Article
Reference
Search for a new heavy gauge boson W′ with event signature electron + missing transverse energy in pp collisions at √s=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a search for a new heavy charged vector boson W′ decaying to an electron-neutrino pair in pp collisions at a center-of-mass energy of 1.96 TeV. The data were collected with the CDF II detector and correspond to an integrated luminosity of 5.3 fb−1. No significant excess above the standard model expectation is observed and we set upper limits on σ⋅B(W′→eν). Assuming standard model couplings to fermions and the neutrino from the W′
boson decay to be light, we exclude a W′ boson with mass less than 1.12 TeV/c2 at the 95%
confidence level.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for a new heavy gauge boson W′ with event signature electron + missing transverse energy in pp collisions at √s=1.96 TeV. Physical Review. D , 2011, vol. 83, no. 03, p. 031102
DOI : 10.1103/PhysRevD.83.031102
Available at:
http://archive-ouverte.unige.ch/unige:39113
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for a new heavy gauge boson W
0with event signature electron þ missing transverse energy in p p collisions at ffiffiffi
p s
¼ 1:96 TeV
T. Aaltonen,21B. A´ lvarez Gonza´lez,1,wS. Amerio,41a,41bD. Amidei,32A. Anastassov,36A. Annovi,17J. Antos,12 G. Apollinari,15J. A. Appel,15A. Apresyan,46T. Arisawa,56A. Artikov,13J. Asaadi,51W. Ashmanskas,15B. Auerbach,59 A. Aurisano,51F. Azfar,40W. Badgett,15A. Barbaro-Galtieri,26V. E. Barnes,46B. A. Barnett,23P. Barria,44c,44aP. Bartos,12
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V. Ramakrishnan,58N. Ranjan,46I. Redondo,29P. Renton,40M. Rescigno,49a,49bF. Rimondi,6b,6aL. Ristori,45,15 PHYSICAL REVIEW D83,031102(R) (2011)
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T. Yoshida,39,kG. B. Yu,14I. Yu,25S. S. Yu,15J. C. Yun,15A. Zanetti,52a,52bY. Zeng,14and 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
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
1Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
10Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
11Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
12Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
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
21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
22University of Illinois, Urbana, Illinois 61801, USA
23The Johns Hopkins University, Baltimore, Maryland 21218, USA
24Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
25Center 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
26Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
27University of Liverpool, Liverpool L69 7ZE, United Kingdom
28University College London, London WC1E 6BT, United Kingdom
29Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
30Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
31Institute 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
T. AALTONENet al. PHYSICAL REVIEW D83,031102(R) (2011)
031102-2
32University of Michigan, Ann Arbor, Michigan 48109, USA
33Michigan State University, East Lansing, Michigan 48824, USA
34Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
35University of New Mexico, Albuquerque, New Mexico 87131, USA
36Northwestern University, Evanston, Illinois 60208, USA
37The Ohio State University, Columbus, Ohio 43210, USA
38Okayama University, Okayama 700-8530, Japan
39Osaka City University, Osaka 588, Japan
40University 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
42LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
43University 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
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 10065, USA
49aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
49bSapienza Universita` di Roma, I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, Italy
52bUniversity of Trieste/Udine, I-33100 Udine, Italy
53University of Tsukuba, Tsukuba, Ibaraki 305, Japan
54Tufts University, Medford, Massachusetts 02155, USA
55University of Virginia, Charlottesville, Virginia 22906, 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 23 December 2010; published 3 February 2011)
aDeceased.
bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.
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aaOn leave from J. Stefan Institute, Ljubljana, Slovenia.
SEARCH FOR A NEW HEAVY GAUGE BOSONW . . . PHYSICAL REVIEW D83,031102(R) (2011)
We present a search for a new heavy charged vector bosonW0decaying to an electron-neutrino pair in pp collisions at a center-of-mass energy of 1.96 TeV. The data were collected with the CDF II detector and correspond to an integrated luminosity of5:3 fb1. No significant excess above the standard model expectation is observed and we set upper limits onBðW0!eÞ. Assuming standard model couplings to fermions and the neutrino from theW0boson decay to be light, we exclude aW0boson with mass less than1:12 TeV=c2at the 95% confidence level.
DOI:10.1103/PhysRevD.83.031102 PACS numbers: 14.70.Pw, 12.60.Cn, 13.85.Rm
TheW0[1] is a postulated charged heavy vector boson which is predicted in models that extend the gauge struc- ture of the standard model. In the left-right (LR) symmetric model [2] considered here, the right-handed W0 boson mass is obtained by the symmetry breaking of the right- handed electroweak gauge group of SUð2ÞRSUð2ÞL Uð1ÞB;L. This provides a natural explanation for the ob- served suppression ofVþAcurrents in low-energy weak processes. The LR symmetric model can also be motivated by the manifestation of a higher symmetry predicted at intermediate energies in grand unified theories [3].
