Article
Reference
Search for electroweak single-top-quark production in pp collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We report on a search for standard model t-channel and s-channel single-top quark production in pp collisions at a center of mass energy of 1.96 TeV. We use a data sample corresponding to 162 pb−1 recorded by the upgraded collider detector at Fermilab. We find no significant evidence for electroweak top quark production and set upper limits at the 95%
confidence level on the production cross section, consistent with the standard model: 10.1 pb for the t-channel, 13.6 pb for the s-channel and 17.8 pb for the combined cross section of t- and s-channel.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for electroweak
single-top-quark production in pp collisions at s√=1.96 TeV. Physical Review. D , 2005, vol. 71, no. 01, p. 012005
DOI : 10.1103/PhysRevD.71.012005
Available at:
http://archive-ouverte.unige.ch/unige:38288
Disclaimer: layout of this document may differ from the published version.
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Search for electroweak single-top-quark production in pp collisions at p s
1:96 TeV
D. Acosta,16J. Adelman,12T. Affolder,9T. Akimoto,54M. G. Albrow,15D. Ambrose,43S. Amerio,42D. Amidei,33 A. Anastassov,50K. Anikeev,31A. Annovi,44J. Antos,1M. Aoki,54G. Apollinari,15T. Arisawa,56J-F. Arguin,32 A. Artikov,13W. Ashmanskas,15A. Attal,7F. Azfar,41P. Azzi-Bacchetta,42N. Bacchetta,42H. Bachacou,28W. Badgett,15 A. Barbaro-Galtieri,28G. J. Barker,25V. E. Barnes,46B. A. Barnett,24S. Baroiant,6M. Barone,17G. Bauer,31F. Bedeschi,44
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J. Zhou,50A. Zsenei,18and S. Zucchelli4
(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
4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
5Brandeis University, Waltham, Massachusetts 02254, USA
6University of California at Davis, Davis, California 95616, USA
7University of California at Los Angeles, Los Angeles, California 90024, USA
8University of California at San Diego, La Jolla, California 92093, USA
9University of California at Santa Barbara, Santa Barbara, California 93106, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14Duke University, Durham, North Carolina 27708, USA
15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
16University of Florida, Gainesville, Florida 32611, USA
17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
18University of Geneva, CH-1211 Geneva 4, Switzerland
19Glasgow University, Glasgow G12 8QQ, United Kingdom
20Harvard University, Cambridge, Massachusetts 02138, USA
21The Helsinki Group, Helsinki Institute of Physics and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00044, Helsinki, Finland
22Hiroshima University, Higashi-Hiroshima 724, Japan
23University of Illinois, Urbana, Illinois 61801, USA
D. ACOSTAET AL. PHYSICAL REVIEW D71,012005 (2005)
012005-2
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 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, 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, University and Scuola Normale Superiore of Pisa, I-56100 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 di Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Texas Tech University, Lubbock, Texas 79409, USA
53Istituto Nazionale di Fisica Nucleare, University of Trieste, Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA (Received 19 October 2004; published 28 January 2005)
We report on a search for standard modelt-channel ands-channel single-top quark production inpp collisions at a center of mass energy of 1.96 TeV. We use a data sample corresponding to162 pb1 recorded by the upgraded collider detector at Fermilab. We find no significant evidence for electroweak top quark production and set upper limits at the 95% confidence level on the production cross section, consistent with the standard model: 10.1 pb for thet-channel, 13.6 pb for thes-channel and 17.8 pb for the combined cross section oft- ands-channel.
DOI: 10.1103/PhysRevD.71.012005 PACS numbers: 14.65.Ha, 12.15.Ji, 13.85.Rm
In pp collisions at 1.96 TeV, top quarks are predomi- nantly produced in pairs via strong interaction processes.
Within the standard model (SM), top quarks are also ex- pected to be produced singly by the electroweak interac- tion involving aWtbvertex [1]. At the Tevatron, the two relevant production modes are the t- and the s-channel exchange of a virtual W boson. The measurement of the single-top cross section is particularly interesting because the production cross section is proportional tojVtbj2, where Vtb is the Cabibbo-Kobayashi-Maskawa (CKM) matrix element which relates top and bottom quarks. Assuming
three quark generations, the unitarity of the CKM matrix implies thatVtbis close to unity [2]. The most recent next- to-leading order (NLO) calculations, assuming jVtbj 1, predict cross sections of1:980:25pbfor thet-channel and 0:880:11pb for the s-channel mode at
ps 1:96 TeV[3]. Using these predictions, a measurement of the single-top cross section will allow for a direct determi- nation ofjVtbj. Single-top searches test also exotic models which predict anomalously altered single-top production rates [4]. Moreover, single-top quark processes result in the same final state as the standard model Higgs boson process . . .
