Article
Reference
Top-Quark Mass Measurement Using Events with Missing Transverse Energy and Jets at CDF
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a measurement of the top-quark mass using a sample of tt events in 5.7 fb−1 of integrated luminosity from pp collisions at the Fermilab Tevatron with s√=1.96 TeV and collected by the CDF II Detector. We select events having large missing transverse energy, and four, five, or six jets with at least one jet tagged as coming from a b quark, and reject events with identified charged leptons. This analysis considers events from the semileptonic tt decay channel, including events that contain tau leptons. The measurement is based on a multidimensional template method. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass of Mtop=172.32±2.4(stat)±1.0(syst) GeV/c2.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Top-Quark Mass Measurement Using Events with Missing Transverse Energy and Jets at CDF. Physical Review Letters , 2011, vol. 107, no. 23, p. 232002
DOI : 10.1103/PhysRevLett.107.232002
Available at:
http://archive-ouverte.unige.ch/unige:38684
Disclaimer: layout of this document may differ from the published version.
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Top-Quark Mass Measurement Using Events with Missing Transverse Energy and Jets at CDF
T. Aaltonen,20B. A´ lvarez Gonza´lez,8,xS. Amerio,41aD. Amidei,31A. Anastassov,35A. Annovi,16J. Antos,11 G. Apollinari,14J. A. Appel,14A. Apresyan,43T. Arisawa,51A. Artikov,12J. Asaadi,47W. Ashmanskas,14B. Auerbach,54 A. Aurisano,47F. Azfar,39W. Badgett,14A. Barbaro-Galtieri,25V. E. Barnes,43B. A. Barnett,22P. Barria,44c,44aP. Bartos,11 M. Bauce,41b,41aG. Bauer,29F. Bedeschi,44aD. Beecher,27S. Behari,22G. Bellettini,44b,44aJ. Bellinger,53D. Benjamin,13 A. Beretvas,14A. Bhatti,45M. Binkley,14,aD. Bisello,41b,41aI. Bizjak,27,bbK. R. Bland,5B. Blumenfeld,22A. Bocci,13 A. Bodek,44D. Bortoletto,43J. Boudreau,42A. Boveia,10L. Brigliadori,6b,6aA. Brisuda,11C. Bromberg,32E. Brucken,20 M. Bucciantonio,44b,44aJ. Budagov,12H. S. Budd,44S. Budd,21K. Burkett,14G. Busetto,41b,41aP. Bussey,18A. Buzatu,30 C. Calancha,28S. Camarda,4M. Campanelli,27M. Campbell,31F. Canelli,10,14B. Carls,21D. Carlsmith,53R. Carosi,44a
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K. Yorita,51T. Yoshida,38,kG. B. Yu,13I. Yu,24S. S. Yu,14J. C. Yun,14A. Zanetti,52aY. Zeng,13and S. Zucchelli6b,6a
(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, ICREA, 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
6University of California, Davis, Davis, California 95616, USA
7University of California, Los Angeles, Los Angeles, California 90024, USA
8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
11Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
12Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
13Duke University, Durham, North Carolina 27708, USA
14Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
15University of Florida, Gainesville, Florida 32611, USA
16Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
17University of Geneva, CH-1211 Geneva 4, Switzerland
18Glasgow University, Glasgow G12 8QQ, United Kingdom
19Harvard University, Cambridge, Massachusetts 02138, USA
20Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
21University of Illinois, Urbana, Illinois 61801, USA
22The Johns Hopkins University, Baltimore, Maryland 21218, USA
23Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
24Center 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
25Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
26University of Liverpool, Liverpool L69 7ZE, United Kingdom
27University College London, London WC1E 6BT, United Kingdom
28Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
29Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
232002-2
30Institute 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
31University of Michigan, Ann Arbor, Michigan 48109, USA
32Michigan State University, East Lansing, Michigan 48824, USA
33Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
34University of New Mexico, Albuquerque, New Mexico 87131, USA
35Northwestern University, Evanston, Illinois 60208, USA
36The Ohio State University, Columbus, Ohio 43210, USA
37Okayama University, Okayama 700-8530, Japan
38Osaka City University, Osaka 588, Japan
39University of Oxford, Oxford OX1 3RH, United Kingdom
41aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
41bUniversity of Padova, I-35131 Padova, Italy
40LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
41University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
44bUniversity of Pisa, I-56127 Pisa, Italy
44cUniversity of Siena, I-56127 Pisa, Italy
44dScuola Normale Superiore, I-56127 Pisa, Italy
42University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
43Purdue University, West Lafayette, Indiana 47907, USA
44University of Rochester, Rochester, New York 14627, USA
45The Rockefeller University, New York, New York 10065, USA
49aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
49bSapienza Universita` di Roma, I-00185 Roma, Italy
46Rutgers University, Piscataway, New Jersey 08855, USA
47Texas A&M University, College Station, Texas 77843, USA
52aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy
52bUniversity of Udine, I-33100 Udine, Italy
48University of Tsukuba, Tsukuba, Ibaraki 305, Japan
49Tufts University, Medford, Massachusetts 02155, USA
50University of Virginia, Charlottesville, Virginia 22906, USA
51Waseda University, Tokyo 169, Japan
52Wayne State University, Detroit, Michigan 48201, USA
53University of Wisconsin, Madison, Wisconsin 53706, USA
54Yale University, New Haven, Connecticut 06520, USA (Received 7 September 2011; published 30 November 2011)
We present a measurement of the top-quark mass using a sample ofttevents in5:7 fb1of integrated luminosity fromppcollisions at the Fermilab Tevatron withpffiffiffis
¼1:96 TeVand collected by the CDF II Detector. We select events having large missing transverse energy, and four, five, or six jets with at least one jet tagged as coming from abquark, and reject events with identified charged leptons. This analysis considers events from the semileptonicttdecay channel, including events that contain tau leptons. The measurement is based on a multidimensional template method. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass ofMtop¼172:32 2:4ðstatÞ 1:0ðsystÞGeV=c2.
DOI:10.1103/PhysRevLett.107.232002 PACS numbers: 14.65.Ha, 12.15.Ff, 13.85.Qk
The top quark (t) is the heaviest known elementary particle, with a mass approximately 40 times heavier than the mass of the bottom quark (b), its partner in the weak isospin doublet. The top-quark mass can be used as a consistency check for standard model (SM) parameters.
For example, the top quark contributes significantly to electroweak radiative corrections relating the mass of the top quark,Mtop, and that of theWboson to the mass of the
predicted Higgs boson within the SM and in new physics models [1,2]. Therefore, combined precision measure- ments of theW mass andMtop provide an important con- straint on the Higgs boson mass. At the Tevatron, the top quark is predominantly produced inttpairs. As in the SM the top quark decays almost exclusively to aWboson and a bquark, the expected signature of attproduction event is tt!WþbWb, assuming unitarity of the three-generation
Cabibbo-Kobayashi-Maskawa matrix [3]. Since theW bo- son subsequently decays either to a quark-antiquark pair or to a lepton-neutrino pair, the final state ofttproduction can be classified by the number of charged leptons produced. In this Letter, we focus on events with large missing trans- verse energy (6ET) [4] as expected for undetected energetic neutrinos, accompanied by jets. We explicitly veto events with identified highpT electrons or muons (leptonþjets events) as well as multijet events where both W bosons decay hadronically (all-hadronic events). This ensures that our result is statistically independent from other CDF top-quark mass measurements [5–8] and allows for a future combination with them. A previous measurement of Mtop in this final state used an integrated luminosity of311 pb1 [9] and yieldedMtop ¼172:315:3ðstatÞ 14:4ðsystÞGeV=c2. Although no identified leptons are ex- plicitly required, our measurement is sensitive to all W leptonic decays. This includes decays toleptons, which constitute approximately 40% of the signal sample in our final selection. The frequency of observed lepton final states is predicted to be enhanced by new physics models such as a charged Higgs decay [10], therefore, a signifi- cantly different measurement ofMtopin this decay channel could indicate contributions from non-SM [11] physics processes.
