Measurements of the top-quark mass using charged particle tracking

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Reference

Measurements of the top-quark mass using charged particle tracking

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

We present three measurements of the top-quark mass in the lepton plus jets channel with approximately 1.9  fb−1 of integrated luminosity collected with the CDF II detector using quantities with minimal dependence on the jet energy scale. One measurement exploits the transverse decay length of b-tagged jets to determine a top-quark mass of 166.9+9.5−8.5(stat)±2.9(syst)  GeV/c2, and another the transverse momentum of electrons and muons from W-boson decays to determine a top-quark mass of 173.5+8.8−8.9(stat)±3.8(syst)  GeV/c2. These quantities are combined in a third, simultaneous mass measurement to determine a top-quark mass of 170.7±6.3(stat)±2.6(syst)  GeV/c2.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Measurements of the top-quark mass using charged particle tracking. Physical Review. D , 2010, vol. 81, no. 03, p. 032002

DOI : 10.1103/PhysRevD.81.032002

Available at:

http://archive-ouverte.unige.ch/unige:38651

Disclaimer: layout of this document may differ from the published version.

Measurements of the top-quark mass using charged particle tracking

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X. Zhang,²⁵Y. Zheng,^9,eand S. Zucchelli^6b,6a

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439

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

6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7Brandeis University, Waltham, Massachusetts 02254

8University of California, Davis, Davis, California 95616

9University of California, Los Angeles, Los Angeles, California 90024

10University of California, San Diego, La Jolla, California 92093

11University of California, Santa Barbara, Santa Barbara, California 93106

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

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

18Fermi National Accelerator Laboratory, Batavia, Illinois 60510

19University of Florida, Gainesville, Florida 32611

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

23Harvard University, Cambridge, Massachusetts 02138

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

26The Johns Hopkins University, Baltimore, Maryland 21218

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

29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720

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

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

36Michigan State University, East Lansing, Michigan 48824

37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

38University of New Mexico, Albuquerque, New Mexico 87131

39Northwestern University, Evanston, Illinois 60208

40The Ohio State University, Columbus, Ohio 43210

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

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

mVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017

lVisitor from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom

kVisitor from University College Dublin, Dublin 4, Ireland

jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus

iVisitor from Cornell University, Ithaca, NY 14853, USA

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gVisitor from University of CA Irvine, Irvine, CA 92697, USA

fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy

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aDeceased

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49Purdue University, West Lafayette, Indiana 47907

50University of Rochester, Rochester, New York 14627

51The Rockefeller University, New York, New York 10021

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

54Texas A&M University, College Station, Texas 77843

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

58Waseda University, Tokyo 169, Japan

59Wayne State University, Detroit, Michigan 48201

60University of Wisconsin, Madison, Wisconsin 53706

61Yale University, New Haven, Connecticut 06520 (Received 7 October 2009; published 10 February 2010)

We present three measurements of the top-quark mass in the lepton plus jets channel with approxi- mately1:9 fb¹of integrated luminosity collected with the CDF II detector using quantities with minimal dependence on the jet energy scale. One measurement exploits the transverse decay length ofb-tagged jets to determine a top-quark mass of166:9^þ9:5_8:5ðstatÞ 2:9ðsystÞGeV=c², and another the transverse mo- mentum of electrons and muons fromW-boson decays to determine a top-quark mass of173:5^þ8:8_8:9ðstatÞ 3:8ðsystÞGeV=c². These quantities are combined in a third, simultaneous mass measurement to determine a top-quark mass of170:76:3ðstatÞ 2:6ðsystÞGeV=c².

DOI:10.1103/PhysRevD.81.032002 PACS numbers: 12.15.y, 13.85.t, 14.60.z, 14.65.Fy

I. INTRODUCTION

An accurate knowledge of the top-quark mass is impor- tant. Combined with other standard model parameters, the top-quark mass can be used to constrain the expected standard model Higgs mass. The most precise constraints place an upper bound on the Higgs mass of154 GeV=c² [1] at the 95% confidence level. Further, since the top quark will be produced in copious quantities at the LHC, its mass can serve as a benchmark for b-jet energy cali- brations, which are otherwise difficult to study in data.

