Article
Reference
Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We present a measurement of the top quark pair production cross section in pp collisions at s√=1.96 TeV using 318 pb−1 of data collected with the Collider Detector at Fermilab. We select tt decays into the final states eν+jets and μν+jets, in which at least one b quark from the t-quark decays is identified using a secondary vertex-finding algorithm. Assuming a top quark mass of 178 GeV/c2, we measure a cross section of 8.7±0.9(stat)+1.1−0.9(syst) pb. We also report the first observation of tt with significance greater than 5σ in the subsample in which both b quarks are identified, corresponding to a cross section of 10.1+1.6−1.4(stat)+2.0−1.3(syst) pb.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2006, vol. 97, no. 08, p. 082004
DOI : 10.1103/PhysRevLett.97.082004
Available at:
http://archive-ouverte.unige.ch/unige:38364
Disclaimer: layout of this document may differ from the published version.
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Measurement of the t t Production Cross Section in p p Collisions at p s
1:96 TeV
A. Abulencia,23D. Acosta,17J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,16D. Ambrose,16S. Amerio,43 D. Amidei,34A. Anastassov,52K. Anikeev,16A. Annovi,18J. Antos,1M. Aoki,55G. Apollinari,16J.-F. Arguin,33 T. Arisawa,57A. Artikov,14W. Ashmanskas,16A. Attal,8F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43 H. Bachacou,28W. Badgett,16A. Barbaro-Galtieri,28V. E. Barnes,48B. A. Barnett,24S. Baroiant,7V. Bartsch,30G. Bauer,32
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J. C. Yun,16L. Zanello,51A. Zanetti,54I. Zaw,21F. Zetti,46X. Zhang,23J. Zhou,52and S. Zucchelli5 (CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California –Davis, Davis, California 95616, USA
8University of California –Los Angeles, Los Angeles, California 90024, USA
9University of California –San Diego, La Jolla, California 92093, USA
10University of California –Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708, USA
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
17University of Florida, Gainesville, Florida 32611, USA
18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
19University of Geneva, CH-1211 Geneva 4, Switzerland
20Glasgow University, Glasgow G12 8QQ, United Kingdom
21Harvard University, Cambridge, Massachusetts 02138, USA
22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
23University of Illinois, Urbana, Illinois 61801, USA
082004-2
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
and SungKyunKwan University, Suwon 440-746, Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
37University of New Mexico, Albuquerque, New Mexico 87131, USA
38Northwestern University, Evanston, Illinois 60208, USA
39The Ohio State University, Columbus, Ohio 43210, USA
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
48Purdue University, West Lafayette, Indiana 47907, USA
49University of Rochester, Rochester, New York 14627, USA
50The Rockefeller University, New York, New York 10021, USA
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155, USA
57Waseda University, Tokyo 169, Japan
58Wayne State University, Detroit, Michigan 48201, USA
59University of Wisconsin, Madison, Wisconsin 53706, USA
60Yale University, New Haven, Connecticut 06520, USA (Received 6 June 2006; published 25 August 2006)
We present a measurement of the top quark pair production cross section in pp collisions atps 1:96 TeVusing318 pb1of data collected with the Collider Detector at Fermilab. We selectttdecays into the final statesejets andjets, in which at least onebquark from thet-quark decays is identified using a secondary vertex-finding algorithm. Assuming a top quark mass of178 GeV=c2, we measure a cross section of8:70:9stat1:10:9systpb. We also report the first observation ofttwith significance greater than5in the subsample in which bothbquarks are identified, corresponding to a cross section of 10:11:61:4stat2:01:3systpb.