We use data corresponding to an integrated luminosity of5:7 fb1 ofpp collisions at the Fermilab Tevatron, and collected by the CDF II Detector [12]. The sample of events used in this measurement is a subset of events that initially passed a trigger requirement, which accepted events with at least four calorimetric clusters [13] of ET>15 GeVand a scalar sumETof these clusters greater than 175 GeV [14]. After the trigger selection, event ob- servables of physical interest are computed. Jets are recon- structed with theJETCLU[15] algorithm using a cone radius ofR¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2þ2
p ¼0:4[16]. Jets are corrected [17]
for nonuniformities of the calorimeter response as a func- tion of, energy contributed by multipleppinteractions in the event, and calorimeter nonlinear response. To deter- mine if a jet originated from abquark, a secondary vertex algorithm [18] is applied. This algorithm identifies jets that are likely to come frombquark hadronization through the presence of a displaced vertex within the jets (btag). We require at least one jet to be identified as abjet (btagged).
We divide the sample of candidate events into two, sepa- rating events with oneb-tagged jet (1 tag) from events with two or moreb-tagged jets (2 tag). Events are required to have at least four and at most six jets with transverse energy ET>15 GeV and jj<2:0. To avoid overlap with other CDF top-quark mass measurements, we reject events with reconstructed electrons or muons with pT >
20 GeV=c and jj<1:0 (leptonþjets final state), and events with6ET significance below3 GeV1=2 (all-hadronic final state), where the6ETsignificance is defined as6ETsig¼
6
ET= jetsET. For further rejection of multijet back- grounds from QCD processes, we require minð6ET;jetÞ>0:4, whereminð6ET;jetÞis the smallest separation in the anglebetween jets andE6 T [14].
Background events withbtags arise from QCD multijet and electroweak production of W bosons associated with heavy flavor jets. In order to improve the ratio ofttsignal in the semileptonic channel to the background of this analysis, an artificial neural network is trained to identify the kinematic and topological characteristics of SM tt events using eight input variables [14]. We apply the neural network to all events passing the above selections, and make a cut on neural network score that retains 81% of the tt signal events while rejecting 91% of background events. The selection criteria listed above define the ‘‘sig- nal region.’’ We follow Ref. [14] and estimate the back- ground rate using a data-driven method. The method uses events with exactly three jets and employs a per-jet pa- rametrization of theb-tagging probability. Because of the presence of tt events in samples with higher jet multi- plicity, we extrapolate the b-tagging probability of the three-jet event sample to higher jet multiplicity events by iteratively removing the ttcontent from the sample [14].
We estimate the background for the 1-tag and 2-tag samples separately. A b-tagging correction factor [19] is applied to take into account the fact that most of the heavy flavor jets are produced in pairs. With this procedure we obtain the estimated number of background events in the signal region shown in TableI. We also show the estimated number ofttsignal events, assuming attproduction cross section of 7.5 pb at Mtop¼172:5 GeV=c2 [20], together with the number of observed events in the data.
Monte Carlo (MC)-simulated tt samples are generated by PYTHIA[21] of 76 differentMtop values ranging from 150 GeV=c2 to 240 GeV=c2, with increments from 0:5 GeV=c2 in the region immediately above and below 172:5 GeV=c2 to 5 GeV=c2 near the extreme mass re- gions. For each of them we reconstruct the events with different values of JES, the difference between the jet energy scale (JES) in the MC simulation and the data. A
TABLE I. Number of expected signal and background events and observed data events with integrated luminosity of5:7 fb1 in the signal region. Thettsignal contribution is estimated with a cross section of 7.5 pb, and the background events are dominant with processes of QCD multijet and electroweak production of W bosons associated with heavy flavor quarks. All selection requirements are applied, and events are separated into 1-tag and 2-tag categories.
Events 1-tag 2-tag
Expectedttsignal 644:3118:7 262:950:3 Expected background 410:631:7 43:811:0 Total expectation 1054:9122:9 306:751:5
Observed data 1147 285
232002-4
total of 27 differentJES values, ranging from3:0c to þ3:0c, wherec is the uncertainty on the JES [22], are used to reduce the systematic effects due to the jet energy uncertainty, as described later.