Historically, top-quark mass measurements have been limited by uncertainties in the simulation of jet energy measurements. For example, in the CDF Run I measure- ment [2] the top-quark mass systematic uncertainty was 4:9 GeV=c², of which the contribution from the uncer- tainty on jet energy measurements was 4:4 GeV=c². In comparison, for a modern CDF Run II measurement [3], the systematic uncertainty is 1:2 GeV=c², of which the contribution from jet energy uncertainties is0:7 GeV=c². This dramatic improvement in the jet energy uncertainty is made possible by exploiting the hadronicW-boson decay.

By constraining the reconstructedW-boson mass to agree between data and simulation, the average simulated jet energy bias is determined and applied to all jets in the event. In this technique, widely used in Run II top-quark mass measurements, most of the jet energy uncertainty becomes a statistical uncertainty rather than a systematic uncertainty. The remaining systematic uncertainty results from the assumption that simulation is biased by the same

amount, on average, for theb-jets as it is for theW-boson jets that are used for the calibration.

While jet energy uncertainties are no longer as large as they once were, they still represent a significant fraction of the total uncertainty on the top-quark mass. Further, this dramatic improvement in the world average mass resolu- tion depends upon the reliability of a single technique.

Thus, it is desirable to develop independent methods to measure the top-quark mass as a cross-check. A novel technique has been proposed for measuring the top-quark mass in a manner that is almost completely independent of the calorimeter using the measured transverse distance [4]

thatb-hadrons from the top quarks travel before decaying (Lxy) [5]. The transverse momenta of the top-quark decay products depend approximately linearly on the top-quark mass. In turn, this means that the lifetime and decay length of theb-hadrons depend approximately linearly on the top- quark mass. When this measurement was performed using 695 pb¹ of integrated luminosity [6], it became the first top-quark mass measurement to be mostly independent of calorimeter-based uncertainties, however it was also very statistically limited. Besides tripling the integrated lumi- nosity with respect to the previous measurement, we also improve the statistical resolution by incorporating more information about the event. Similarly to b-hadrons, the transverse momenta (pT) of leptons fromW-boson decays also depend linearly on the top-quark mass, and are mostly independent of the calorimeter jet energy scale. Since the momentum of the leptons is mostly uncorrelated to that of theb-quarks, this is complementary information, and it is

an ideal variable to add to the measurement, as has been previously proposed [5,7].

In this paper we present measurements of the top-quark mass using both the lepton transverse momentum and the mean decay length variables. Both measurements are per- formed upon the same events which pass an event selection that is designed to isolate ttevents where the W-boson from one top-quark decays to two quark jets, and the other decays leptonically to an electron or a muon plus a neutrino (the lepton plus jets channel). This decay channel was chosen because it occurs more often than any other final state that can be isolated with a high signal purity. We compute the mean Lxy of the leading twob-tagged jets and the mean transverse momentum of the leptons identified in the events. We measure the top-quark mass through com- parisons with the mean Lxy and mean lepton p_T from analysis simulations (‘‘pseudoexperiments’’) performed for a variety of top-quark mass hypotheses. To illustrate the dependence of these variables on the top-quark mass, the expected distributions forttsignal events passing our event selection are shown in Fig.1. It should be noted that our measurements are sensitive to very different event characteristics than typical mass measurements, and thus require unique treatments. In particular, it is more impor- tant for us to correctly model the boost of the top quarks than for other measurements. Thus, we reweight our simu- lated signal events to a more accurate parton distribution function. Further, the Lxy values that are measured in our simulation are sensitive to a variety of possible modeling inaccuracies. We determine correction factors to be applied to the simulated Lxy values by comparing the Lxy mea- surements between data and simulation for bb events, parametrized as a function of the jet energies.