DOI:10.1103/PhysRevLett.97.082004 PACS numbers: 13.85.Ni, 13.85.Lg, 13.85.Qk, 14.65.Ha
The top quark completes the third quark generation in the standard model (SM). Because of its large mass, nearly 40 times greater than that of the next heaviest quark, the top quark can at present be studied only at the Fermilab Tevatron, app collider with a center-of-mass energy (
ps ) of 1.96 TeV. In these collisions, the SM top quark is mostly produced in pairs throughqqannihilation and gluon fusion with a total theoretical cross section of6:10:60:8 pb[1] for a
mass of178 GeV=c2 [2]. Previous measurements of thett production cross section (tt) performed at the Tevatron [3,4] were consistent with SM expectations but suffered from large uncertainties due to small event samples. With significant enhancements in the integrated luminosity and our sensitivity tot-quark decay products, we are no longer limited by statistical uncertainties, now reaching a preci- sion comparable to that of the theory.
The top quark decays predominantly to aWboson and a bottom quark. We expect the top quark signal significance to be greatest in the leptonjets channel, in which top quark pairs decay through the chain tt!WWbb!
‘qq0bb. A typical tt event in this decay channel will include an electron or muon with large transverse momen- tum (pT), four jets corresponding to the four final-state quarks, and an imbalance in the total transverse energy in the event (6ET) from the undetected neutrino [5]. We dis- tinguish ttevents from background by requiring at least one jet to be identified as a bottom quark (btagged) using the secondary vertex-finding algorithm (SECVTX) described in Ref. [3]. We have reoptimized the algorithm to enhance the efficiency forbtagging two jets per event; the sample of events with this signature is dominated by tt(>90%
pure), making it an ideal environment for direct determi- nation of top quark properties. In this Letter, we present a measurement of thett cross section inb-tagged lepton jets events, and we also report the first observation oftt with significance greater than 5 in doubly b-tagged events.
Results reported here are obtained using 318 pb1 of integrated luminosity collected between March 2002 and August 2004 by the Collider Detector at Fermilab (CDF II). The CDF II detector [6] is a general-purpose particle detector located at one of the two interaction points at the Tevatron Collider. Inside a 1.4 T solenoidal magnetic field, a large open-cell drift chamber, the central outer tracker (COT) [7], and an eight-layer silicon system [8]
provide tracking information. The COT covers the pseu- dorapidity rangejj<1:1and provides a long lever arm for track curvature measurements. The silicon system pro- vides three-dimensional hit information between radii of 1.3 and 28 cm, with anrimpact parameter resolution of 40m (including a 30m contribution from the beam spot). Outside the solenoid, electromagnetic and hadronic calorimeters surround the tracking volume in a projective tower geometry, identifying jets and electrons with jj<3:6. Electron energies are measured in the central electromagnetic calorimeter for jj<1:1 and in the end-plug calorimeters for1:1<jj<3:6. Beyond the calorimeters, drift chambers provide muon identification in the regionjj<1:0.
The data were collected with an inclusive high-pTlepton trigger that requires an electron (muon) with transverse energyET>18 GeV(pT >18 GeV=c). The trigger effi- ciency, including lepton identification, is 95:91:5%
(87:71:5%) for electrons (muons). Following the event selection used in Ref. [3], we require all leptons to be isolated [9] andET>20 GeV(pT>20 GeV=c) for elec- trons (muons). We remove events with multiple high-pT leptons, a cosmic ray muon, a photon conversion electron, or a track that forms the Z mass with the lepton. The position of the primary vertex along the beam is required to be within 60 cm of the nominal interaction position and consistent with thezposition of the high-pT lepton.
After leptons are selected, we require the presence of at least three jets with jj<2 and ET>15 GeV (ET is corrected as described in Ref. [10]). Jets are clustered with a cone-based algorithm with a cone size R
22 p 0:4.