The manifest LR symmetric model assumes that the right-handed Cabibbo-Kobayashi-Maskawa matrix and the gauge coupling constants are identical to those of the standard model [4]. TheW0 can decay in the same way as the standard modelW, with the exception that the tb[5]
decay channel is accessible if theW0 is heavy enough and that the diboson decay channel (W0!WZ) is suppressed in the extended gauge model [1].
TheW0boson has been previously searched for in high- energy physics experiments using final state signatures such as leptons, jets, and/or missing energy. The most recent direct searches for a charged heavy vector boson have been performed at the Tevatron collider at Fermilab.
The CDF experiment previously set limits on the cross section times branching fraction in the decay mode W0 !tb and excluded a W0 boson mass below 800 GeV=c2 at the 95% confidence level (C.L.) using 1:9 fb1 data ofpp collisions [6]. The D0 experiment set limits on the product of the cross section and branching fraction in the decay mode W0 !eand excluded a W0 boson mass below 1:00 TeV=c2 at the 95% C.L. using 1:0 fb1of data [7]. Both of these recent mass limits assume that the couplings between the new vector boson and the fermionic final states are the same as in the standard model.
In this paper, we present the results of a search for aW0 boson in the edecay mode, assuming the manifest LR symmetric model and the right-handed neutrino from the boson decay to be light (m mW0) and stable. Under these assumptions, the results in this paper can be useful in the generic model [1] since the kinematics of the left- and right-handed W0 bosons is not different. We use a data sample corresponding to5:3 fb1integrated luminosity of ppcollisions at ffiffiffi
ps
¼1:96 TeVrecorded by the upgraded Collider Detector at Fermilab (CDF II). We select events that are consistent with the production of the standard
model W and the heavier W0 boson that decay to an electron and neutrino in the final state. The analysis tech- nique applied is the same as in a previous search [8].
The CDF II detector is described in detail elsewhere [9].
CDF II is a general purpose solenoidal detector which combines precision charged-particle tracking with fast projective calorimetry and fine-grained muon detection.
Tracking systems are contained inside a superconducting solenoid, 1.5 m in radius and 4.8 m in length, which generates a 1.4 T magnetic field parallel to the beam axis. Calorimeters and muon systems surround the sole- noid and the tracking system. Electron candidates are identified by an energy deposit in the electromagnetic calorimeter with a track pointing to it. A set of charged- particle detectors surrounding the calorimeters identify muon candidates. The energy of the electron candidate is measured by the calorimeter and its direction is determined from the tracking system. The component of the neutrino momentum transverse to the beam line is inferred to be equal to the missing transverse energy ET [10], which is derived from the transverse energy imbalance of all the deposited energy in the calorimeters.
The on-line selection requires either one electron can- didate in the electromagnetic calorimeter with transverse energyET>18 GeVthat has a matching track with trans- verse momentum pT >9 GeV=cor an electron candidate in the electromagnetic calorimeter with transverse energy ET>70 GeV. No restrictions on the amount of energy leakage into the hadronic calorimeter were imposed, in order to ensure high efficiency for high-ET electrons. We select the candidate event sample off-line by requiring an isolated electron candidate with ET >25 GeV and the existence of an associated track with pT >15 GeV=c that is contained in the fiducial region of the tracking system ofjj<1:0[11]. Electron candidates are selected based on an ET-dependent isolation cut [12] in order to maximize the efficiency in the high-ET region. The elec- tron shower profile is required to be consistent with that of test-beam electrons in order to match with the expected electromagnetic shower [13]. In events with high-energy muons, theET is adjusted by adding the muon momentum and removing the expected ionization energy deposition in the calorimeter. The ET is corrected further for - and energy-dependent nonuniformities of the calorimeter re- sponse. In the final selection, the correctedET is required to be greater than 25 GeV. Dilepton events coming from
T. AALTONENet al. PHYSICAL REVIEW D83,031102(R) (2011)
031102-4
Drell-Yan, tt, and diboson backgrounds are vetoed by rejecting events with a second isolated lepton, either an electron or a muon, with pT >15 GeV=c. QCD multijet events are a background to W=W0 !e when a jet is misidentified as an electron and mismeasured jets lead to significantET. The electron candidateETand the eventET
are likely to significantly differ in magnitude in this case.