Our measurements will be mostly independent of calorimeter-based jet energy uncertainties. However, some small dependence will remain because a loose jet energy cut is applied when selecting events, as will be explained below. Further, this parametrization of the Lxy

correction as a function of jet energy introduces a jet energy uncertainty to the measurement. In order to mini- mize this latter effect, we develop and apply an algorithm to measure jet energies for our Lxy correction parametri- zation that is based upon charged particle tracking.

This paper is organized as follows. We first present the relevant parts of the CDF detector in Sec.II, and the event selection used in this analysis in Sec. III. In Sec. IV, we explain the procedures for determining the normalizations as well as the shapes of the Lxy and leptonp_Tdistributions for our backgrounds. In Sec. V we explain how we cali- brate our signal distributions. In Sec. VI we present the procedures we apply to extract the top-quark mass from our final Lxy and lepton momentum distributions and present our results. Finally, in Sec.VIIwe explain in detail how our systematic uncertainties are estimated. We con- clude with some projections of the future potential for this type of measurement.

II. THE CDF II DETECTOR

We describe the most important parts of the CDF detec- tor for this analysis. A more complete description can be found elsewhere [8]. The CDF detector consists of cylin- drically symmetric layers of hardware, each designed to perform specific functions. The CDF tracking system con- sists of a silicon tracker, inside of a wire drift chamber, immersed in a 1.4 T magnetic field produced by a super- conducting solenoid. Calorimeters encircle the tracking system and consist of alternating layers of scintillator and absorber. The calorimeter system is split into an inner calorimeter that is designed to absorb and measure the energies of photons and electrons, and an outer calorimeter that is designed to absorb and measure the energies of hadrons.

The wire drift chamber consists of 96 wire planes grouped into eight superlayers covering the radial region between 0.40 and 1.37 m from the beam axis, and less than

Lxy [cm]

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

0.08 m_t = 150 GeV/c2

= 200 GeV/c2

mt

Entries / (0.05 cm)

(a)

Lepton p [GeV/c]

-0.5 0 0.5 1 1.5 2 2.5 3 0^{0 20 40 60 80} 10 0 12 0 14 0 16 0 18 0 200

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0.09 m_t = 150 GeV/c²

= 200 GeV/c2

mt

Fraction / (4 GeV/c)

T

(b)

FIG. 1 (color online). Distributions of our measurement variables for simulatedttevents after passing our event selection for top- quark masses separated by50 GeV=c². These distributions are normalized to unit area.

1.0 in pseudorapidity. The superlayers alternate between axial and 2 stereo angle to provide both axial and longitudinal hit information. The inner tracker consists of eight layers of double-sided silicon, covering the radial range between 1.35 and 29 cm, and less than 2.0 in pseu- dorapidity. Again, to provide longitudinal hit information, some layers contain double-sided silicon that combines axially oriented strips with strips oriented either at 90 or 1.2 relative to the axial direction. The silicon tracker is vital for vertexing, providing a two-dimensional impact parameter resolution of about 40m for isolated tracks with high transverse momentum, including uncertainties on both the track trajectory and the location of the collision vertex. The wire tracker is vital for the lepton measure- ments in this analysis, providing a transverse momentum resolution of about 5% for50 GeV=cmuons. Leptons and b-jets are reliably identified out to a pseudorapidity of about 1.0, after which the efficiency drops rapidly due to tracks falling outside of the range of full tracker coverage.

Muon candidates are identified by matching tracks in the inner tracking detector with track stubs in the outer muon drift chambers. Electron candidates are identified by matching tracks in the inner tracking detector with showers in the electromagnetic calorimeter. An energy resolution of about 3% is achieved for electrons with 50 GeV of trans- verse energy.