To account for the expected neutrino, we require large missing transverse energy 6ET>20 GeV. The invariant mass of the lepton and the neutrino, with the longitudinal neutrino momentum set to zero, is required to exceed 20 GeV=c2; this rejects50%of background events that do not contain a real W boson. Finally, asttevents typi- cally have larger total transverse energy than background events, we require that the scalar sum of the 6ET and the lepton and total jet transverse energies (HT) be greater than 200 GeV. This leaves 310 (190) events in the electron (muon) data sample, dominated byWbosons with associ- ated production of light-flavor jets (WLF). Table I in- cludes the event count beforebtagging (pretag) sorted by the number of jets in the event. The one- and two-jet bins have been included as an assumed control sample, although without theHT requirement.
We improve the tt signal significance by requiring at least one jet to bebtagged with theSECVTXalgorithm [3].
Because of their long lifetime, bquarks typically decay a measurable distance from the primary interaction point.
We reconstruct the decay vertices using a minimum of two or three tracks with an impact parameter significance greater than 3.0 or 2.0, respectively. Contributions from K0S and decays and interactions in the inner detector material are reduced through additional requirements on the invariant mass of the secondary vertex and an upper limit on the track impact parameter d0<0:15 cm. We measure the two-dimensional displacement of the second- ary vertex from the primary interaction point projected along the jet axis (L2D); a jet isbtagged if the vertex has L2Dsignificance larger than 6.0, where the uncertainty on L2D includes contributions from both the primary and secondary vertex fits. The probability of misidentifying a light-flavor jet as a b-quark jet due to detector resolution (mistag rate) is estimated from secondary vertices recon- structed behind the primary vertex with L2D significance less than6:0.
The tt acceptance is calculated from a combination of data and Monte Carlo simulation. We use the PYTHIA
Monte Carlo generator [11] with CTEQ5L parton distribu- tion functions [12] and assume Mtop178 GeV=c2, the world average top quark mass measurement from run I [2].
The CLEO QQ Monte Carlo program [13] models the decays of bottom and charm hadrons. These events are passed through a GEANT [14] simulation of the CDF II detector and subjected to the same selection requirements as the data. The total acceptance, including the branching fraction, is calculated as the product of geometric and kinematic acceptances (including lepton identification) and the trigger efficiency. The efficiency to identify iso- 082004-4
lated high-pT leptons in the simulation is scaled to the value measured in theZ!‘‘data. The acceptance for electron (muon) events before b tagging is 3:80:3%
(2:80:2%).
Theb-tagging efficiency for the fullttevent is measured in attsimulation that has been tuned to match the single jet b-tagging efficiency in the data. A multiplicative scale factor (Sb0:9270:066), measured in heavy-flavor- enriched samples of nonisolated, low-ET leptons, corrects for the per-jet efficiency difference between data and simulation [3]. Inttevents, we measure ab-tagging effi- ciency of 484%for b-quark jets with a corresponding mistag rate of 1:20:1%; we therefore expect 695%
(233%) of these events to have at least onebtag (twob tags).
The systematic uncertainty in the cross section due to the Sbcorrection is 6%, dominated by the extrapolation from
the charm-contaminated low-ET sample to more energetic ttevents. Other leading systematic uncertainties arise from the integrated luminosity measurement (6%) [15], jet en- ergy corrections (5%), lepton isolation (2%), lepton iden- tification (2%), parton distribution functions (2%), choice of Monte Carlo generator (2%), and modeling of initial- and final-state radiation (1%). These systematic uncertain- ties are uncorrelated forttand are added in quadrature for the signal expectation.
The background tottis mostly due to direct production of a W boson with multiple jets (Wjets). Smaller con- tributions come from QCD jet production, in which theW signature is faked by jets appearing as electrons or by semileptonic b-hadron decays (non-W), and electroweak processes such as single top quark production, diboson (WW, WZ, and ZZ) production, and Z boson decays to tau pairs. We describe these backgrounds in turn and
TABLE II. Summary of event yields and background (Bkgd) expectations sorted by the number of jets in the event, for events with at least twob-tagged jets, assuming the measuredttcross section of 10.1 pb. TheHTrequirement is released for events with 2 jets. The Z!background is negligible.