WH!‘bband therefore impact future searches for the Higgs boson at the Tevatron [5]. In this article we report results of the first search for single-top production in Run 2 at the Tevatron. Results of searches for single-top produc- tion at
ps
1:8 TeV(Run 1) can be found in Refs. [6,7].
The experimental signature of single-top events consists of theW decay products plus two or three jets, including one b quark jet from the decay of the top quark. To suppress QCD multijet background we select only W ! and W!ee candidates. In s-channel events we expect a second b quark jet from the Wtb vertex. In t-channel events a second jet originates from the recoiling light-quark and a third jet is produced through the splitting of the initial-state gluon into abbpair. Mostly, this third jet escapes detection, since it is produced in the high pseudor- apidity () regions and at low transverse energy (ET). The polar angleis measured with respect to the proton beam direction. The pseudorapidity is defined as lntan=2.ET Esin.1
This article describes two analyses: (1) a combined search for thet-channel plus s-channel single-top signal, (2) a separate search, where we measure the rates for the two single-top processes individually. The data sample corresponds to an integrated luminosity of 162 10pb1 collected with the upgraded collider detector at Fermilab (CDF II), which is described elsewhere [8]. The common event preselection of our two analyses resembles closely the one used in the CDF measurement of the tt cross section reported in Ref. [9]. We accept events with evidence for a leptonic W decay: (a) missing transverse energyET= >20 GeV from the neutrino and (b) an iso- lated central electron with ET>20 GeV or an isolated central muon candidate withpT>20 GeV=c. An electron or muon candidate is considered isolated if the nonlepton ET in an – cone of radius 0.4 centered around the lepton is less than 10% of the leptonET orpT. To remove dilepton events from tt-production and leptonic Z boson decays, we accept events with only one well identified lepton. In addition, we veto events if we find a second, loosely identified lepton candidate that forms an invariant mass with the primary lepton between 76 GeV=c2<
M‘‘<106 GeV=c2. The jet reconstruction uses a fixed
cone of radius R
22
p 0:4. We count jets with transverse energyET 15 GeV and jj 2:8. We only accept W2 jets events. At least one of these jets must be identified as likely to originate from a b quark (b-tag) by requiring a displaced secondary vertex within the jet as measured using silicon tracker information. The effective coverage of theb-tagging ranges up tojj&1:4.
To optimize our sensitivity, we apply a cut on the invariant mass M‘b of the charged lepton, the neutrino
and the b-tagged jet: 140 GeV=c2M‘b210 GeV=c2. The transverse momentum of the neutrino is set equal to the missing transverse energy vectorE6~T;pzis obtained up to a two-fold ambiguity from the constraintM‘MW. From the two solutions we pick the one with lowerjpzj.
If the pz solution has nonzero imaginary part as a consequence of resolution effects in measuring jet ener- gies, we use only the real part of pz. For the separate search, we subdivide the sample into events with exactly oneb-tagged jet or exactly twob-tagged jets. For the 1-tag sample, we require at least one jet to haveET 30 GeV.
We determine the total event detection efficiency evt for the signal from events generated by the matrix element event generator MadEvent [10], followed by parton show- ering with PYTHIA [11] and a full CDF II detector simu- lation. MadEvent features the correct spin polarization of the top quark and its decay products. Fort-channel single- top production we generated two samples, one bq! tq0 and one gq!tbq0 which we merged to- gether to reproduce the pT spectrum of thebas expected from NLO differential cross section calculations. This is an improved model compared to the Pythia modelling used in the Run I analyses. The event detection efficiency evt includes the kinematic and fiducial acceptance, branching ratios, lepton and b-jet identification as well as trigger efficiencies. We combineevt as given in Table I with the cross sections predicted by theory [3] and thereby obtain the number of expected single-top events listed in Table II.