The Tevatron proton-antiproton bunches cross one an- other at a rate of 2.5 MHz. During each crossing one or more collisions are likely to occur. In order to reduce this flow of information to a manageable level and to select collision events of interest to particular analyses, CDF employs a three level triggering system to sequentially reject events that are less relevant. The final event stream is read out at a rate of roughly 100 Hz. Details of the triggering system are provided in [8].

III. EVENT SELECTION

In comparison to the previous publication using only Lxy [6], we tighten the event selection in the analyses presented here to reduce the systematic uncertainties.

The statistical sensitivities of both the lepton momentum and the decay length measurements depend linearly on the fraction of events in which top quarks are produced [9], and scale as the square root of the number of events selected.

This tightened selection improves the former and worsens the latter, and has little impact on the final statistical sensitivity.

The data used in this analysis were collected between March 2002 and May 2007, and correspond to an inte- grated luminosity of 1:9 fb¹. Events passing the full trigger and event selections described below are studied to determine the expected event counts and uncertainties for each signal and background type. Top quarks almost always decay toWb. Our selection criteria are designed to accept events where oneW-boson decays to an electron or

muon plus neutrino and the other decays to two jets. We start from a triggering stream that requires one electron (muon) to have transverse energy (momentum) greater than 18 GeV (GeV=c); these requirements will later be tightened slightly. Once events are accepted by the trigger, they are saved, reconstructed, and studied in greater detail.

Calorimeter towers are clustered together within a cone of radiusR¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðtowjetÞ²þ ðtowjetÞ²

q ¼0:4[10] to

form jets. At least three jets must be found withjj<2:0, and transverse energy greater than 20 GeV after correcting for multiple interactions, calorimeter response, noise, and other nonuniformities. No attempt is made to correct for interactions from spectator partons (‘‘underlying event’’) or out-of-cone effects [10], but systematic uncertainties are assigned for these effects. To account for the neutrino and suppress the QCD background, a quantity called the miss- ing transverse energy is used, which is defined as the transverse component of the four-momentum vector that is needed to conserve momentum in the event [11]. The missing transverse energy in the event must be greater than 20 GeV. Additionally, an electron (muon) must be identi- fied with transverse energy (momentum) greater than 20 GeV (GeV=c). Electrons are identified from a track pointing at a cluster in the calorimeter which matches the expected shape profile. Most of the energy of this cluster is required to be confined in the electromagnetic calorimeter, the track momentum is required to agree with the measured calorimeter energy to within a factor of 2, and if the track is consistent with the electron having originated from a pho- ton conversion, the electron candidate is vetoed. Muons are formed from tracks in the muon chambers which are matched to tracks found in the inner tracking system.

The calorimeter energy deposits along the muon trajectory must be consistent with that of a minimum ionizing parti- cle. Calorimeter isolation is based upon the fraction of the lepton’s energy (f_iso) in a cone of radiusR¼0:4centered on the lepton, excluding the energy in the calorimeters from the lepton itself. Both electrons and muons must satisfy f_iso<0:1. This cut eliminates most jets that are mistakenly identified as leptons (‘‘fake leptons’’) as well as real leptons which result fromb-hadron decays. Further, at least one collision vertex must be reconstructed from tracks in the event, and the track of the lepton must pass within 3 cm along the beam axis of the highest momentum vertex to minimize contamination from multiple interactions and tracking errors. One electron or muon is required to pass these cuts, but to suppress events from the dilepton channel the event is vetoed if any other leptons are found passing a much looser set of cuts which also allow nonisolated and forward leptons. Finally, one or more jets must be identi- fied (tagged) as originating from a b-quark, as explained below. In the case where only three jets pass our selection, two of them must be tagged in order to reduce the large Wþjets and non-W QCD backgrounds in this kinematic region.

Jets containingb-quarks are identified using a tagging algorithm known as SecVtx [12], which relies upon the long lifetime of hadrons originating from b-quarks. This algorithm attempts to construct a secondary vertex using tracks that are likely to have originated from ab-hadron decay. If a vertex can be found which is significantly displaced from the primary vertex, then the jet is tagged.

