Article
Reference
Strong Evidence for ZZ Production in pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We report the first evidence of Z boson pair production at a hadron collider with a significance exceeding 4 standard deviations. This result is based on a data sample corresponding to 1.9 fb−1 of integrated luminosity from pp collisions at s√=1.96 TeV collected with the Collider Detector at Fermilab II detector. In the ℓℓℓ′ℓ′ channel, we observe three ZZ candidates with an expected background of 0.096+0.092−0.063 events. In the ℓℓνν channel, we use a leading-order calculation of the relative ZZ and WW event probabilities to discriminate between signal and background. In the combination of ℓℓℓ′ℓ′ and ℓℓνν channels, we observe an excess of events with a probability of 5.1×10−6 to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is σ(pp¯→ZZ)=1.4+0.7−0.6(stat+syst) pb, consistent with the standard model expectation.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Strong Evidence for ZZ Production in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2008, vol. 100, no. 20, p. 201801
DOI : 10.1103/PhysRevLett.100.201801
Available at:
http://archive-ouverte.unige.ch/unige:38503
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Strong Evidence for ZZ Production in p p Collisions at p s
1:96 TeV
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Y. Zheng,8,band S. Zucchelli5 (CDF Collaboration)a
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138, USA
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
201801-2
24University of Illinois, Urbana, Illinois 61801, USA
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27Center 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
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, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
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
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
45Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
47Purdue University, West Lafayette, Indiana 47907, USA
48University of Rochester, Rochester, New York 14627, USA
49The Rockefeller University, New York, New York 10021, USA
50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
51Rutgers University, Piscataway, New Jersey 08855, USA
52Texas A&M University, College Station, Texas 77843, 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 1 February 2008; published 22 May 2008)
We report the first evidence of Z boson pair production at a hadron collider with a significance exceeding 4 standard deviations. This result is based on a data sample corresponding to1:9 fb1 of integrated luminosity from pp collisions at
ps
1:96 TeV collected with the Collider Detector at Fermilab II detector. In the‘‘‘0‘0channel, we observe threeZZcandidates with an expected background of0:0960:0920:063events. In the‘‘channel, we use a leading-order calculation of the relativeZZandWW event probabilities to discriminate between signal and background. In the combination of‘‘‘0‘0and‘‘
channels, we observe an excess of events with a probability of5:1106 to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is pp!ZZ 1:40:70:6statsystpb, consistent with the standard model expectation.
DOI:10.1103/PhysRevLett.100.201801 PACS numbers: 12.15.Ji, 13.40.Em, 13.85.Qk, 14.70.Hp
Measurements of heavy vector boson pair production (WW,WZ,ZZ) are of great importance because they test the electroweak sector of the standard model (SM).
Diboson production provides a sensitive probe of new
physics, including anomalous trilinear gauge couplings [1], new particles such as the Higgs boson [2], and large extra dimensions [3]. Important tests of electroweak phys- ics have been made recently at the Fermilab Tevatron with
the first observation ofWWproduction in hadron collisions [4,5] and the first observation ofWZ production [6]. The production ofZpairs has been observed ineecollisions at LEP [7], but not in hadron collisions. As a window to new physics,ZZ production is particularly interesting be- cause of the absence ofZZandZZZcouplings in the SM, and because of the very low backgrounds in the four charged-lepton channel.
The full process that we consider in this search ispp ! Z=Z=. For brevity, we denote Z=Z= as ZZ throughout this Letter. The most sensitive previous search forZZproduction in hadron collisions was reported by the D0 Collaboration using data corresponding to 1 fb1 of integrated luminosity. That search used only the four charged-lepton channel and set a limit on the cross section ofZZ<4:4 pbat 95% C.L. [8]. The next-to-leading- order (NLO) ZZ cross section for pp collisions at
ps 1:96 TeV is 1:40:1 pb in the zero-width Z boson ap- proximation [9].
In this Letter, we report a signal forZZ production in hadron collisions which has a probability of5:1106to be due to the expected background. The production is observed inpp collisions at
ps
1:96 TeVusing a data sample corresponding to1:9 fb1of integrated luminosity collected by the CDF II detector. We consider bothZZ!
‘‘‘0‘0 andZZ!‘‘ decays, where‘ and‘0 are elec- trons or muons directly fromZdecay or from the leptonic decay of’s when one or bothZbosons decay toleptons.
