Article
Reference
Measurement of the top quark mass using the invariant mass of lepton pairs in soft muon b -tagged events
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present the first measurement of the mass of the top quark in a sample of tt→ℓνbbqq events (where ℓ=e,μ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon b tagging). The pp collision data used correspond to an integrated luminosity of 2 fb−1 and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the W boson of the t→Wb decay and the muon from a semileptonic b decay. We fit template histograms, derived from simulation of tt events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180.5±12.0(stat)±3.6(syst) GeV/c2, consistent with the current world average value.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Measurement of the top quark mass using the invariant mass of lepton pairs in soft muon b -tagged events. Physical Review. D , 2009, vol. 80, no. 05, p. 051104
DOI : 10.1103/PhysRevD.80.051104
Available at:
http://archive-ouverte.unige.ch/unige:38638
Disclaimer: layout of this document may differ from the published version.
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Measurement of the top quark mass using the invariant mass of lepton pairs in soft muon b -tagged events
T. Aaltonen,24J. Adelman,14T. Akimoto,56B. A´ lvarez Gonza´lez,12,uS. Amerio,44b,44aD. Amidei,35A. Anastassov,39 A. Annovi,20J. Antos,15G. Apollinari,18A. Apresyan,49T. Arisawa,58A. Artikov,16W. Ashmanskas,18A. Attal,4 A. Aurisano,54F. Azfar,43W. Badgett,18A. Barbaro-Galtieri,29V. E. Barnes,49B. A. Barnett,26P. Barria,47c,47aP. Bartos,15 V. Bartsch,31G. Bauer,33P.-H. Beauchemin,34F. Bedeschi,47aD. Beecher,31S. Behari,26G. Bellettini,47b,47aJ. Bellinger,60
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PHYSICAL REVIEW D80,051104(R) (2009)
A. Nagano,56J. Naganoma,56K. Nakamura,56I. Nakano,41A. Napier,57V. Necula,17J. Nett,60C. Neu,46,x M. S. Neubauer,25S. Neubauer,27J. Nielsen,29,hL. Nodulman,2M. Norman,10O. Norniella,25E. Nurse,31L. Oakes,43 S. H. Oh,17Y. D. Oh,28I. Oksuzian,19T. Okusawa,42R. Orava,24K. Osterberg,24S. Pagan Griso,44b,44aC. Pagliarone,55a
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(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798, USA
6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy
6bUniversity of Bologna, I-40127 Bologna, Italy
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
22Glasgow University, Glasgow G12 8QQ, United Kingdom
T. AALTONENet al. PHYSICAL REVIEW D80,051104(R) (2009)
051104-2
23Harvard University, Cambridge, Massachusetts 02138, USA
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
25University of Illinois, Urbana, Illinois 61801, USA
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
28Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea; Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;
Chonnam National University, Gwangju,500-757, Korea; Chonbuk National University, Jeonju 561-756, Korea
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
34Institute of Particle Physics: McGill University, Montre´al, Que´bec, Canada H3A 2T8;
Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;
University of Toronto, Toronto, Ontario, Canada M5S 1A7; and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
38University of New Mexico, Albuquerque, New Mexico 87131, USA
39Northwestern University, Evanston, Illinois 60208, USA
40The Ohio State University, Columbus, Ohio 43210, USA
41Okayama University, Okayama 700-8530, Japan
42Osaka City University, Osaka 588, Japan
43University of Oxford, Oxford OX1 3RH, United Kingdom
44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44bUniversity of Padova, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
47bUniversity of Pisa, I-56127 Pisa, Italy
47cUniversity of Siena, I-56127 Pisa, Italy
47dScuola Normale Superiore, I-56127 Pisa, Italy
48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
49Purdue University, West Lafayette, Indiana 47907, USA
zOn leave from J. Stefan Institute, Ljubljana, Slovenia.