For each MC-simulated sample in this analysis, we reconstruct three variables using the leading four or five jets to form the templates. The first variable,mjj, defined as the invariant mass of the two jets from the hadronically decayingWboson, serves as anin situconstraint of the JES through the likelihood fit described later. We calculatemjj from the two non-b-tagged jets whose invariant mass produces the closest value to the world average W boson mass of80:40 GeV=c2 [23]. We also reconstruct the top- quark mass (mhad;recot ) from the invariant mass of the three jets whose momentum sum yields the largestpT since the invariant mass constructed this way has a large correlation with the hadronically decaying top-quark mass. To en- hance the probability that the jets used to compute mhad;recot come from the hadronically decaying top quark, we add two constraints to the calculation ofmhad;recot : first, mhad;recot must contain the two jets that form mjj, and second, in 2-tag events the third jet of mhad;recot must be b tagged. A third variable, mhad;recoð2Þ
t , is defined as the invariant mass of three jets, two of which are the pair which definesmjj. The third jet ofmhad;recoð2Þ
t is required to be the most energetic jet of those not formingmhad;recot , and is also required to be b tagged in a 2-tag event. The variable mhad;recoð2Þ
t plays a complementary role to mhad;recot in ex- tracting information on the top-quark mass, and is particu- larly important in events where the three jets used to compute mhad;recot were not the actual decay products of the hadronically decaying top quark.
The template method used in the extraction of Mtop requires that a probability density function (pdf ) be built for each template. For each MC signal and background sample, we estimate the pdfs using the kernel density estimation (KDE) [24,25] that employs a nonparametric method to construct pdfs. For each sample, we build a three dimensional pdf from the reconstructed observables (mjj, mhad;recot , and mhad;recoð2Þ
t ), taking their correlations into account. To measureMtop, we fit the signal and back- ground pdfs to the distributions of the observables in the data using an unbinned maximum likelihood fit [26] where we minimize the negative logarithm of the likelihood using
MINUIT [27]. The likelihood fits Mtop and JES simulta- neously, and is built separately for each subsample, 1-tag and 2-tag events, in order to improve the usage of statistical information. References [25,28] provide detailed informa- tion about this technique.
The mass fitting procedure is tested with pseudoexperi- ments for a set of MC tt samples with 14 different Mtop values ranging from159 GeV=c2to185 GeV=c2. For each pseudoexperiment we select the number of background
events from a Poisson distribution with a mean equal to the expected total number of background events in the sample and the number of signal events from a Poisson distribution with a mean equal to the expected number of signal events normalized to attproduction cross section of 7.5 pb [20]. The distributions of the average mass residual (the deviation from the input top-quark mass) and the width of the pull (the ratio of the residual to the uncer- tainty) for simulated experiments are corrected to be zero and unity, respectively. The correction is Mtcorr¼1:24 Mtmeas40:6 GeV=c2, whereMtmeasis the raw value from the likelihood fit and Mcorrt is the corrected value of the measurement. The measured uncertainty is correspond- ingly increased by 20% to correct the width of the pull distribution.
We examine various sources of systematic uncertainties that could affect the measurement by comparing the results of pseudoexperiments in which we vary relevant parame- ters within their uncertainties. One of the dominant sources of systematic uncertainty is the residual JES [6,22]. We vary the JES components within their uncertainties in the generated signal MC events and interpret the shifts in the returned top-quark mass as uncertainties. Thebjet energy scale systematic uncertainty that arises from the modeling of b fragmentation, b hadron branching fractions, and calorimeter response captures the additional uncertainty not taken into account in the light quark jet energy scale [6]. The uncertainty arising from the choice of MC gen- erator is estimated by comparing results from MC- simulated samples generated with PYTHIA and HERWIG
[29]. We estimate the systematic uncertainty due to im- perfect modeling of initial-state gluon radiation (ISR) and final-state gluon radiation (FSR) by varying the amounts of ISR and FSR in simulated events [30]. We estimate the systematic uncertainty due to parton distribu- tion functions (PDF’s) of the proton by varying the inde- pendent eigenvectors of the CTEQ6M [31] PDF’s, varying QCD(228 MeV vs 300 MeV), and comparing CTEQ5M [32] with MRST72 [33] PDF’s. To estimate the systematic uncertainty associated with uncertainties in the top-quark production mechanism, we vary the fraction of the top quarks produced by gluon-gluon annihilation from the default 6% to 20%, corresponding to a 1 standard deviation upper bound on the gluon fusion fraction [34]. We also evaluate the uncertainty due to background modeling effects by reweighting the background shape up and down and comparing the resulting measurements. We ap- ply an additional uncertainty to account for the effect of the trigger simulation in the signal MC samples, in a similar way to the background shape systematic uncertainty estimation. We also estimate an uncertainty due to the effect of multiple hadron interactions, which takes into account the increasing instantaneous luminosity in this data set. The color reconnection (CR) systematic uncer- tainty [35] is evaluated using MC samples generated with
and without CR effects adopting different tunes [36] of
PYTHIA. Table II summarizes the individual systematic uncertainties considered, giving a total systematic uncer- tainty of1:0 GeV=c2for the measurement ofMtop.
