Article
Reference
Search for the Production of Narrow tb Resonances in 1.9 fb
−1of pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present new limits on resonant tb production in pp collisions at s√=1.96 TeV, using 1.9 fb−1 of data recorded with the CDF II detector at the Fermilab Tevatron. We reconstruct a candidate tb mass in events with a lepton, neutrino candidate, and two or three jets, and search for anomalous tb production as modeled by W′→tb. We set a new limit on a right-handed W′ with standard model-like coupling, excluding any mass below 800 GeV/c2 at 95% C.L. The cross section for any narrow, resonant tb production between 750 and 950 GeV/c2 is found to be less than 0.28 pb at 95% C.L. We also present an exclusion of the W′
coupling strength versus W′ mass over the range 300–950 GeV/c2.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for the Production of Narrow tb Resonances in 1.9 fb
−1of pp Collisions at s√=1.96 TeV. Physical Review Letters , 2009, vol. 103, no. 04, p. 041801
DOI : 10.1103/PhysRevLett.103.041801
Available at:
http://archive-ouverte.unige.ch/unige:38600
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for the Production of Narrow t b Resonances in 1:9 fb
1of p p Collisions at ffiffiffi p s
¼ 1:96 TeV
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S. S. Yu,18J. C. Yun,18L. Zanello,51b,51aA. Zanetti,54aX. Zhang,25Y. Zheng,9,eand 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, 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
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
22Glasgow University, Glasgow G12 8QQ, United Kingdom
23Harvard University, Cambridge, Massachusetts 02138, USA
041801-2
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
25University of Illinois, Urbana, Illinois 61801, USA
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
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, 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
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
43aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43bUniversity of Padova, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
46bUniversity of Pisa, I-56127 Pisa, Italy
46cUniversity of Siena, I-56127 Pisa, Italy
46dScuola 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
51aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
51bSapienza Universita` di Roma, I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy
54bUniversity of Trieste/Udine, I-33100 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
61Center 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 (Received 16 February 2009; published 20 July 2009) We present new limits on resonanttbproduction inppcollisions atpffiffiffis
¼1:96 TeV, using1:9 fb1of data recorded with the CDF II detector at the Fermilab Tevatron. We reconstruct a candidatetbmass in events with a lepton, neutrino candidate, and two or three jets, and search for anomaloustbproduction as modeled byW0!tb. We set a new limit on a right-handed W0 with standard model-like coupling, excluding any mass below 800 GeV=c2 at 95% C.L. The cross section for any narrow, resonant tb production between 750 and950 GeV=c2is found to be less than 0.28 pb at 95% C.L. We also present an exclusion of theW0coupling strength versusW0 mass over the range300–950 GeV=c2.
DOI:10.1103/PhysRevLett.103.041801 PACS numbers: 14.70.Pw, 13.85.Rm
Many modifications of the standard model (SM) of particle physics include new, massive, short-lived particles with two-body decays to known fermion pairs. A classic search strategy for these states looks for resonant signals in the spectra of two-body mass distributions. Recent tech- niques developed to observe electroweak single-top-quark production are well suited to a search for unexpected tb resonances [1]. A tb resonance (inclusion of the charge conjugate is implied throughout the text) is predicted by a wide range of models containing a massive charged vector boson, generically referred to asW0. The classic model is a simple extension of the SM to the left-right symmetric group SUð2ÞLSUð2ÞRUð1Þ [2], which adds a right- handed charged boson WR with universal weak coupling strength and unknown mass. TheW0 may arise in models with other symmetry extensions: as the excitation of theW boson in Kaluza-Klein extra dimensions [3], as the techni-of technicolor theories [4], or as a bosonic partner in little Higgs scenarios [5].
The classic limits onW0are derived from searches in the W0 !‘ decay channel [6]. For large W0 masses, the sensitivity in this channel is diminished by the broad Jacobian line shapes for the lepton momentum and W0 transverse mass. Searches in thetb channel [7] avoid this difficulty and also probe models where the couplings are free parameters and the leptonic decay modes may be suppressed. Although we quantify our results using the model of a right-handed W0 with SM-like coupling [8], this analysis is sensitive to any narrow state decaying totb, including, e.g., a charged Higgs boson or bound states arising from new dynamics in the third generation.
Searches in thetbchannel complement searches for neu- tral states coupling tott[9].