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50University of Rochester, Rochester, New York 14627, USA
51The Rockefeller University, New York, New York 10021, USA
52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
52bSapienza Universita` di Roma, I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, Italy
55bUniversity of Trieste/Udine, I-33100 Udine, Italy
56University of Tsukuba, Tsukuba, Ibaraki 305, Japan
57Tufts University, Medford, Massachusetts 02155, USA
58Waseda University, Tokyo 169, Japan
59Wayne State University, Detroit, Michigan 48201, USA
60University of Wisconsin, Madison, Wisconsin 53706, USA
61Yale University, New Haven, Connecticut 06520, USA (Received 29 June 2009; published 8 September 2009)
We present the first measurement of the mass of the top quark in a sample oftt!‘b bq qevents (where‘¼e; ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muonb tagging). The pp collision data used correspond to an integrated luminosity of2 fb1and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from theWboson of thet!Wbdecay and the muon from a semileptonicbdecay.
We fit template histograms, derived from simulation ofttevents and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180:512:0ðstatÞ 3:6ðsystÞGeV=c2, consistent with the current world average value.
DOI:10.1103/PhysRevD.80.051104 PACS numbers: 14.65.Ha, 12.15.Ff
A massive top quark plays an important role in the standard model (SM). The mass of the top quark (mt) enters electroweak precision observables as an input pa- rameter via quantum effects, i.e., loop corrections, and its large numerical value gives rise to sizable corrections that behave as powers ofmt[1]. For example, in the theoretical prediction of theWboson mass (mW) within the SM, when these corrections are combined with the logarithmic de- pendence on the mass of the postulated Higgs boson (mH), a relationship emerges that provides a constraint on mH
from experimental determinations of mW and mt [2].
Indeed, the strong dependence of the SM radiative correc- tions onmtmade it possible to predict the value ofmt[3]
prior to its experimental determination [4,5]. Thus, a pre- cision value ofmtis crucial for constraining SM parame- ters, for high-sensitivity searches for effects of new physics, and for stringent consistency tests of models beyond the SM (e.g., supersymmetry). Furthermore, inde- pendent measurements ofmtin all final states ofttdecay provide an important consistency check of the top quark sector of the SM, and might reveal new physics with top- like signatures.
Significant progress has been made recently in reducing the uncertainty in measurements of mt and in devising alternative and independent techniques. The current best single measurement is determined by reconstructing the full decay chain and computing the invariant mass of the decay products in tt!‘b bq q events, and yields mt¼ 172:11:6 GeV=c2 [6,7]. However, this and all the most
precise of the current techniques are limited by the com- mon systematic uncertainty in the calorimeter jet energy calibration [jet energy scale (JES)]. To provide indepen- dent measurements, several techniques with minimal de- pendence on the JES have been proposed. For example, the flight distance of thebhadron from the top decay can be used to infer the mass of the top quark [8], but this method also requires precision track reconstruction to determine the decay length. A proposal has been made [9] for ex- ploiting the correlation betweenmtand the invariant mass of the system composed of aJ=c (from the decay of ab hadron) and the lepton from theWdecay. The advantage is a stronger correlation of this system mass withmtthan that of individual decay products of the top quark, and thus a better sensitivity to the top quark mass, but the overall branching fraction for this final state is onlyOð105Þ.
We present the first measurement of the mass of the top quark in a sample oftt!‘b bq qevents (where‘¼e; ) selected by identifyingbjets with a candidate muon from semileptonic decay of heavy-flavor hadrons. We have de- veloped a novel technique that exploits the invariant mass of the lepton from theWboson of thet!Wbdecay, and the muon from a semileptonic b decay. The selection method is complementary to that taking advantage of the long lifetime ofbhadrons through the presence of a decay vertex displaced from the primary interaction. Since only 50%of the sample ofttcandidates with a semileptonicb decay overlaps the top samples selected by the identifica- tion of a displaced vertex, and a still smaller fraction is in
T. AALTONENet al. PHYSICAL REVIEW D80,051104(R) (2009)
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common with traditional samples that require all four jets for the mass reconstruction, our technique provides an essentially independent measurement of mt from these data. Moreover, our observable is largely independent of the JES, because the calorimeter information is used solely for the selection of event candidates, and therefore the result can add a significant amount of information when averaged with those from other measurements. Including sequential decays of charm, the branching fraction forb! X’20%[2] is sizable and since this technique does not require precision secondary vertex reconstruction to sup- press backgrounds, it could be an attractive option for the early phase of experiments at the Large Hadron Collider (LHC). Finally, the observable has a higher correlation to the top quark mass than the momentum of the lepton from theW decay alone. A partial reduction in sensitivity will arise from b-W mispairing, when the lepton from theW decay and the muon from thebsemileptonic decay do not originate from the same top quark.