In theZZ!‘‘‘0‘0channel, we have a small sensitivity to the contribution and its interference with theZboson, while this sensitivity is negligible for the ZZ!‘‘
channel.
The CDF II detector is described elsewhere [10]. We specify the geometry using the azimuthal angleand the pseudorapidity lntan=2, where is the polar angle with respect to the proton beam axis. The pseudor- apidity of a particle assumed to originate from the center of the detector is referred to asd. The branching fraction of theZZ state to foureor leptons, including those from leptonicdecays, is only 0.51%. When coupled with the small SM cross section, only a small number (14) of ZZ!‘‘‘0‘0are expected to be produced in1:9 fb1at the Tevatron. The finite CDF II acceptance further reduces the expected number of observedZZ!‘‘‘0‘0 events.
We use the lepton identification strategy developed for the WZ observation analysis [6], which was designed to have high acceptance for multilepton final states. The lepton candidates are divided into seven exclusive catego- ries: three for muons, three for electrons, and one for tracks that project to detector regions with insufficient calorime- ter coverage for energy measurement. One of the muon categories uses the muon chambers and the other two consist of minimum ionizing tracks that use either the central (jdj<1:1) or forward (1:2<jdj<2:0) calorim- eters. The three electron categories are central (jdj<
1:1), forward (1:2<jdj<2:0) with a matched silicon- based track, and forward (1:2<jdj<2:8) without a matched silicon-based track.
To suppress jets being misidentified as leptons, all lepton candidates are required to pass a calorimeter-based isola- tion requirement such that the sum of the ET for the calorimeter towers not associated with the lepton in a
cone of R
2 2
p <0:4 around the lepton direction is less than 10% of theETfor electrons orpT for muons and track lepton candidates. The transverse energy ET of an EM cluster or calorimeter tower isEsin, where E is the associated energy. Similarly, pT is the track momentum transverse to the beam line.
To infer the presence of neutrinos inZZ!‘‘decay, we use missing transverse energyE6~T P
iEin^T;i, where
^
nT;iis the transverse component of the unit vector pointing from the interaction point to calorimeter tower i. The E6~T calculation is corrected for muons and track leptons, which do not deposit all of their energy in the calorimeter. We analogously define the scalar sumET P
iET;iapplying the same corrections.
We require events to pass at least one of four trigger selection criteria. The central electron trigger requires an EM energy cluster withET>18 GeVmatched to a track withpT>8 GeV=c. For theZZ!eechannel only, a trigger for forward electrons requires an EM energy cluster withET>20 GeVand an uncorrected, calorimeter-based measurement of E6 T>15 GeV. Two muon triggers are based on stubs from the corresponding muon detectors matched to a track withpT>18 GeV=c. Trigger efficien- cies are measured in leptonicW andZboson data samples [11].
The ZZ!‘‘‘0‘0 candidates are selected from events with exactly four charged-lepton candidates using require- ments that were optimized with Monte Carlo simulation without reference to the data. At least one lepton is re- quired to satisfy the trigger criteria and haveET>20 GeV (pT >20 GeV=c) for electrons (muons). We loosen this requirement to 10 GeV (GeV=c) for the other leptons to increase theZZkinematic acceptance. We require at least two same-flavor, opposite-sign lepton pairs in the event.
Trackless electrons are considered to have either charge, and track-only leptons either flavor. One pair must have invariant massM‘‘in the range76;106 GeV=c2, while the requirement for the other pair is extended to 40;140 GeV=c2 to increase the acceptance for off-shell Zdecays.
The acceptance for theZZprocess is determined using
PYTHIA[12] followed by aGEANT-based simulation [13] of the CDF II detector. An efficiency correction, of up to 10%
per lepton, is applied to the simulation based on measure- ments of the lepton reconstruction and identification effi- ciencies using observedZ!‘‘ events.