In contrast, aW=W0!eevent will have an electron and neutrino emitted in opposite directions which results in the electron ET and ET being of comparable magnitude, re- spectively, assuming thepT of the boson is much smaller than its mass. Thus, in order to reduce the QCD multijet background, we require the candidate events to satisfy 0:4< ET=ET<2:5. The efficiency of this requirement is larger than 99% for W=W0 events whereas the rejection fraction is 40% for QCD multijet events with ET >
100 GeV. After all selection requirements, the transverse mass of a candidate event is calculated as
mT ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ETETð1coseÞ q
; (1)
where e is the azimuthal opening angle between the electron candidate and theET direction.
TheW0!esignal events are generated with PYTHIA
[14] using the CTEQ5L [15] parton distribution functions (PDFs) and a simulation of the CDF II detector [16]. Since the cross sections calculated by PYTHIA are at leading order, next-to-next-to-leading-orderK factors are applied to the leading order cross sections. Mass-dependent next- to-next-to-leading-orderK factors from Ref. [17] are ob- tained with an approximate magnitude around 1.3. The total acceptance times efficiency of the event selection cuts ranges from 45% to 35% and decreases above aW0 boson mass of800 GeV=c2. Figure1shows the expected W0 boson transverse mass distributions for various input masses with the background predictions. The on-shell production of heavy bosons near the kinematic limit is suppressed due to the smallness of the PDFs at large momentum fraction, which results in the low acceptance rate of W0 events at high mass above800 GeV=c2 after applying the kinematic selection requirements.
The background sources toW0!eare primarily pro- cesses with an electron and missing energy in the final state. These sources of background are W !e, W ! !e, Z=!!eX, tt, and diboson (WW, WZ) production. TheZ=!eeprocess can also produce missing energy when one of the electrons escapes detec- tion. ThemT distributions and acceptance times efficiency of the nonmultijet backgrounds are obtained usingPYTHIA
and a simulation of the CDF II detector. Theoretical cross section predictions are used to estimate the expected back- ground yields [17–19]. For the QCD multijet background estimation, a data-driven method is applied that uses the distribution of the azimuthal angle between the primary electron candidate and the vector sum of the jet energy. For
the multijet case, a jet misidentified as an electron candi- date will appear to recoil against the rest of the jet in the event. Therefore, a back-to-back distribution is expected in the azimuthal opening angle. The W=W0!e process, however, does not have a strong correlation in this angle.
The QCD multijet contribution is estimated by a likelihood fit to the data using the different angular shapes. The multijetmT distribution is obtained using a QCD enriched sideband sample with the isolation cut inverted. The data and the total backgroundmTdistributions are compared in Fig.2. The contributions fromW !e, QCD multijet, and
2) (GeV/c mT
0 200 400 600 800 1000 1200 1400
2Events / 10 GeV/c
10-2
10-1
1 10 102
103
104
105
106
107
Total Background = 500 GeV/c2
mW’
= 800 GeV/c2
mW’
= 1000 GeV/c2
mW’
= 1200 GeV/c2
mW’
FIG. 1 (color online). The transverse mass distributions for W0!esignal events generated usingPYTHIAwith total back- ground expectation.
2) (GeV/c mT
10 102 103
2 Events / 5 GeV/c
10-3
10-2
10-1
1 10 102
103
104
105
106
DATA Total background
ν
→ e W Multijet Other background
FIG. 2 (color online). The transverse mass distributions ofe candidate events compared to the total backgrounds.
SEARCH FOR A NEW HEAVY GAUGE BOSONW . . . PHYSICAL REVIEW D83,031102(R) (2011)
the other backgrounds in the mass region above mT ¼200 GeV=c2 are listed in TableI. This comparison shows good agreement between the data and the total backgrounds.
In order to quantify the size of the potential signal contributions in the data sample, a binned maximum like- lihood fit was performed on the observedmT distribution between 0 and 1500 GeV=c2, using the background predictions and the expected W0 boson contribution for
different mass values ranging from 500 to1300 GeV=c2. The fit results are shown in TableII, normalized to
BðW0!eÞ
BðW0 !eÞLR; (2) where the numerator is the observed cross section times the branching fraction and the denominator is that expected from the manifest LR symmetric model. The expected signal yield was normalized to the observedWboson yield obtained from the fit. This removes several sources of systematic uncertainty such as the integrated luminosity, TABLE I. The event yields for the background sources inmTabove200 GeV=c2compared to
the observed data.