Top quarks are produced at the Tevatron proton- antiproton collider predominantly in pairs oft andt, and are identified by the SM decayt!Wb, providing a final state that includes twoW bosons and two bottom quarks.
W’s are identified through their decay to leptons or quarks.
Quarks hadronize and are observed as jets of charged and neutral particles. The CDF II detector is described in detail elsewhere [10]. The components relevant to this analysis include the central outer tracker (COT), the central elec- tromagnetic and hadronic calorimeters, the central muon detectors and the luminosity counters. The data sample, produced inpp collisions at ffiffiffi
ps
¼1:96 TeVduring run II of the Fermilab Tevatron, was collected between March 2002 and May 2007 and corresponds to an inte- grated luminosity of 2:00:1 fb1. We select events where one of theW bosons decays to an isolated electron (muon) carrying large transverse energy (ET) [momentum (pT)] [11] with respect to the beam line, plus a neutrino.
We refer to these high-pT electrons or muons as primary leptons (PLs). The neutrino escapes the detector causing an imbalance of the total transverse energy vector, referred to as missingET(E6 T). The otherWboson in the event decays hadronically to a pair of quarks. We take advantage of the semileptonic decay ofBhadrons by searching for muons within final-state jets [soft-lepton tagging (SLT)], in order to identify those jets that result from hadronization of the bottom quarks.
The event selection starts with an inclusive lepton trig- ger requiring an electron (muon) withET>18 GeV(pT >
18 GeV=c). Further selection requires that candidate elec- tron (muon) PLs are isolated and have ET >20 GeV (pT>20 GeV=c) and jj<1:1. We define an isolation parameterIas the calorimeter transverse energy in a cone of openingR ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðÞ2þ ðÞ2
p ¼0:4around the lep-
ton (not including the lepton energy itself ) divided by the electron ET or muon pT. We select isolated electrons
(muons) by requiring I <0:1. The event must haveE6 T>
30 GeV, consistent with the presence of a neutrino from the W boson decay. Jets are identified using a fixed-cone algorithm with a cone opening ofR¼0:4and are con- strained to originate from thepp collision vertex. Muons inside jets are identified by matching the tracks of the jet, as measured in the COT, with track segments in the muon detectors. Such a muon with pT>3 GeV=c and within R <0:6of a jet axis is called anSLT. The probability of misidentifying a hadron as an SLT, denoted as the SLTmistag probability, is measured using a data sample of pions, kaons and protons from D and 0 decays. A Monte Carlo (MC) simulation ofWþjets events, whose details are given below, is used to model the ,K andp admixture in light-quark jets [12]. The SLT mistag probability is parametrized as a function of the track pT and, and is shown to describe within5%the number of false SLT tags in candidate light-flavor jets of QCD multijet andþjet events.
To reduce background from dimuon resonances and double-semileptonic Bhadron decays, we remove events in which the PL muon and SLTare oppositely charged and have an invariant mass consistent with a Z, or, irrespectively of the PL flavor, less than 5 GeV=c2. We further reject events as candidate radiative Drell-Yan andZ bosons if the tagged jet has an electromagnetic energy fraction above 0.8 and only one track with pT>
1:0 GeV=cwithin a cone ofR¼0:4about the jet axis.
The jet energies are corrected to account for variations of the detector response inand time, calorimeter gain drifts, nonlinearity of calorimeter energy response, multiple pp interactions in an event and for energy loss in uninstru- mented regions [13]. Finally, the sample is partitioned according to the number of jets with ET>20 GeV and jj<2:0 in the event, and at least one jet is required to contain an SLT(defining the SLT-tagged Wþnjets sample). The subset of W plus at least 3 jets is the tt candidate sample, and to reduce background from QCD production ofW boson with multiple jets, we additionally require the total transverse scalar energy in the event (HT
[14]) to be greater than 200 GeV.