The dominant backgrounds to the ZZ!‘‘‘0‘0 selec- tion are the Drell-Yan Z= process (DY) with two jets 201801-4
misidentified as leptons (Zjets) and DY with an addi- tional photon and a jet, both misidentified as leptons (Z jets). TheZjets andZjets background contributions are estimated from data by extrapolating from a sample of events that contain three identified leptons and a jet jl containing a track or EM energy cluster similar to those required in the lepton identification. The contribution of each event to the total yield is scaled by the probability that the jl is identified as a lepton. This probability pjl is determined from multijet events collected with jet-based triggers and is a function of thejlpTand type of lepton. A correction to pjl is applied for the small real lepton contribution using Monte Carlo simulation of single W and Z boson processes. In this background sample, one of the three identified leptons is likely to be either a jet or a photon misidentified as a lepton. An event with two leptons and two jl jets enters the three lepton plus jl sample if either of the jl jets is misidentified as a lepton, but will enter the four identified lepton sample only if bothjl jets are misidentified as leptons. Therefore, the contribution from this category of events is double counted. A correc- tion for this is made by subtracting the yield of two leptons plus two jl jets scaled by pjl;1 pjl;2. As the three lepton plusjlsample has significant contributions from the ZZ!‘‘‘0‘0 signal itself when one of the leptons is not fully identified but is counted as a jl, an anti-isolation requirement of >20% (see previous definition of calorimeter-based isolation) is applied to thejlselection.
As a cross-check, we estimate theZjet background contribution in an alternative way using the yield of three lepton plusjlevents in simulatedZjet data scaled by pjl. We correct this with the ratio of data to simulation for an analogous calculation of two identified leptons and onejlscaled bypjlinZjets events. This estimate of the Zjet background is in good agreement with the nominal estimate based solely on the data.
We separate the ZZ!‘‘‘0‘0 candidates into two ex- clusive categories based on whether or not they contain at least one forward electron without a track because the background from Zjets is much larger in candidates with a forward trackless electron. The expected signal and background yields, assuming ZZ 1:40:1 pb, and the observed yields are shown in TableI.
The statistical significance of theZZ!‘‘‘0‘0 yield is determined using a maximum likelihood fit with two bins,
one for each of the ZZ!‘‘‘0‘0 categories. We define lnL as the logarithm of the likelihood ratio between this fit and the no signal hypothesis. In 107 background- only Monte Carlo experiments, only 109 have larger lnL than that observed in data. This corresponds to a background-only probability (pvalue) of 1:1105 and a signal significance equivalent to 4.2 standard deviations.
The ZZ!‘‘ candidates are selected from events with exactly two lepton candidates excluding events with forward electrons without a track which are contaminated by largeWbackgrounds. At least one lepton is required to satisfy the trigger and have ET>20 GeV (pT>
20 GeV=c) for electrons (muons). This requirement is loosened to 10 GeV (GeV=c) for the other lepton. We apply a track-based isolation selection in which the sum of thepTof the tracks not associated with the lepton within a cone ofR <0:4around the lepton is required to be less than 10% of the momentum of the track associated with the lepton.
Aside fromZZproduction, other SM processes that can lead to two high-pT leptons include events from DY, aW decay with photon (W) or jet (Wjets) misidentified as a lepton, andtt, WW, and WZproduction. The DY back- ground is suppressed by requiring sufficiently largeE6 T in the event to remove contributions from mismeasured lep- tons and/or jets. This is achieved by requiring E6 T;rel>
25 GeV, where E6 T;rel
E6 T if E6 T;‘;jet>2 E6 TsinE6 T;‘;jet if E6 T;‘;jet<2
(1) and E6 T;‘;jet is the angle between E6~T and the nearest lepton or jet. To suppress events with E6 T from mismeas- ured unclustered energy, we require E6 T=
p
ET>
2:5 GeV1=2. We suppressttbackground by requiring fewer than two reconstructed jets withET>15 GeVandjdj<
2:5in the event. We require the lepton pair to be consistent with the same-flavor, opposite-sign property of Z decay and have dilepton invariant mass M‘‘>16 GeV=c2 to suppress QCD backgrounds.
The acceptances for the WW, WZ, ZZ, W, and tt processes are determined using the same detector simula- tion as described for theZZ!‘‘‘0‘0 channel. Events are simulated with theMC@NLOprogram forWW[14],PYTHIA
forWZ,ZZ, andtt[12], and the generator described in [15]
TABLE I. Expected and observed number ofZZ!‘‘‘0‘0 candidate events. The first uncer- tainty is statistical and the second one is systematic.
Category Candidates without a trackless electron Candidates with a trackless electron
ZZ 1:9900:0130:210 0:2780:0050:029
Zjets 0:0140:0100:0070:003 0:0820:0890:0600:016 Total 2:0040:0160:0150:210 0:3600:0890:0600:033
Observed 2 1
for W. An additional correction is applied to the W background estimate based on a measurement of the pho- ton conversion veto efficiency in data. The background fromWjets is estimated from the yield of one identified lepton plus onejl scaled bypjl. As the sample size is sufficiently large, a loose calorimeter-based isolation cut (<30%) is applied to thejlsamples to reduce the magni- tude of the extrapolation from thejlto the fully identified lepton.