We distinguish between two background components:tt and nontop background. We estimate the tt background based on events generated with PYTHIA, normalized to the theoretically predicted cross section of tt 6:70:70:9 pb[12]. The primary source (62%) of the nontop background is the W+heavy flavor processes qq 0!Wg withg!bborg!cc, andgq!Wc. Additional sources are ‘‘mistags’’ (25%), in which a light-quark jet is erro- neously identified as heavy flavor, ‘‘non-W’’ (10%), e.g.,
TABLE I. Event detection efficiencies in %.
Process Combined 1-tag 2-tag
t-channel 0:890:07 0:860:07 0:0070:002 s-channel 1:060:08 0:780:06 0:230:02
TABLE II. Expected number of signal and background events compared with observations.
Process Combined 1-tag 2-tag
t-channel 2:80:5 2:70:4 0:020:01
s-channel 1:50:2 1:10:2 0:320:05
tt 3:80:9 3:20:7 0:600:14
Nontop 30:05:8 23:34:6 2:590:71
Total background 33:85:9 26:54:7 3:190:72 Total expected 38:15:9 30:34:7 3:530:72
Observed 42 33 6
1The polar angle is measured with respect to the proton beam direction. The pseudorapidity is defined as lntan=2.ETEsin.
D. ACOSTAET AL. PHYSICAL REVIEW D71,012005 (2005)
012005-4
directbbproduction, and diboson (WW,WZ,ZZ) produc- tion (3%). The non-W and mistag fractions are estimated using CDF II data. TheW+heavy flavor rates are extracted from ALPGEN [13] Monte Carlo (MC) events normalized to data [9]. The diboson rates are estimated from PYTHIA events normalized to theory predictions [14]. The numbers of expected signal and background events are summarized in Table II. Having applied all selection cuts we observe 42 events for the combined search, 33 events in the 1-tag sample and six events in the 2-tag sample. Within the uncertainties, the observations are in good agreement with predictions.
To extract the signal content in data, we use a maximum likelihood technique. We separatet- ands-channel events by using the Q distribution which exhibits a distinct asymmetry for t-channel events, see Fig. 1(a). Q is the charge of the lepton and is the pseudorapidity of the untagged jet. In Fig. 1(b) we show the data versus stacked MC templates weighted by the expected number of events in the 1-tag sample. The separate search defines a joint likelihood function for theQdistribution in the 1-tag sample and for the number of events in the 2-tag sample.
Lsig1;...;4;!1;...;!7 edndd nd! YNbin
k11
eknkk nk!
Y4
j1 jsig
Gj;SM;j;j
Y7
i1
G!i;0;1:
Four processes are considered and labeled by the indexj:
t-channel (j1),s-channel (j2),tt(j3), and non- top (j4). The corresponding cross sections are denoted j. The background cross sections are constrained to their SM prediction SM;j by Gaussian priors of width 3 23%SM;3 for tt and 4 20%SM;4 for nontop. The index ‘‘sig’’ denotes the signal process, which is s- or t-channel, respectively. The k are the mean number of events in binkof theQhistogram (Nbin number of bins), whiled is the mean number of events in the 2-tag sample.nkandndare the event numbers observed in data, respectively. Seven sources of systematic uncertainties are considered in the likelihood function: (1) jet energy scale (JES), (2) initial-state radiation (ISR), (3) final state radia- tion (FSR), (4) parton distribution functions (PDF), (5) the choice of signal MC generator, (6) the top quark mass, (7) trigger, identification andb-tagging efficiencies and the luminosity. The relative strength of a systematic effect due to sourceiis parameterized by the variable!i. Systematic effects change the acceptance and influence the shape of theQdistribution. When calculatingk=d we take the systematic shifts in the acceptance and in the shape of the template histograms, and their full correlation into ac- count. All variables except the signal cross section sig
are constrained to their expected values by Gaussian func- tions Gx;x0;x of mean x0 and width x. The largest uncertainties are on the b-tagging efficiency (7%), lumi- nosity (6%), top quark mass (4%), and JES (4%). The effect of uncertainty in the JES is evaluated by applying energy corrections that describe 1 variations.