Standard model processes that result in the same signa- ture as thettsignal are backgrounds to this measurement.
There are three dominant backgrounds: the largest one is mistags of Wþlight-flavor events, and a smaller contri- bution is due to the W boson in association with heavy- flavor jetsðWbb; Wc c; WcÞ . Events withoutWbosons that pass the event selection are typically QCD multijet events where one jet is reconstructed as a high-pT lepton, mis- measured jet energies produce apparent E6 T and an addi- tional jet contains anSLT. A fraction of these events is frombbandcc, where the candidate PL may result from a semileptonic decay of one of the fragmenting heavy quark and the SLT from a semileptonic decay of the other.
Other minor backgrounds that can mimic aW boson and . . .
an SLT signature include diboson ðWW; ZZ; WZÞ, Drell-Yan!, single top quark, and residual Drell-Yan!events not removed by the dimuon reso- nance removal. The composition of the data sample used in this analysis has been studied extensively in [12], where we have measured the production cross section forpp !ttX, and is summarized in TableI. TheWþjets, QCD multijet and Drell-Yan background are determined using the data, while the remaining backgrounds are estimated from MC simulations. The Wþ1;2 jets samples contain little tt events and have a composition similar to the background of thettcandidate sample. The simulation ofttevents is performed usingPYTHIA[15] andHERWIG [16]. The gen- erators are used with the CTEQ5L[17] parton distribution functions (PDFs). Modeling of b and c hadron decay is provided byEVTGEN[18]. Modeling ofWþjets produc- tion is performed usingALPGEN[19], coupled withPYTHIA
for the shower evolution andEVTGENfor the heavy-flavor hadron decays. Diboson production ðWW; ZZ; WZÞ and Drell-Yan! are determined using PYTHIA. Drell-Yan!þjets events are modeled using
ALPGEN while single top production is modeled with
MADEVENT [20], both with PYTHIA showering. The CDF II detector simulation models the response of the detector to particles produced inpp collisions. The detec- tor geometry used in the simulation is the same as that used for reconstruction of the collision data. Details of the CDF II simulation, based on theGEANT3package, can be found in [21].
We compute the invariant mass (M‘) between the PL and the SLT in the tt candidates sample. In rare cases where there is more than oneSLTtag in the same jet, or more than oneSLTtagged jet in the same event, we use the SLTcandidate that has the best match between the COT track and the track segment in the muon detectors. No attempt is made to choose the correct pairing from the decay chain of the two top quarks. The electric charge of theSLT, for instance, is not an effective flavor selector due to abundant sequentialb!c!decays. When the
wrong pairing is chosen, there is still sensitivity to the top quark mass due to the boost of theSLTand the PL. The distribution ofM‘ is given by the contribution ofttand background events. For the background, theM‘distribu- tion of QCD multijet events is derived from the data themselves in the kinematic region of I >0:15, E6 T>
30 GeV, topologically close to the signal region, while for other background sources we use MC simulation. We check the background model inWþ1;2jetSLT-tagged data events, a sample with a similar composition as the background to tt candidates. We find the predicted and observed distributions of M‘(Fig.1) to be in agreement with a p value of 55%, as given by the Kolmogorov- Smirnov test.
We construct a set of template histograms of the M‘
distribution using the background model and a simulation ofttevents. Thettsamples are generated with different top quark mass values in the range 150–195 GeV=c2, incre- menting by steps of up to 0:5 GeV=c2, and the full M‘ spectra are determined by adding the signal and expected background histograms in the ratio shown in Table I.
Figure 2 shows the mean value of the M‘ distributions versus the input top quark mass, indicating a linear rela- tionship between the two quantities. Also shown is hM‘i ¼35:61:1ðstatÞ GeV=c2, measured in the data.