Events inmTbins (GeV=c2)
200–250 250–350 350–500 500–700 700–1000
W!e 711þ5050 359þ2525 85þ66 13þ11 1:1þ0:10:1 Multijet 9þ22 6þ11 2þ22 0:2þ1:60:2 0:01þ1:100:01 Other background 70þ96 33þ43 8þ11 1þ0:10:1 0:09þ0:010:01 Total background 790þ6158 398þ3130 94þ98 14þ31 1:2þ1:20:1
Data 784 426 88 18 1
TABLE II. The expected numbers of events from theW0!e process,Nexp, assuming the manifest LR symmetric model and normalized by the observedW boson yield. We also show the observed relative rate of theW0 boson production from the fit described in the text and the 95% C.L. upper limit on this relative rate. The uncertainties are statistical only and do not include systematic uncertainties. The 95% upper limits include both statistical and systematic uncertainties.
mW0 Nexp ð¼BðWBðW0!eÞ0!eÞLRÞ (GeV=c2) (events) Fit (102) Upper limit 500 5828 0:08þ0:210:08 5:38103 550 3407 0:18þ0:260:18 7:16103 600 2037 0:28þ0:360:28 1:01102 650 1218 0:43þ0:540:43 1:52102 700 731 0:36þ0:830:36 2:22102 750 433 0:15þ1:070:15 2:80102 800 263 0:03þ1:360:03 3:82102 850 160 0:00þ1:890:00 5:68102 900 100 0:00þ2:800:00 8:79102 950 62 0:00þ4:530:00 1:49101 1000 41 0:00þ6:640:00 2:48101 1050 27 0:00þ10:80:00 4:36101 1100 19 0:00þ17:70:00 7:62101 1150 14 0:00þ32:50:00 1.39 1200 10 0:00þ62:70:00 2.47
1250 8.1 0:00þ1140:00 3.96
1300 6.7 0:00þ2240:00 6.24
2) (GeV/c mW’
500 600 700 800 900 1000 1100 1200 1300
) (pb)ν e→ B(W’ ×σ
10-3
10-2
10-1
1 10
Br(NNLO)
× σ obs. limit exp. limit
exp. limit σ
± 1
exp. limit σ
± 2
FIG. 3 (color online). The 95% C.L. limits on the cross section times the branching fraction as a function ofW0boson mass and the expected limits from the simulated experiments with back- ground only. The black solid lines represent the median ex- pected; the shaded bands indicate the 1and2invervals on the expected limits. The region above the red dashed line (observed limit) is excluded at the 95% C.L. The cross section times the branching fraction assuming the manifest LR symmet- ric model, BðW0!eÞLR, is shown along with its uncer- tainty. The intercept of the cross section limit curve and the lower bound of the theoretical cross section yields mW0>
1:12 TeV=c2 at the 95% C.L.
T. AALTONENet al. PHYSICAL REVIEW D83,031102(R) (2011)
031102-6
the trigger, and the identification efficiencies, all of which cancel in the ratio.
Systematic uncertainties on the signal and the back- ground rates were considered for the PDFs, the jet energy scale, the theoretical cross sections, the multijet back- ground, the initial and final state radiation of the signal, and the energy scale of the electromagnetic calorimeter.
The dominant contribution to the systematic uncertainty comes from the PDFs. The total systematic uncertainty varies from5%to10%forW0 boson masses ranging frommW0 ¼500to1300 GeV=c2.
To determine the limit on, we use a Bayesian approach [20] by constructing a marginalized posterior probability distribution [pðÞ] from the likelihood function. Sources of systematic uncertainty are included as nuisance parame- ters in the definition of the likelihood function. The 95%
C.L. upper limits on the ratio of the observed to the expected cross section are obtained from the fit. We use the resulting likelihood function, and the obtained upper limits are summarized in TableIIand plotted in Fig.3as a function mW0 together with the expected limits obtained from simulated experiments with background only. Using theoretical predictions that assume the manifest LR sym- metric model [4], the limits on the cross section times the branching fraction are converted into limits on the mass of theW0boson. The lower mass limit can be set at the mass value for which95¼1, whereR95
0 pðÞd¼0:95. We
take the lower bound of the theoretical cross section to obtain the mass limit. Hence, the 95% C.L. is found to be mW0>1:12 TeV=c2.
In summary, we have performed a search for a new heavy charged vector boson decaying to an electron- neutrino pair with a light and stable neutrino in pp collisions at ffiffiffi
ps
¼1:96 TeV. We do not observe any sta- tistically significant excess over the background expecta- tions. We use a fit to themT distribution to set upper limits on the production and decay rate of a W0 boson as a function of mW0 and exclude a W0 boson with mW0<
1:12 TeV=c2 at the 95% C.L., assuming the manifest LR symmetric model.
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 Balduin Una Forschung, Germany; the World Class University Program, the National Research Foundation of Korea;
the Science and Technology Facilities Council and the Royal Society, United Kingdom; the Institut National de Physique Nucleaire et Physique des Particules/CNRS and Universite Pierre et Marie Curie; 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.
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