Article
Reference
Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV Using Missing E
T+ jets Events with Secondary Vertex b
Tagging
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al .
Abstract
We present a measurement of the tt production cross section in pp collisions at s√=1.96 TeV which uses events with an inclusive signature of significant missing transverse energy and jets. This is the first measurement which makes no explicit lepton identification requirements, so that sensitivity to W→τν decays is maintained. Heavy flavor jets from top quark decay are identified with a secondary vertex tagging algorithm. From 311 pb−1 of data collected by the Collider Detector at Fermilab, we measure a production cross section of 5.8±1.2(stat)+0.9−0.7(syst) pb for a top quark mass of 178 GeV/c2, in agreement with previous determinations and standard model predictions.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the tt Production Cross Section in pp Collisions at s√=1.96 TeV Using Missing E
T+ jets Events with Secondary Vertex b Tagging. Physical Review Letters , 2006, vol. 96, no. 20, p. 202002
DOI : 10.1103/PhysRevLett.96.202002
Available at:
http://archive-ouverte.unige.ch/unige:38341
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 Using Missing E
Tjets Events with Secondary Vertex b Tagging
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T. Yoshida,40I. Yu,27S. S. Yu,43J. C. Yun,16L. Zanello,49A. Zanetti,52I. Zaw,21F. Zetti,44X. Zhang,23 J. Zhou,50and 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
202002-2
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
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
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
46Purdue University, West Lafayette, Indiana 47907, USA
47University of Rochester, Rochester, New York 14627, USA
48The Rockefeller University, New York, New York 10021, USA
49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
53University of Tsukuba, Tsukuba, Ibaraki 305, Japan
54Tufts University, Medford, Massachusetts 02155, USA
55Waseda University, Tokyo 169, Japan
56Wayne State University, Detroit, Michigan 48201, USA
57University of Wisconsin, Madison, Wisconsin 53706, USA
58Yale University, New Haven, Connecticut 06520, USA (Received 22 March 2006; published 23 May 2006) We present a measurement of thettproduction cross section inppcollisions atps
1:96 TeVwhich uses events with an inclusive signature of significant missing transverse energy and jets. This is the first measurement which makes no explicit lepton identification requirements, so that sensitivity toW! decays is maintained. Heavy flavor jets from top quark decay are identified with a secondary vertex tagging algorithm. From311 pb1of data collected by the Collider Detector at Fermilab, we measure a production cross section of5:81:2stat0:90:7systpbfor a top quark mass of178 GeV=c2, in agreement with previous determinations and standard model predictions.
DOI:10.1103/PhysRevLett.96.202002 PACS numbers: 14.65.Ha, 13.85.Ni
At the Tevatron pp collider, top quarks are produced mainly in pairs through quark-antiquark annihilation and gluon-gluon fusion processes. In the standard model (SM), the calculated cross section for pair production is 6:10:60:8 pb[1] for a top mass of178 GeV=c2[2] and varies
linearly with a slope of 0:2 pb=GeV=c2 with the top quark mass in the range 170 GeV=c2< mt<
190 GeV=c2. Because the Cabibbo-Kobayashi-Maskawa matrix element Vtb is close to unity and mt is large, the SM top quark decays to a W boson and abquark almost
100% of the time. The final state of top quark pair produc- tion thus includes two W bosons and two b-quark jets.
When only one W decays leptonically, thettevent typi- cally contains a charged lepton, missing transverse energy from the undetected neutrino, and four high transverse energy jets, two of which originate from b quarks [3].
Previous cross section analyses [4 –6] select this distinct ttsignature by requiring well-identified leptons (e; ) with high transverse momentum. In this Letter, we describe att production cross section measurement which is sensitive to leptonicW decays regardless of the lepton type and has a sizable acceptance todecays of theWboson. The direct identification offromW !decays suffers from a very low efficiency; thus, our measurement, using data collected by a multijet trigger, selects top decays by inclusively requiring a high-pT neutrino signature rather than charged lepton identification. Events with well-identified high-pT electrons or muons are explicitly vetoed in order to avoid statistical overlap and provide complementary results with respect to lepton-based measurements.