We perform a binned-likelihood fit to theM‘histogram of the data, in 20 bins between 4 and 100 GeV=c2, with the binning and range chosena prioriappropriately to the size of the data sample. The likelihood is defined as
TABLE I. Composition of theSLT-taggedWþnjets can- didate sample [12]. TheHT>200 GeVrequirement is released for events with fewer than 3 jets.
Source Wþ1jet Wþ2jet Wþ 3jets
Wþlight flavor 62231 22612 52:32:6 Wþheavy flavor 14555 6725 14:35:4
QCD multijet 9216 4510 6:91:5
WWþWZþZZ 3:80:4 7:00:7 1:90:3 Drell-Yan! 2:60:6 1:50:4 0:60:3 Drell-Yan! 6:01:2 4:10:9 0:80:5
Single top 4:40:4 9:00:7 2:70:2
Total background 87654 35924 79:55:3 tt(tt¼9:1 pb) 3:50:2 31:81:0 168:55:3
Data 892 384 248
2] [GeV/c
µ
M l
10 20 30 40 50 60 70 80 90 100 2 Events/4.8 GeV/c
0 20 40 60 80 100 120 140 160 180 200
Data, 2 fb-1
W+jets QCD multijet
other W+1,2 jets sample
FIG. 1. The predicted and observed M‘ distributions in the sample of Wþ1;2 jet SLT-tagged events. The predicted distributions are stacked.
T. AALTONENet al. PHYSICAL REVIEW D80,051104(R) (2009)
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lnLðmtÞ ¼ NXbins
i¼1ndatai ln
nTPi ðmtÞ nTPtot
; (1) wherendatai andnTPi ðmtÞare the number of entries in eachi bin of the data and template histograms, respectively, the total number of entries isnTPtot ¼ndatatot , andnTPi ðmtÞ=nTPtot PiðmtÞ is the probability of the ith bin, normalized such thatP
iPi¼1. The background normalization is fixed and its value is varied in the evaluation of the systematic uncertainty. A parabolic function is fit to the values of lnLðmtÞ derived from each mass template, and the mea- sured top quark mass is determined from the minimum of the likelihood function, while the statistical uncertainty is given by the range corresponding to an increase in the lnL of 0.5 units above the minimum. For each mass point within the full mass range, we generate 5000 pseu- doexperiments with the same sample size as that of the data and verify that the fitting procedure is unbiased and that the statistical uncertainty returned by the fits represents the 68% confidence level. From 248 tt candidate events, we measure:
mt¼180:512:0ðstatÞ 3:6ðsystÞ GeV=c2: (2) Figure3shows theM‘distribution of the data, the back- ground, and the templates corresponding to the best fit and the statistical uncertainty.
The sources of systematic uncertainty that affect the measured value of the top quark mass are summarized in TableII. The limited size of thettsamples simulated with different values ofmt, input to the fitting procedure, yields an uncertainty of0:3 GeV=c2. Several components enter the uncertainty on the modeling of the background. The uncertainty on the Wþheavy- and light-flavor normal- izations yields an uncertainty of0:5 GeV=c2. The uncer-
tainty on the shape of theWþjets histogram is evaluated by varying the distribution, to within the statistical accu- racy associated with the comparison in the Wþ1;2jets sample between the data and the background model, and yields an uncertainty of1:4 GeV=c2. The normalization of the QCD multijet background contributes 0:8 GeV=c2. The shape of the QCD multijet distribution accounts for0:6 GeV=c2, as determined by replacing the nominal sample with dijet enriched data selected by I <
0:1 and E6 T<15 GeV, and by varying the distribution according to its statistical uncertainty. The shift on the measured top quark mass due to the uncertainties on the remaining backgrounds is negligible. The total uncertainty from background modeling is1:9 GeV=c2.
Monte Carlo modeling of the signalM‘ distributions includes effects of PDFs, initial-state radiation (ISR), final- state radiation (FSR), and JES. The uncertainty due to the MC modeling of tt production and decay, including b fragmentation, is determined by comparing the simulation
2] Top quark mass [GeV/c
150 160 170 180 190
]2 〉 [GeV/cµ l 〈M
33 34 35 36 37
MC simulation Fit to simulation
, 2 fb-1
σ
±1 Data
FIG. 2. The correlation between the mean value of the M‘ histograms from simulatedttand background samples, and the inputmt. The continuous line shows a linear fit to the points.