In this Letter we present a new search for ans-channel W0 !tb resonance produced in pp collisions at ffiffiffi
ps 1:96 TeV at the Fermilab Tevatron. The data set of¼ 1:9 fb1was recorded with the CDF II detector; a standard coordinate system [10] is used. A detailed explanation of this analysis can be found in [11]. Our selection is based on the leptonic decay mode tb! ð‘bÞb, which has been well understood in the search for electroweak single-top- quark production [1]. Events are expected to have a high transverse momentum (pT) electron or muon candidate, missing transverse energy (E6 T) from a neutrino [12], and two or three jets, at least one of which is a b-quark candidate. The dominant background is fromWþjet pro- cesses and electroweak top-quark production. We recon- struct each event according to our signal hypothesis W0 !tb! ð‘bÞb, then search the mass spectrum for a narrow resonance. If no signal is detected, we set limits on theW0 !tbcross section and on theW0coupling strength gW0.
The CDF II detector [13] is a cylindrically symmetric general-purpose detector. Precision charged-particle track-
ing is accomplished by layers of silicon microstrip detec- tors surrounded by a large open-cell drift chamber within a 1.4 T solenoidal magnetic field. Outside the magnet are the electromagnetic and hadronic calorimeters, steel for had- ronic shielding, and an exterior layer of muon detectors.
The luminosity of thepp collisions is measured using gas Cherenkov detectors at small angles.
We select data using online selection criteria which require a high-pT lepton or large E6 T [14]. We identify tb!‘bb candidates as having an electron or muon with pT 20 GeV=c. We also require E6 T 25 GeV and two or three hadronic jets with pT 20 GeV=cand jffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffij 2:8. Jets are clustered in cones of fixed radiusR
ðÞ2þ ðÞ2
p 0:4, and at least one jet is required to be ‘‘b tagged,’’ i.e., the jet contains a secondary vertex consistent with the decay of a hadron containing abquark [15]. We reduceZdecays andttcontamination by exclud- ing events with a second charged lepton. Events consistent with cosmic ray or photon interactions are also excluded.
QCD multijet background, which does not involve a W boson, is rejected with a specific set of requirements [11].
The primary background process is the associated pro- duction of a W boson and jets with subsequent leptonic decay of theW boson (Wþjets). Approximately 70% of our sample are Wþjets events containing heavy flavor (Wbb, Wcc, Wcj) or incorrectly b-tagged light flavor (mistags). We establish the normalization of these pro- cesses from data, and estimate the fraction of the candidate events with bottom or charm flavor using the ALPGEN
Monte Carlo event generator [16]. The mistagging rate for light-flavor jets is estimated from inclusive generic jet data [17]. Additional backgrounds including tt pair pro- duction,s-channel andt-channel single-top-quark produc- tion, and diboson processes (WW, WZ,ZZ) are modeled using thePYTHIAMonte Carlo event generator [18] and are normalized to the next-to-leading-order cross sections pre- dicted by theory. A small multijet background without leptonicW decay (‘‘non-W’’) arises when a jet is misiden- tified as a lepton andE6 T results from jet energy mismea- surement; this background is modeled using data. The predicted SM background is detailed in Table I. The un- certainties are dominated by imprecise knowledge of the heavy-flavor fractions and pertain to background rate esti- mates only; other systematic uncertainties are discussed later. In data we observe 1362 events with two jets and 617 events with three jets.
According to the proposedW0hypothesis, theW0mass is given by reconstructingMtbfrom the four-momenta of the lepton, neutrino, and two jets. The unmeasured longitudi- nal neutrino momentumpz is quadratically constrained by assigning M‘¼MW ¼80:448 GeV=c2 [19]. We assign pz to the smallest real solution or to the real part of complex solutions [20]. We assume the two highest ET jets arise from thebquarks, even for the three-jet case in 041801-4
which the third jet has beenbtagged. The reconstructedW is then combined with these two leading jets, corrected to reproduce parton-level energies, to formMtb.