Specifically, the SecVtx algorithm selects tracks associated with the jet that are displaced from the fitted primary collision vertex, and that are well resolved in both the silicon and the outer tracking chamber. As a first pass the algorithm uses relatively less stringent cuts on track selec- tion, and attempts to fit at least three tracks into a displaced vertex. If no vertex is found the tracking requirements are tightened significantly and the algorithm searches for a two-track vertex. Further cuts are applied to eliminate tracks that are likely to originate from material interactions or long lived strange particles. If a secondary vertex is found the jet is tagged as ab-jet if the transverse distance from the primary vertex projected onto the jet direction (Lxy) and its uncertainty () satisfy Lxy= >7:5. It should be noted that charmed daughters of the b-hadron are also likely to travel a significant distance before decay- ing. The SecVtx algorithm is deliberately designed to be loose enough to attach some tracks from these tertiary decays into one ‘‘pseudovertex’’ at a position that is aver- aged between two real vertices. Since the boosts of the charm hadrons depend on the boost of theb-hadron, this extra information does not dilute our mass resolution.

IV. SAMPLE COMPOSITION

For this analysis we will need to know the Lxy and lepton p_T distributions for each of our signal and back- ground samples, as well as their relative normalizations after full event selection is enforced. Specifically, we nor- malize our signal and background distributions using the results of a cross section measurement that was performed on the same data using identical event selection. The procedures for this cross section measurement are briefly described in Sec. IVA. The procedures that are used to determine our Lxy and lepton p_T shapes for the back- grounds are deferred to Sec.IV B.

A. Sample normalization

In this section we give a brief outline of how the cross section measurement is performed to determine our signal and background normalizations along with the associated uncertainties. Further information can be found in the publications of similar cross section measurements [13]

[14], or in the Ph.D. thesis [15] which explains the tech- nique we apply in detail. For this measurement technique we begin by determining the expected backgrounds assum- ing the expected standard model ttcross section. The tt cross section will then be revised and the backgrounds redetermined in an iterative manner until the number of

expected events matches the observed event counts both before and afterb-tagging.

First we determine the numbers of events for some of the rarest processes from simulation. Backgrounds modeled from simulation include single top production as well as the electroweak diboson (WW, WZ, ZZ) and Zplus jets final states. The diboson events are simulated usingPYTHIA

version 6.216 [16]. The single top andZplus jets samples are simulated using other programs (MadEvent [17] for single top, andALPGENversion 2.10 prime [18] forZplus jets). Hadronic showers are simulated using PYTHIA. For each sample, the event count is determined and scaled according to the theoretical cross section, branching ratio, and detector and trigger acceptances. Thettsignal is also simulated using PYTHIA. Its cross section is initially set equal to its standard model expectation. For all of these samples, EvtGen [19] is used to determine the lifetimes and masses of the variousb-hadron species within jets. A further scaling is applied to correct theb-tagging efficiency modeling based upon data-driven studies [14].

Since fake leptons are difficult to simulate accurately, the non-W QCD contribution is determined from data.

Missing transverse energy templates are made for tt, W plus jet, and QCD distributions, and used in a fit to deter- mine the QCD normalization. Templates for the fake elec- trons are filled from events where isolated electron candidates are selected and required to have a calorimeter shower profile that is consistent with QCD fakes rather than true electrons. For muons the standard cuts are kept except that the isolation cut is inverted to require greater than 20% isolation instead of less than 10%. Different binnings, fit ranges, and cuts are applied and the differ- ences in the results are taken as a systematic uncertainty.

This fit is done separately in the tagged and pretagged cases.