W2jets W3jets W4jets W 5jets
Dibosons 0:680:12 0:110:02 0:0410:010 0:0170:005
t 2:20:5 0:530:15 0:120:04 0:0260:010
Wbb 11:33:6 1:30:4 0:210:08 0:0100:004
Wcc 0:920:41 0:220:11 0:070:04 0:0030:002
Wc 0:510:15 0:050:03 0:0210:012 0:0010:001
WLF 2:51:3 0:020:11 0:000:12 0:160:23
Non-W 0:590:59 0:280:28 0:50:5 0:350:35
Bkgd 18:74:3 2:51:2 1:00:8 0:60:5
tt 7:51:3 18:53:2 23:74:1 7:91:4
Total 26:34:5 21:03:4 24:64:2 8:41:4
Data 30 23 25 6
TABLE I. Summary of event yields and background (Bkgd) expectations sorted by the number of jets in the event. Event totals beforebtagging (Pretag) are listed in the first row; all other entries correspond to the sample with at least oneb-tagged jet, assuming the measuredttcross section of 8.7 pb. TheHTrequirement is released for events with fewer than 3 jets.
W1jet W2jets W3jets W4jets W 5jets
Pretag 30 283 4676 324 142 34
Dibosons 6:30:9 11:81:7 1:70:3 0:460:12 0:150:04
t 8:12:9 13:24:0 1:80:5 0:390:12 0:080:03
Z! 9:71:9 2:50:5 0:180:04 0:0170:003 0:0080:002
Wbb 11435 6319 6:11:6 1:40:7 0:170:09
Wcc 4613 309 3:81:2 1:10:4 0:130:04
Wc 12833 349 3:00:8 0:810:21 0:100:03
WLF 26157 10122 14:43:2 4:41:0 0:610:13
Non-W 5812 245 2:80:7 2:50:8 0:200:18
Bkgd 632100 27943 33:65:8 11:12:4 1:430:59
tt 3:50:4 27:93:2 52:95:8 56:76:1 18:22:0
Total 635100 30743 86:58:2 67:96:6 19:62:1
Data 722 346 80 71 23
summarize the results in Tables I and II. The total un- certainty accounts for correlations between the indi- vidual background sources and the tt prediction where appropriate.
We separate the contribution fromWjets into events with and without heavy-flavor jets. For the former, the fractions of Wjets events attributable to Wbb, Wcc, and Wc are estimated with ALPGEN and HERWIG
Monte Carlo programs [16,17], then scaled by a multi- plicative factor of1:50:4 to reproduce the b-tag rates observed in a control sample of inclusive jet data. The expected number of events is estimated by multiplying these fractions by the number of pretag events, after re- moving the pretag expectations for all other backgrounds andttsignal.
We estimate the background contribution fromWevents with only light-flavor jets by applying the mistag rate, measured in the inclusive jet data set and parametrized in jet ET, , , and the number of tracks and the total jet energy in the event, to the leptonjets data set. The mistag rate is adjusted higher by 3613% to account for heavy-flavor contamination in the jet data and residual contributions from material interactions andKS0=decays.
Finally, this result is adjusted down by the fraction of the data sample attributed to physics processes with heavy- flavor production.
The expectation for the non-W background is deter- mined using data. Assuming that the isolation of the lepton and the 6ET in these events are uncorrelated [3], we ex-
trapolate from the low-6ET and nonisolated regions (which contain fewer realW bosons) to predict the non-W content of the pretag and signal sample.
Finally, we use Monte Carlo calculations to estimate the backgrounds due to single top (PYTHIA and MADEVENT
[18]) and dibosons=Z! (PYTHIA), normalizing the expectations to their respective theoretical cross sections [19]. Here the b-tagging efficiency is evaluated analo- gously to thettsignal prediction.