Results reported in this Letter are obtained from 311 pb1 of data from pp collisions at
ps
1:96 TeV recorded by the Collider Detector at Fermilab (CDF II).
The CDF II detector consists of a magnetic spectrometer surrounded by calorimeter systems and muon chambers and has been described in detail elsewhere [7]. The beam luminosity is determined, with an uncertainty of 6% [8], using gas Cherenkov counters which measure the average number of inelasticpp collisions per bunch crossing.
The data sample used in this analysis is collected by a multijet trigger, which requires four or moreET 15 GeV clusters of contiguous calorimeter towers and a total trans- verse energy clustered in the calorimeter of PET 125 GeV. The initial data sample consists of 4:2106 events. The understanding of signal acceptances and effi- ciencies relies on a detailed simulation of the production processes and the detector response. Inclusively decaying ttevents, assuming a top quark mass of178 GeV=c2, are simulated using PYTHIA v6.2 [9] and HERWIG v6.5 [10]
generators in conjunction with the CTEQ5L [11] parton distribution functions, QQV9.1 [12] for the modeling of b andchadron decays, and a full simulation of the CDF II detector. Jets are identified as groups of calorimeter tower energy deposits which fall within a cone of radiusR
22
p 0:4. Jet energies, after the absolute en- ergy scale setting, are corrected for calorimeter nonlinear- ity, losses in the gap between towers, and multiple interactions [13].
The tt signature used in the present study (6ETjets) consists of large missing transverse energy6E~T, associated with the neutrino from the leptonic decay of aW boson, and jets. Since the 6ET resolution 6ET is observed to degrade as a function of the total transverse energy of the event, in the form 6ET /pPET
[14], the missing ET significance, defined by 6EsigT j6E~Tj= P
ET
p , is used for
event selection. Besides the requirement of large 6EsigT , further background rejection can be achieved by exploiting 6E~T-related geometrical properties. The neutrino direction fortt! 6ET jets events is, in general, uncorrelated with the jet directions in the transverse plane. On the contrary, background events for which the 6E~T is mainly due to jet energy mismeasurement or tob-quark semileptonic decays are more likely to have6E~Taligned with a jet direction. The minimumdifference between the6E~T and any jet in the event, min6ET;jets, is used as an analysis cut. Events containing identified high-pT electrons or muons, as de- fined in Ref. [5], are removed, in order to increase the relative contribution of W ! decays and to provide a statistically independent sample with respect to other lepton+jets cross section analyses.
To reject events with only light quark or gluon jets, we require at least one jet to be identified as originating from a bquark. The presence ofbjets (‘‘tags’’) is established by the identification of secondary decay vertices using the
SECVTXalgorithm [5]. An optimization procedure, aimed at minimizing the relative statistical uncertainty on the cross section measurement, sets NjetET 15 GeV;jj 2:0 4, 6EsigT 4:0 GeV1=2 and min6ET;jets 0:4 radas the best kinematical selection cuts. After these requirements, the data sample is reduced to 597 events with an expected pretag signal to background ratioS=B1=5.
Background events with btags arise from QCD heavy flavor production, electroweak production of W bosons associated with heavy flavor jets, and from false identifi- cation by the SECVTX algorithm. The overall number of backgroundbtags is estimated from the multijet sample by applying a parametrization of the per-jet tagging probabil- ity. The tagging probability is calculated using879 000 multijet data events with exactly three jets having ET 15 GeVandjj 2:0. It is parametrized on a per-jet basis as a function of the jet ET, track multiplicity, and the projection of the 6E~T along the jet direction, defined by 6EprjT 6E~Tcos6ET;jet. The tt contamination in this control sample is negligible. The jetb-tagging rate, calcu- lated as the ratio of the number of b-tagged jets to the number of jets with at least two good tracks with hits in the silicon detector, is shown in Fig. 1. The 6EprjT parametriza- tion accounts for background b tags frombb production processes, whose b-tag rate is enhanced at high positive values of the 6EprjT due to the correlation of 6E~T andb-jet direction in the case of a semileptonicb-quark decay. The extrapolation of the 3-jet b-tag rate to higher jet multi- plicity events is checked by comparing the predicted and observedb-tag rates as a function of the number of jets in the multijet sample without the kinematical selection on 6EsigT and min6ET;jets, as shown in Fig. 2(a). The capability of the parametrization to track possible sample composition changes introduced by the kinematical 202002-4
selection is established by constructing the ratio of ob- served over expected b tags in two separate control samples of multijet data, depleted of signal contamina- tions, as shown in Figs. 2(b) and 2(c), respectively. The first control sample is defined by 6EsigT 3:0 GeV1=2, min6ET;jets 0:3 rad; the second as 6EsigT 3:0 GeV1=2, min6ET;jets 0:3 rad. The b-tag rate parametrization is found to predict the non-ttbackground to within 10% in the4 Njet 6region where 96.4% of thettsignal is expected. This value is thus adopted as the systematic uncertainty on the background prediction.