The remainder of the observed pretagged events are all taken to originate fromWplus jets. TheWplus jets sample is simulated usingALPGENversion 2.10 prime [18], and the resulting partons are showered usingPYTHIA. This sample is simulated in various bins of jet multiplicity for theWbb, Wcc, Wc, and W plus light-flavor final states. These samples are then weighted by their theoretical cross sec- tions and combined. It is important to make sure that the proper number of heavy flavor jets are simulated before b-tagging is applied. Pythia showering will double count heavy flavor production from the ALPGEN simulation, so this overlap must be removed. Further, since the samples are only generated at leading order, a correction is needed to reweight the heavy flavor fraction. This correction is taken from comparisons between data and simulation in related samples with higher statistics [14].

By construction, this procedure yields pretagged back- ground and signal normalizations that exactly match the observed number of data events. Afterb-tagging, however, fewer events were predicted than were observed, and this

discrepancy is attributed tottevents. Thus, the signal cross section was increased and the analysis was repeated in an iterative fashion. The ttcross section at which we found agreement was 8:20:7 pb (ignoring luminosity uncer- tainties). The resulting event counts passing full event selection and tagging requirements are shown in TableI, along with the number of jets in these events which were b-tagged and included in our Lxy analysis.

B. Background Lxy and leptonp_Tshapes The dominant backgrounds for our analysis areW plus heavy flavor (bandcjets),Wplus light-flavor mistags, and non-W QCD events. Along with single top and diboson events, these distributions and the signal account for about 99% of events passing selection. The remaining events come from the Z plus jets background, for which the relatedW plus jets background Lxy and leptonpT distri- butions were used.

Single top samples (with masses mt¼165, 170, 175, and 180 GeV=c²) were generated in MadEvent and de- cayed inPYTHIA, in both the s- and t- channels. Results for the s- and t- channels were combined, weighted according

to their expected theoretical cross sections, and the final decay length and lepton momentum distributions were fit to Gaussian plus exponential distributions. The trends in the fit parameters were extrapolated to other mass points, and were used to generate new Lxy and lepton p_T distri- butions for each mass hypothesis. The W plus jets back- ground is selected in exactly the same way as for the cross section measurement, as is the background for non-W QCD electrons. For non-W QCD background in the muon channel, instead of using nonisolated muons, we selected muon candidates with high energy deposition in the calorimeter, or that were associated with tracks that were highly displaced from the collision vertex. Any in- accuracies introduced by these cuts are covered by our estimation of the background uncertainties as explained below.

Our final Lxy and leptonp_T distributions are shown in Fig.2for events passing full selection. While the distribu- tion of leptonp_T is shown for the combined electron and muon samples, the backgrounds and uncertainties for each lepton type are evaluated separately. A more detailed dis- cussion of the differences between the electron and muon channels that are relevant to this measurement can be found elsewhere [20]. As cross-checks the same distribu- tions in the background dominated one and two jet bins are shown in Figs.3and4. These cross-check samples are used in evaluating the background based systematic uncertainty as described in Sec.VII.

V. CORRECTIONS TO THE SIGNAL SIMULATION Ourttevents are simulated inPYTHIAfor various hypo- thetical top-quark mass values. A number of corrections are applied to the simulated results as discussed below. In Sec. VA we will discuss corrections that are needed to account for simulated parton distribution function inaccur- TABLE I. Estimated signal and background contributions after

full event selection.

Source Events Recordedb-Tags

Wbb 25:47:0 37:59:7

WccorWc 13:94:6 15:94:7

Wplus light flavor 16:93:7 17:93:7

non-W QCD 18:812:7 20:213:2

Electroweak 9:00:4 11:30:5

Single Top 8:40:4 13:40:7

tt 478:340:3 659:345:5

Total 570:844:3 775:550:2

Lxy [cm]

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60

70 ^ttbar, Electroweak^{σ = 8.2 pb}

Single Top W+jet QCD data

Entries / (0.05 cm)

(a)

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0 50 0 150 200 250 300

0 10 20 30 40 50

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 00 50 100

10 20 30 40 50

= 8.2 pb σ ttbar, Electroweak Single Top W+jet QCD data

T

Entries / (4 GeV/c)

(b)