Backgrounds that depend on the assumed ttcross sec- tion are calculated iteratively. The signal and background contributions to the b-tagged data sample, sorted by jet multiplicity, are summarized in TableIand Fig.1(a). The total corrected background in the signal region with at least onebtag is469events, where we observe 174 events.
We interpret the excess of events with three or more jets as pure tt signal, corresponding to a cross section of 8:7 0:9stat1:10:9systpb.
We observe 54 events with multiplebtags, the first time the event yield is inconsistent with the no-top hypothe- sis with significance greater than5. After correcting for tt, we expect 4:12:5 background events, a significant improvement in signal purity (Table III). We summarize these results in Table II and Fig. 1(b); the agreement in five-jet events underscores our ability to model initial- and final-state gluon radiation. We measure a cross section of 10:11:61:4stat2:01:3syst pb in the multiply b-tagged sample, consistent with the result for events with a single btag.
TABLE III. Summary of b-tagging information for the algorithms used, where b is the b-tagging efficiency forb-quark jets inttevents andtbt is the per-event efficiency fortt.
SECVTX TSECVTX JET PROBABILITY
b (%) 484 403 353
Mistag rate (%) 1:200:07 0:480:04 1:220:08
btags per event 1 2 1 1
tbt (%) 695 233 603 544
Observed signal=background 2.8 12.2 4.5 4.7
Number of Jets
1 2 3 4 ≥ 5
1 2 3 4 ≥ 5
Number of Events
0 100 200 300 400 500 600 700 800
Number of Jets
1 2 3 4 ≥ 5
1 2 3 4 ≥ 5
Number of Events
0 100 200 300 400 500 600 700
800 Data
t t
Diboson/Single Top W+Light Flavor Non-W W+Heavy Flavor
(a)
Number of Jets
2 3 4 ≥ 5
2 3 4 ≥ 5
Number of Events
0 5 10 15 20 25 30 35
Number of Jets
2 3 4 ≥ 5
2 3 4 ≥ 5
Number of Events
0 5 10 15 20 25 30
35 (b)
FIG. 1. Summary of background and signal event yields versus number of jets in the event when requiring (a) at least oneb-tagged jet and (b) at least two b-tagged jets. The tt contribution is normalized to the measured cross section in each sample. The HT requirement is released for events with fewer than 3 jets. The hashed region shows the uncertainty on the total expectation.
082004-6
The acceptance and efficiency both have a small depen- dence on the top quark mass; the cross section measure- ments change by0:08 pbfor each1 GeV=c2change in the assumed top quark mass from the initial value of 178 GeV=c2, in the range of160–190 GeV=c2.
We perform two cross-checks using alternateb-tagging algorithms. Table III compares the b-tagging character- istics of these checks with those of the main analyses. In the first, we repeat the analysis with an update of the original SECVTX algorithm described in Ref. [3]
(TSECVTX). We observe 138 events with at least one b tag, over an expected background of 255 events. The measured cross section is 8:70:9stat1:10:9systpb. In the second cross-check, we use theJET PROBABILITYalgo- rithm [20]. We observe 120 events with at least onebtag compared to a background of213events, corresponding to tt8:91:0stat1:11:0syst pb. Both cross-checks are in agreement with the lead result above, albeit with highly correlated uncertainties.
In summary, we have measured a tt production cross section of8:70:9stat1:10:9syst pbin pp collisions at
s
p 1:96 TeV using318 pb1 of data with at least one secondary vertexb tag. The result is consistent with the SM expectation of 6:10:60:8 pb for a top quark mass of 178 GeV=c2, which changes by 0:2 pb for every 1 GeV=c2 shift in the assumed mass. Additionally, we have studied a large, very pure sample ofttevents with at least twobtags, which will form the foundation for future high-precision measurements of top quark properties. In this sample, we measurett10:11:61:4stat2:01:3syst pb.
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 Particle Physics and Astronomy Research Council and the Royal Society, United Kingdom; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN- CT-2002-00292; and the Academy of Finland.
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