Additional checks, using low jet multiplicity events from leptonic W data samples, show agreement between ob- served and predicted b tags to within the quoted uncertainty.
The sample selected with the optimized kinematical requirements described above and the requirement of at least oneb-tagged jet consists of 106 events, containing a total of 127 b-tagged jets. The number of b-tagged jets yielded by background processes in that sample is expected to beNexp67:42:76:7. The first uncertainty con- tribution is due to the limited number of events in the 3-jet sample used for the tagging rate parametrization, while the second contribution is the 10% systematic uncertainty on the b-tag rate parameterization discussed above (Fig. 2).
The 597 pretag data events are expected to contain a non-
negligible tt component, due to the S=B enhancement provided by the kinematical selection, which yields a background overestimate. The Nexp value is corrected, to N0exp57:48:1, to account for this effect. Table I sum- marizes the number of b-tagged jets expected from Monte Carlo simulation for eachttdecay mode satisfying the kinematical and1b-tag requirements, as well as the predicted and observed btags in the selected data sample as a function of the jet multiplicity of the events. In Table I, the numbers ofbtags from events with only three jets are provided as a cross-check of the background. The excess in the number of btags, forNjet4, is ascribed to top pair production. Final states with W !decays account for 44%of the signal acceptance.
In order to further establish thettsignal in the selected data, we perform binned likelihood fits to kinematical distributions. The stability of the fitting technique is checked using simulations with known signal fractions.
We fit 6ET and 6ET;tagged jet data distributions to the sum of attand a background template. The former is obtained from Monte Carlottinclusive events; the latter is derived from the tagging rate parametrization applied to the data. Results for the 6ET;tagged jet fit are dis- FIG. 2 (color online). Ratio of observed to expectedbtags as a function of the number of jets in the data before the optimized kinematical selection on 6EsigT and min6ET;jets (a) and in control samples: 6EsigT 3:0 GeV1=2, min6ET;jets 0:3 rad (b) and 6EsigT 3:0 GeV1=2, min6ET;jets 0:3 rad(c).
[GeV]
Jet ET
20 40 60 80 100 120 140 160 180 200 220
b-tag rate
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
CDF Run II, L = 311 pb-1
(a) CDF Run II, L = 311 pb-1
Jet track Multiplicity
2 3 4 5 6 7 8 9 10 11
b-tag rate
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
(b)
projection [GeV]
Jet Missing ET
-60 -40 -20 0 20 40 60
b-tag rate
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
(c)
FIG. 1 (color online). Tagging rate probability parametrization in the multijet sample as a function of (a) jet ET, (b) track multiplicity, and (c) missingET projection.
played in Fig. 3. The fittedtt components in the selected data [6812% and 4412% for the 6ET and 6ET;tagged jet fits, respectively] are in agreement with the overall prediction calculated fromb-tag counting, before any correction to account for thettcontamination in the pretagging data sample (475%determined compar- ing the number of expected and observedbtags, forNjet 4, in Table I).