FIG. 2 (color online). Signal, background, and data for the Lxy and lepton p_T distributions passing full event selection under hypothesized top-quark masses that are close to the measured results. The left plot is for the Lxy measurement, using top-quark mass 168 GeV=c², and the right plot is for the leptonp_T measurement, using top-quark mass173 GeV=c².

acies. It is also necessary to correct the simulated Lxy measurements to match observations from data. An over- view for this procedure is given in Sec. V B. We will parametrize the Lxy correction as a function of the jet transverse energy as measured using tracking. The proce- dure for this will be discussed in Sec.V C. We will sum- marize the complete decay length calibration procedure in Sec.V D.

A. Parton distribution functions

For this analysis it is vital to have accurate models of the lepton and jet boosts. Since these quantities depend on the energy of the colliding partons, an accurate modeling of parton distribution functions (PDFs) for particles within the proton is very important. Thettsamples were gener- ated using the leading order CTEQ5L parton distribution function [21]. One drawback of this PDF is that it under- estimates the rate ofttproduction by gluon fusion (5% in

simulation observed versus about 15% expected [22]).

Since gluon fusion produces events with slightly smaller boosts on average, this will bias the analysis. In addition, since the CTEQ5L PDF overestimates the fraction of the proton momentum carried by colliding quarks, thettevents produced within the quark-antiquark annihilation channel have artificially high boosts. To compensate for this effect, different parton distribution functions were studied. The Les Houches Accord PDF Interface [23] was used to determine the probabilities for each parton involved in the collision to have the generated momentum for a given PDF. One can then reweight the simulated events to con- struct distributions appropriate to a different PDF as ex- plained in [24]. We use this prescription to reweight all of our ttevents to the next-to-leading order CTEQ6M [25]

parton distribution function for gluon fusion and quark annihilation events separately. A further weighting is then applied to gluon fusion events to scale the gluon

Lxy [cm]

Entries / (0.05 cm)

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

0 20 40 60 80 100 120 140 160 180 200 220

240 ttbar

Single Top Electroweak QCD W+Jet

(a)

Lepton p [GeV/c]

Entries / (4 GeV/c)

0 20 40 60 80 100 120 140 160 180 200 0

50 100 150 200 250

300 ttbar

Single Top Electroweak QCD W+Jet

T

(b)

FIG. 3 (color online). Background prediction compared with data (black points) in the one-jet control region for Lxy (left) and lepton p_T(right).

Lxy [cm]

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

Entries / (0.05 cm)

0 20 40 60 80 100 120

140 ttbar

Single Top Electroweak QCD W+Jet

(a)

Lepton p [GeV/c]

Entries / (4 GeV/c)

0 20 40 60 80 100 120 140 160 180 200 0

20 40 60 80 100 120 140

160 ttbar

Single Top Electroweak QCD W+Jet

T

(b)

FIG. 4 (color online). Background prediction compared with data (black points) in the two-jet control region for Lxy (left) and leptonp_T (right).

fusion fraction to the value expected for the sample’s top- quark mass (20% for m_t¼150 GeV=c², 10% for m_t¼ 200 GeV=c²). These combined reweightings result in new distributions to be used in our mass measurement and lead to a1:7 GeV=c²shift in the top-quark mass for the lepton pT analysis and a0:9 GeV=c² shift for the Lxy analysis, relative to the values obtained using CTEQ5L. A similar prescription is used to reweight to other PDFs and gluon fractions to evaluate systematic uncertainties, as will be explained in Sec.VII.

B. Lxy calibration strategy

A number of effects may bias the decay length measure- ment in simulation. Inaccuracies in the EvtGen [19] values for the hadron lifetime or in the simulated production fractions would have a direct impact. Similarly, inaccura- cies in thePYTHIAfragmentation model would lead to the wrong boost, and thus the wrong average decay length, of theb-hadrons. Entirely different problems may arise from any inaccuracies in the modeling of the tracking system, which could lead to biases in the vertexing results. Our approach is to calibrate the simulation directly to the data to compensate for all of these biases simultaneously.