By applying a likelihood fit to the data using the three observables described above and the corrections obtained from the simulated experiments, the top-quark mass is measured to be
Mtop¼172:32:4ðstatÞ 1:0ðsystÞ GeV=c2
¼172:32:6 GeV=c2: (1)
Figure1shows the distribution of the observables used for the Mtop measurement overlaid with their probability
density functions from tt signal events with Mtop¼ 172:5 GeV=c2 and the estimated background.
In conclusion, we have performed a measurement of the top-quark mass in events with large E6 T and jets, corre- sponding to an integrated luminosity of5:7 fb1. The data sample has been chosen in such a way as to exclude events used in other CDF top-quark mass measurements. The result, Mtop¼172:32:6 GeV=c2, is approximately a factor of 6 improvement from the previous measurement in this channel [9], and is in agreement with other mea- surements which contribute to the world average ofMtop¼ 173:20:9 GeV=c2[37].
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 Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, U.K.; 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; the Academy of Finland;
and the Australian Research Council (ARC).
2) (GeV/c mjj
40 60 80 100 120 140
)2 Events/(6.25 GeV/c 0
50 100 150 200 250 300 350
-1) Data (5.7 fb Signal+Bkgd Bkgd only
1-tag mjj
2) (GeV/c
had,reco
mt
50 100 150 200 250 300 350 400 450 )2 Events/(16 GeV/c
0 50 100 150
200 Data (5.7 fb-1)
Signal+Bkgd Bkgd only
1-tag
had,reco
mt
2) (GeV/c
had,reco(2)
mt
50 100 150 200 250 300 350 400 450 )2 Events/(16 GeV/c
0 50 100 150
200 Data (5.7 fb-1)
Signal+Bkgd Bkgd only
1-tag
had,reco(2)
mt
2) (GeV/c mjj
50 100 150 200
)2 Events/(8 GeV/c
0 20 40
60 Data (5.7 fb-1)
Signal+Bkgd Bkgd only 2-tag mjj
2) (GeV/c
had,reco
mt
50 100 150 200 250 300 350 400 450 )2 Events/(16 GeV/c
0 10 20 30 40 50
60 Data (5.7 fb-1)
Signal+Bkgd Bkgd only
2-tag
had,reco
mt
2) (GeV/c
had,reco(2)
mt
50 100 150 200 250 300 350 400 450 )2 Events/(16 GeV/c
0 10 20 30 40
50 Data (5.7 fb-1)
Signal+Bkgd Bkgd only
2-tag
had,reco(2)
mt
FIG. 1 (color online). Distribution of kinematic variables mjj,mhad;recot , and mhad;recoð2Þ
t from data (points), overlaid with their corresponding one dimensional pdfs from signal MC sample (Mtop¼172:5 GeV=c2, hashed area) plus the estimated background (filled area). Both 1-tag (top) and 2-tag (bottom) events are displayed.
Systematic sources Uncertainty (GeV=c2)
Residual JES 0.5
b-JES 0.3
MC generator 0.7
Radiation 0.2
PDFs 0.2
ggfraction <0:1
Background 0.1
Trigger modeling 0.1
Multiple hadron interaction 0.2
Color reconnection 0.2
Total systematic uncertainty 1.0
232002-6
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