The efficiency of the trigger, the kinematical selection, and theb-tagging algorithm are evaluated using inclusive tt Monte Carlo events. The combined efficiency of the trigger and kinematical selection amounts to kin 4:88%0:43% for a top mass of mt178 GeV=c2, where the dominant uncertainty is determined by the com- parison of the results from the PYTHIA andHERWIG gen- erators. Other sources of systematic uncertainty are
evaluated by varying Monte Carlo generation settings with respect to the default values and by applying system- atic variations of the jet energy correction factors; the trigger acceptance uncertainty is determined by comparing trigger turn-on curves between Monte Carlo and data events (Table II). The average number of b tags per Monte Carlo tt event is found to be avetag 0:789 0:046, and it is corrected according to the Monte Carlo
SECVTXefficiency scale factor of0:9090:060to repro- duce the datab-tagging efficiency [5].
The cross section is calculated with a Poisson likelihood function in which the maximum likelihood solution for tt is given by: tt N obsNexp0 =kinavetag Lint, where Nobs and N0exp are the number of b-tagged jets observed and expected from background events by the tagging rate parametrization in the selected data, andLint
is the integrated luminosity of the multijet data sample. The input parameters Lint, kin, avetag, and Nexp0 are subject to Gaussian constraints. With these input val- ues, we measure a top pair production cross section of 5:81:2stat0:90:7syst pb. Additional samples of inclu-
TABLE II. Relevant sources of systematic uncertainty.
Source Relative error
kinsystematics
Generator dependence 8.2%
Trigger acceptance 2.0%
Gluon radiation 2.0%
Parton distribution functions 1.6%
Jet energy scale 1.5%
Others
Background prediction method 10.0%
Luminosity measurement 6.0%
avetag (SECVTXscale factor) 5.8%
(MET, tagged jet) [rad]
φ
0 0.5 1 1.5∆ 2 2.5 3
tagged jets/0.16 rad
0 5 10 15 20
25 CDF Run II data, L = 311 pb-1
MC: 0.44 +/- 0.12 t
t
BKG: 0.56 +/- 0.12 /NDF = 0.91 χ2
Fit result,
FIG. 3 (color online). 6ET;tagged jetdistribution for data after kinematical selection and with at least one b-tagged jet.
The data are fit to the sum ofttsignal and background templates as described in the text.
TABLE I. Number ofb-tagged jets expected from Monte Carlottproduction usingtt 6:1 pb(mt178 GeV=c2), predicted by the tagging rate parametrization, and observed in the selected sample, as a function of the jet multiplicity. The total uncertainty on the number of predicted backgroundbtags is the sum in quadrature of the statistical uncertainty and of a 10% systematic uncertainty.
The number of backgroundbtags corrected for thettcontamination in the pretagging data sample is also provided for the signal region. Uncertainties on signal contributions are statistical only.
Number of jets 3 4 5 6
tt!ll0l; l0e= 0:150:02 0:750:04 0:300:02 0:100:02
tt!l(le==) 0:220:02 1:800:05 0:790:04 0:260:02
tt!ejets 0:680:04 6:610:11 8:700:13 4:250:09
tt!jets 1:070:04 11:920:15 6:560:11 2:470:07
tt!jets 1:000:04 10:980:14 11:710:15 5:530:10
tt!jets 0:010:01 0:090:01 0:140:02 0:220:02
tt!X 3:130:08 32:150:24 28:140:23 12:830:15
Backgroundb-tagged jets 32:683:46 37:534:14 21:442:76 8:471:40
Corrected backgroundb-tagged jets 33:144:01 17:582:85 6:712:78
Observedb-tagged jets 31 53 55 19
202002-6
sive tt Monte Carlo events generated with different mt values in the range130;230 GeV=c2are used to compute the cross section measurement dependence on mt. The cross section measurement changes by0:05 pbfor each 1 GeV=c2 change in the top quark mass from the initial value of178 GeV=c2. For instance, we measure tt 6:01:2stat0:90:7systpbassuming a top quark mass of 175 GeV=c2. The change is due to the varying signal selection efficiency with top quark mass.
In conclusion we report the first measurement of the top pair production cross section oftt 5:81:51:4 pb using an inclusive selection of6ETjetsttdecays. The result is complementary to other cross section analyses [4 – 6], maintains high sensitivity with respect toW ! ttde- cays, and is in good agreement with SM calculations [1]
and previous measurements.
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 Foun- dation; 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 by the Academy of Finland.
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