Systematic uncertainties on this calibration will be dis- cussed in Sec. VII F. We select our calibration sample so that the tagged jets will be almost exclusivelyb-jets. We select dijet events where the jets are required to open at a wide angle with >2:0. Both jets must beb-tagged, and one of the jets must contain a well resolved muon with at least 9:5 GeV=c transverse momentum. We also apply additional cuts to minimize overlap between jets which will be discussed later. We estimate these samples to be about 95%bb, with the remaining 5% coming from charm contamination, which is accounted for as a small system- atic uncertainty.

Since jets frombb events tend to have a much lower transverse energy than those fromttevents, the possibility that a different calibration is needed for higher energy jets must be taken into account. To this end, we bin ourbbjets according to their energy, and derive the needed correction bin-by-bin, which we apply based on the measured energy of the tagged signal jet. Great care must be taken in doing this, however, as this kinematic based correction directly introduces a jet energy scale uncertainty to the analysis. If, for example, the simulation underestimates the energies of jets, then the average decay length in a given energy bin will be too high in the simulation, throwing off the cali- bration. This is an unavoidable uncertainty for any kind of calibration using dijets. In fact, even if we could convince ourselves that it was unnecessary to parametrize this decay length calibration as a function of energy, a significant jet energy uncertainty would still be needed to cover the determination of which jets pass selection in the simula- tion. As our goal is to minimize the calorimeter jet energy uncertainties, we choose to parametrize our calibration

based upon the energies of jets measured using tracking rather than the calorimetry.

C. Track-based jet energies

We develop a straightforward algorithm for computing the track-based energy of a jet. We start with one of our jets that has been clustered in the calorimeter in the usual manner. We select tracks that are within R¼0:4 of the calorimeter jet direction inspace. The tracks them- selves are required to be well resolved in both the wire tracking chamber and in the silicon, and they must have a transverse momentum of at least1 GeV=c. We also require that the tracks pass within 3 cm along the beam axis of the fitted primary collision vertex. This cut eliminates about 85% of the contamination from multiple interactions. We then take the total transverse component of the sum of our track four-vectors as our tracking transverse energy. We make no attempt to correct for the missing neutral parti- cles. Our goal is not to measure the true jet energy, but rather to measure a quantity that is proportional to the true energy (in this case, the charged transverse energy) in a manner that agrees well between data and simulation.

We calibrate the energies of our track jets with a similar approach to the one used for calorimeter jets at CDF [10].

We select events where one photon and one jet are found back-to-back ( >3radians), where no extra jets in the event above 3 GeV in E_T are allowed, and strict cuts on photon quality are applied to minimize fakes. Under such a selection, the transverse momentum of the photon should reflect the true transverse momentum of the jet. As a first step we consider the ratio of the track-based transverse energy of the jet to the measured transverse energy of the photon. The distribution of this ratio shows good agree- ment between data and simulation as seen in Fig.5(a)for a particular range of photon energies, and this agreement also holds for higher energy photons. The mean fraction of the photon transverse energy that is carried in the track- based jet transverse energy is shown for a range of true jet transverse energies (determined by the energy of the pho- ton) in Fig. 6. The extent of the agreement between data and simulation is given by the ratio of these trends and is shown in black in the same figure. A line is fitted to these ratios and represents the calibration that will be applied to the track jet energies that we measure in our simulation.

Specifically, for a given tagged jet in our simulatedbbortt sample, we begin by measuring the corrected transverse energy of the jet in the calorimeter and its associated track- based jet transverse energy. We then correct this track- based transverse energy according to its calorimeter-based transverse energy and the fitted function in Fig. 6(b).

Uncertainties in the simulation of the calorimeter-based jet energy measurements are found to have a negligibly small impact on the correction factor that is determined in this manner. There are, however, significant statistical un- certainties on the fitted function that translate into a sys-