Our signal model is a W0 with purely right-handed decays and SM-like coupling, simulated using PYTHIA. The model assumes a top-quark mass of 175 GeV=c2. The left-handed case is not considered since the conse- quentWW0 interference has not been observed in any precisionWmeasurements. Figure1shows theMtbdistri- bution in data superimposed with the expected signal shape for a600 GeV=c2W0produced with a total cross section of 9 pb (4the prediction for aW0with SM-like coupling [8]). The reconstructed width of the signal is dominated by resolution effects, particularly the jet-energy resolution [21] and the incorrect assignment of jets from initial or final state radiation. Our test signal is therefore applicable for anyW0-like object whose width is small compared to the experimental resolution. The binning is chosen so that background models have a sufficient number of entries in each bin, including the overflow bin for all values above 700 GeV=c2.
Unlike single-top-quark production, W0 production is entirely an s-channel process; contributions from the t anduchannels are suppressed by the largeW0 mass. We simulate a narrow right-handedW0 with SM-like coupling and a mass between 300 and 950 GeV=c2 in steps of 100 GeV=c2 below600 GeV=c2 and steps of50 GeV=c2 above. This is the mass range to which our analysis is sensitive to changes in the signal distribution: above 950 GeV=c2 the signal events simply pile into the Mtb
overflow bin. Since there is very little high-mass back- ground, we are sensitive to excesses of just a few events in the tail. ForMW0 ¼800 GeV=c2, our selection efficiency in thetbchannel is approximately2:81:0%. An excess of ten events, for example, would correspond to a Tevatron cross section of 0.18 pb.
The branching ratios (BR) of a right-handedW0depend on whether decay to R is allowed; we consider both possibilities. If leptonic decay is forbidden, as for a lep-
tophobic W0 or when MW0< MR, the Mtb prediction simply has a slightly larger normalization. For example, ifMW0 ¼800 GeV=c2,BRðW0 !tbÞ is predicted to be 0.337 pb if leptonic decays are forbidden and 0.262 pb if they are allowed.
We set frequentist limits onW0!tbusing the measure CLs from [22], which is defined as the probability of background plus a specified signal fraction matching the data (PSþB) divided by the probability of a background-
2) (GeV/c
b
Mt
0 100 200 300 400 500 600 700
Events
0 50 100 150 200 250 300
CDF Run II: 1.9 fb-1
Data
Single top quark t
t
Mistags, Non-W , Wcj, WW c
Wc
, WZ, ZZ, Z+jets b
Wb
W′ (9 pb) 600 GeV/c2
FIG. 1. Mtbfor events with two jets and onebtag, comparing the shapes between background and signal. Backgrounds are stacked and grouped according to similar shape. A600 GeV=c2 W0 model is shown withBRðW0!tbÞ ¼9 pb(4the prediction for aW0with SM-like coupling).
TABLE I. Predicted SM background contribution with two jets and with three jets.
Background 2 jets 3 jets
Wbb 409:4123:4 125:637:9 WccþWcj 412:4127:2 109:333:6
Mistags 276:535:0 82:510:7
Non-W 53:221:3 17:36:9
tt 126:513:4 291:836:7 Single top quark (tchannel) 53:37:8 15:72:3 Single top quark (schannel) 35:45:0 11:61:6 WWþWZþZZ 54:44:2 18:41:5 Zþjets 22:63:3 9:31:4 Total BG prediction 1443:8254:6 681:683:0
Observed 1362 617
TABLE II. 95% C.L. limits onBRðW0!tbÞas function of MW0 for a right-handed W0 with SM-like coupling. The expected limit is quoted with the range of values into which our observation should fall 68% of the time assuming no signal is present.
MW0 (GeV=c2) Expected limit (pb) Observed limit (pb)
300 1:56þ00::6245 1.59
400 1:04þ00::4430 1.17
500 0:74þ00::3522 0.84
600 0:54þ00::2417 0.44
650 0:46þ00::2113 0.39
700 0:40þ00::1712 0.32
750 0:33þ00::1509 0.28
800 0:30þ00::1309 0.26
850 0:28þ00::1308 0.25
900 0:28þ00::1308 0.26
950 0:30þ00::1309 0.28
only model matching the data (PB). Sources of uncertainty are treated using a large series of trials (50103) for both cases. Each trial is produced by randomly varying all uncertain parameters in the model prediction within a Gaussian constraint about their nominal values. PSþB is determined from the fraction of the SþB trials with a minimized 2¼2ðdatajSþBÞ 2ðdatajBÞ larger than in data; PB is analogous. The 95% C.L. limit is set by adjusting the signal fraction assumed in the SþB model untilCLs¼0:05.