Systematic uncertainties due to the modeling of ISR and FSR are obtained from MC samples that describe varia- tions in these effects. To evaluate the uncertainty associ- ated with the choice of a specific parametrization of PDF we investigated several PDF sets and took the maximum deviation (MRST72) from our standard PDF set
TABLE III. Fractional changes inevt of single-top processes in %. trigis the trigger efficiency, ID the lepton identification efficiency.
i Source t-channel s-channel Combined
1 JES 2:46:7 0:43:1 0:14:3
2 ISR 1:0 0:6 1:0
3 FSR 2:2 5:3 2:6
4 PDF 4:4 2:5 3:8
5 Generator 5 2 3
6 Top quark mass 0:76:9 2:3 4:4
7 trig,ID, luminosity 9:8 9:8 9:8
a)
η
•
-3 -2 -1 0 1 2 Q 3
Entries norm. to unit area
0 0.05 0.1 0.15 0.2
t-channel s-channel
t t non-top
η
•
-3 -2 -1 0 1 2 Q 3
Entries norm. to unit area
0 0.05 0.1 0.15 0.2
b)
η
•
-3 -2 -1 0 1 2 Q 3
Events per 0.4 units
0 2 4 6 8
η
•
-3 -2 -1 0 1 2 Q 3
Events per 0.4 units
0 2 4 6
8 CDF II data t-channel s-channel
t t nontop
FIG. 1. Qdistributions for a) MC templates normalized to unit area, b) Data in the 1-tag sample (33 events) vs stacked MC templates normalized to the SM prediction.
. . .
(CTEQ5L). We estimate the uncertainty associated with the choice of single-top MC generator using samples gen- erated with TopReX [15]. The values of acceptance un- certainties for the single-top processes are summarized in Table III.
To measure the combined t- plus s-channel signal in data, we use a kinematic variable whose distribution is very similar for the two single-top processes, but is different for background processes:HT, which is the scalar sum ofET= and the transverse energies of the lepton and all jets in the event. We use a likelihood function similar to that in the separate search. One difference is that in the combined search the HT distributions are smoothed. In Fig. 2 we show theHTdistribution observed in data compared to the SM prediction.
We perform Monte Carlo experiments to estimate oura priorisensitivity assuming the SM signal cross sections.
For each experiment we integrate out all nuisance parame- ters (i.e., all variables exceptsig) from the full likelihood function and thereby construct the marginalized likelihood Lsigsig.Lsig is normalized and interpreted as posterior probability density function psig. We calculate the upper limit at the 95% C.L. using a Bayesian method assuming a prior probability density, which is 0 ifsig<
0 and 1 if sig0. The median of the expected upper limits defines our sensitivity and is stated in Table IV. We calculate the posterior probability densities for CDF II data and obtain the most probable values (MPV) and highest
posterior density (HPD) intervals [2] as given in Table IV.
Within the statistical uncertainty these results are compat- ible with SM predictions. We find upper limits of 10.1 pb at the 95% C.L. for thet-channel cross section and 13.6 pb for the s-channel. For the combined search we find an upper limit of 17.8 pb at the 95% C.L.
In summary, we find no significant evidence for electro- weak single-top quark production in 16210 pb1 of integrated luminosity recorded with CDF II. We set the first limits on single-top cross sections inppcollisions at
ps 1:96 TeVin Run 2 at the Tevatron. If compared with Run one results the upper limits on t-channel and s-channel single-top quark production are considerably improved by 28% (t-channel) or 20% (s-channel), respectively. We have introduced improved Monte Carlo modeling for single-top and a fully Bayesian treatment of systematic uncertainties in the likelihood function which are important steps for future analyses aiming for the observation of single-top quark production.
We wish to thank the Fermilab staff and the technical staffs of the participating institutions for their vital contri- butions. This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Instituto 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 Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foundation of Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; and in part by the European Community’s Human Potential Program under Contract No. HPRN-CT-20002, Probe for New Physics. We ac- knowledge the help of T. Stelzer and S. Slabospitsky for the generation of MadEvent and TopReX MC samples. We thank S. Mrenna and Z. Sullivan for useful discussions.
TABLE IV. Upper limits at the 95% C.L. and most probable values (MPV) of single-top cross sections in pb.
t-channel s-channel Combined
expected limit 11.2 12.1 13.6
observed limit 10.1 13.6 17.8
MPVHPD 0:04:70:0 4:63:83:8 7:75:14:9
[GeV]
HT
100 150 200 250 300 350 400 450
Events / 15 GeV
0 2 4 6 8
CDF II data single top
t t non-top SUM
[GeV]
HT
100 200 300 400
Normalized to unit area
s-channel t-channel
FIG. 2. HT distribution for data (42 events) in the combined search compared with smoothed MC predictions for signal and background.
D. ACOSTAET AL. PHYSICAL REVIEW D71,012005 (2005)
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