mean L2D is converted to a most-probable top mass (and corresponding 1 sigma confidence interval) using correlation functions derived from Monte Carlo simiulations of signal (tt events with varied top mass) and background. Details of the analysis are described in the conference note linked below.


Paper submitted to PRD (Rapid Communication):

  • hep-ex/0612061 - Measurement of the Top Quark Mass in ppbar Collisions at sqrt(s) = 1.96 TeV using the Decay Length Technique


    Public Tables and Plots:

    Transverse Decay Length Simulaton vs. Data
    L2D for jets with positive secondary vertex tags obtained from CDF data and CDF Monte Carlo simulation are overlaid. The CDF data sample is b-enriched dijet data recorded on an 8 GeV lepton trigger. The simulation is Herwig dijet MC. Events are required to have two jets in opposite hemispheres, both of which are secondary vertex tagged and one of which contains an identified lepton. This selection prodresults in samples which are nearly 100% bb.
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    Tranverse Decay Length - Control Sample
    Points are the Lxy distribution of positive tags in selected W plus 1 or 2 jet data, for which little top contribution is expected. Expected contributions from background Monte Carlo are overlaid in the solid stacked histogram. N.B. to facilitate the shape comparison, the MC is normalized to the observed data. A Kolmogorov-Smirnov test is performed on these distributions, with a resultant probability of 30.6%.
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    Tranverse Decay Length - Signal Sample
    Points are the L2D distribution of positive tags in selected W plus 3 or more jet data, from which the mean transverse deacy length used to measure the top mass is extracted. Expected contributions from signal background Monte Carlo are overlaid in the solid stacked histogram. N.B. to facilitate the shape comparison, the MC is normalized to the observed data. A Kolmogorov-Smirnov test is performed on these distributions, with a resultant probability of 16.7%.
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    Top Mass Result
    Most probable top mass and 1 sigma Neyman confidence intervals as a function of mean transverse decay length. The mean decay length measured in 695 pb-1 of CDF data is overlaid as the dashed line, from which the measured top mass and its uncertainty can be read off. N.B. the uncertainty represented in the plot is statistical only.
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    Systematic Uncertainties
    Summary of sources of systematic error and their estimated uncertainties. Note the small systematic error arising from the jet-energy scale.
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    Backgrounds
    Background sources and their estimated contributions to the tagged lepton+jets sample used in this analysis with 3 or more jets.
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    Note: Please click on the thumbnail plots to get higher resolution .gif images.


    Contacts:

    Last updated: December 17, 2006, chill@fnal.gov