Our event selection introduces various sources of sys- tematic uncertainty. These are manifest as errors in both the rates and shapes of the mass distributions for our signal and background models. They include jet-energy scale (JES),b-tagging efficiencies, lepton identification and trig- ger efficiencies, recorded luminosity, quantity of initial and final state radiation, parton distribution functions, factori- zation and renormalization scale, and MC modeling. Our limit procedure evaluates their impact by making reason- able variations in the model parameters and resimulating the analysis [11].
The systematic uncertainties are dominated by JES and theb-tagging rate uncertainties for the signal. JES uncer- tainty is modeled by calculating 1 shifts in each jet- energy correction and adding the results in quadrature.
The uncertainty in b-tagging efficiency is determined by binning the b-tagging rate as a function of energy for multijet data. The uncertainty is found to be proportional to the jet energy, allowing extrapolation to the higher energies common for our W0 signal. This jet-energy weighted uncertainty on theb-tagging rate leads to accep- tance errors as large as 40% for a 950 GeV=c2 W0.
Including all such sources of uncertainty in our model results in the expected upper limit on the cross section increasing by 30%–40%.
Applying the full limit procedure, we set 95% C.L.
upper limits on BRðW0!tbÞ as listed in Table II for a right-handed W0 with SM-like coupling. Predicted cross sections for such aW0[8] are shown in Fig.2: we set new 95% C.L. limits of MW0>800 GeV=c2 including leptonic decays, and MW0>825 GeV=c2 if leptonic de- cays are forbidden. The best prior result used0:9 fb1and found MW0 768 GeV=c2 if leptonic decays are forbid- den [7]. These results are quoted for a top-quark mass of 175 GeV=c2 and thus are slightly conservative: using the smaller world average would increase the tb branching fraction.
For a simple s-channel model with effective coupling gW0, the cross section is proportional tog4W0. Relaxing the assumption of the universal weak coupling, our cross section limits can be rewritten as upper limits ongW0 as a function of MW0. The excluded region of the gW0MW0
plane is shown in Fig.3, withgW0in units ofgW. AtMW0 ¼ 300 GeV=c2, we limit (95% C.L.) the effective coupling to be less than 0.40 of the W boson coupling. In this more general case, the effective cross section for any narrow, resonant tb production between 750 and 950 GeV=c2 is found to be less than 0.28 pb at 95% C.L.
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
2) W′ Mass (GeV/c
300 400 500 600 700 800 900
) (pb)b t→ BR(W′×σ
0 0.2 0.4 0.6 0.8 1 1.2 1.4
1.6 Expected Limit
Expected Limit σ
± 1
Observed Limit Allowed ν
→ Theory: W′
Forbidden ν
→ Theory: W′
CDF Run II: 1.9 fb
FIG. 2 (color online). Expected and observed 95% C.L. limits on BRðW0!tbÞ as function of MW0 for 1:9 fb1, along with theoretical predictions. A right-handed W0 with SM-like couplings is excluded forW0masses below800 GeV=c2.
2) W′ Mass (GeV
300 400 500 600 700 800 900
W / gW′g
0 0.2 0.4 0.6 0.8 1 1.2
Excluded Region
Forbidden ν
→ Obs Limit: W′
Allowed ν
→ Obs Limit: W′
CDF Run II: 1.9 fb
FIG. 3 (color online). Observed 95% C.L. limits on the cou- pling strength of a right-handed W0 compared to the SM W boson coupling, gW0=gW, as function ofMW0 for1:9 fb1. The shaded region above each dashed line is excluded.
041801-6
Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Founda- tion; 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 Science and Technology Facilities Council and the Royal Society, UK; 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; and the Academy of Finland.
aDeceased.
bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.
cVisitor from Universiteit Antwerpen, B-2610 Antwerp, Belgium.
dVisitor from University of Bristol, Bristol BS8 1TL, United Kingdom.
eVisitor from Chinese Academy of Sciences, Beijing 100864, China.
fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
gVisitor from University of California Irvine, Irvine, CA 92697, USA.
hVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.
iVisitor from Cornell University, Ithaca, NY 14853, USA.
jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.
kVisitor from University College Dublin, Dublin 4, Ireland.
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mVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.
nVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.
oVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.
pVisitor from University of Iowa, Iowa City, IA 52242, USA.
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zOn leave from J. Stefan Institute, Ljubljana, Slovenia.
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