2] [GeV/c
µ
M l
10 20 30 40 50 60 70 80 90 100 2 Events/4.8 GeV/c
0 5 10 15 20 25 30 35 40 45
Data, 2 fb-1
Fit to data
Background (stat.) σ Fit +1
2)
= 180.5 GeV/c (mt
(stat.) σ Fit -1
FIG. 3. The distribution of invariant massM‘ of the lepton from theWdecay and theSLT, from a sample of 248 candidate ttevents with 79.5 background.
TABLE II. Summary of systematic uncertainties.
Source mt½GeV=c2
MCttsamples statistics 0:3
Background 1:9
ttproduction and decay model 2:1
Parton distribution functions 1:0
Initial- and final-state radiation 1:3
Jet energy scale 0:3
PL energy/momentum scale 0:9
SLTmomentum 0:9
Pileup 0:5
Total 3:6
. . .
using PYTHIAwith that using HERWIG and gives mt¼ 2:1 GeV=c2. The PDF uncertainty is evaluated by adding in quadrature the contribution of four effects: variations of the PDFs according to the 20CTEQeigenvectors [22], the difference between the standard tt simulation using the
CTEQ5L PDF and one derived using MRST98 [23] in the default configuration or with two alternative choices for
s, and the variation of the contribution of gluon fusion in ttproduction between 5% and 20%. The overall estimated uncertainty from PDF is1:0 GeV=c2. We vary both ISR and FSR simultaneously in thettMonte Carlo simulation, within constraints set by studies of radiation in Drell-Yan events in the data, and assign a systematic uncertainty on mtof1:3 GeV=c2.
The jet reconstruction is used in this analysis only for the selection of event candidates and therefore the uncertainty on the calibration of the jet energies enters the measure- ment solely through the event selection, via the jet count- ing and theE6 Trequirement. The uncertainty due to the JES is measured by shifting the energies of the jets in thettMC simulation by1of the JES [13] and results inmt¼ 0:3 GeV=c2. The uncertainty of1%on the difference between data and simulation of the PL energy and momen- tum scales gives an uncertainty of 0:9 GeV=c2. The differences in the data versus simulation for the SLT pT spectrum depend on theb-quark fragmentation model- ing and the momentum calibration. In addition to the different fragmentation models inHERWIGversusPYTHIA, we consider comparisons of the data with MC simulation of the muon pT spectra in B!D0X [24] and bb! X[25] which indicate an uncertainty on the muonpTof 0:8%, corresponding to mt¼ 0:9 GeV=c2. The uncertainty on the pT dependence of the SLT tagging efficiency yields a shift on the top quark mass of 0:2 GeV=c2. Finally, a source of systematic uncertainty is due to the modeling of pileup events from multiplepp interactions and it is estimated to affect the measured mass by 0:5 GeV=c2.
In summary, we have performed the first measurement of the top quark mass in a sample oftt!‘b bq q events
selected by identifyingbjets with a muon candidate from the semileptonic decay of heavy-flavor hadrons. The result, mt¼180:512:0ðstatÞ 3:6ðsystÞGeV=c2, is in agree- ment with the current world average value of 173:1 1:3 GeV=c2 [6], providing a consistency check of the top quark sector with soft muon b-tagged events. Our mea- surement technique exploits the correlation between the parent top quark mass and the invariant mass of the system composed of the lepton from the W decay and the muon from the semileptonicBdecay. The uncertainty at present is dominated by the statistical component. The method has a minimal dependence on the jet energy calibration, mak- ing it suitable for averaging the result with those from other techniques, and its dominant systematic uncertainties are likely reducible, e.g., by improving the calibration of the leptons’ pT to better than 1% with J=c, and Z reso- nances, by using improved tuning for the MC modeling of tt production and decay, and with high statistics data samples for the background model.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Science and Technology Facilities Council and the Royal Society, United Kingdom; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.
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