We observe 276 events in the selected region which is expected to contain25621events of which only142 are from the ZZ!‘‘ process in the SM.
Approximately half of the yield is due to theWWprocess.
However, ZZ!‘‘andWW have different kinematic properties which are exploited to statistically separate the contribution of these two processes to the dataset. We calculate an event-by-event probability density for the observed lepton momenta andE6 T using leading-order cal- culations of the differential decay rate for the processes [9].
The event probability density is Pxobs 1
hi
Z dLOy
dy yGxobs; ydy; (2) where the elements of y (xobs) are the true (observed) values of the lepton momenta and E6 T, ddyLO is the parton level cross section differential in those observables,yis the detector acceptance and efficiency function, and Gxobs; yis the transfer function representing the detector resolution. The constant hi normalizes the total event probability to unity. The missing information due to the fact that we have two neutrinos in the final state is inte- grated over in this calculation. We then form a likelihood ratio discriminant LR which is the signal probability di- vided by the sum of signal and background probabilities LRPZZ=PZZPWW. The distribution of log101 LR for the data compared to the summed signal and background expectation is shown in Fig.1.
For both‘‘‘0‘0 and‘‘channels, the systematic un- certainties associated with the Monte Carlo simulation affect theWW,WZ,ZZ,W, andttexpectations similarly.
The uncertainties from the lepton selection and trigger efficiency measurements are propagated through the analy- sis, giving uncertainties from 1.0% to 1.4% and 2.1% to 6.1% for the respective efficiencies of the different signal and background processes. The detector acceptance varia- tion due to PDF uncertainties is assessed to be in the range 1.9% – 4.1% using the 20 pairs of PDF sets described in [16]. We assign a 6% luminosity uncertainty to signal and background estimates obtained from simulation [17]. The cross section uncertainties are 10% forWW [9],WZ[9], andW[18], and 15% fortt[19,20].
The systematic uncertainty on theWjets background toZZ!‘‘candidates and theZjets andZjets background to the ZZ!‘‘‘0‘0 is estimated to be 20%
from differences in the observed probability that a jet is
identified as a lepton for jets collected using different jet ET trigger thresholds. These variations correspond to changing the parton composition of the jets and the relative amount of contamination from real leptons.
For the ZZ!‘‘, the systematic uncertainty on the DY background yield due to theE6 Tresolution modeling is estimated to be 20% from comparisons of the data and Monte Carlo simulation in a sample of dilepton events. For the W background contribution, there is an additional uncertainty of 20% from the detector material description and conversion veto efficiency.
We define four independent control samples based on theZZ!‘‘selection where one of the cuts is removed or inverted to test our modeling of background-dominated data. Removing the E6 T requirement produces a DY- dominated sample which tests luminosity accounting, lep- ton reconstruction efficiency, and non-E6 T related trigger efficiencies that apply to both the ‘‘‘0‘0 and‘‘ final states; we observe (expect) 160 980 (160 00018 000) events, where the uncertainty combines statistical and systematic contributions. Inverting the charge sign require- ment to select same-sign dilepton events tests our modeling of photons and jets misidentified as leptons; we observe (expect) 161 (13819) events. Inverting theE6 T=
p ET requirement but selectingE6 T;rel>25 GeVevents tests our modeling of the effect of unclustered energy on theE6 T; we observe (expect) 55 (599) events. Finally, inverting the E6 T;rel>25 GeVrequirement but selecting E6 T>25 GeV events tests our modeling of mismeasured leptons or jets at highE6 T; we observe (expect) 151 (17830) events. The observed yields are in good agreement with the expecta- tions in each selection set.
log(1-LR)
-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
Events
10-1
1 10 102
103
WW
,W+jets γ
,W t t DY Data ZZ WZ
Syst. Uncertainty
L dt = 1.9 fb-1
∫
FIG. 1 (color online). Distribution of the discriminating vari- ablelog101LRfor theZZ!‘‘search.
201801-6
We combine the ZZ!‘‘ with the ZZ!‘‘‘0‘0 results by extending the likelihood fit previously described to include thelog101LRdistribution. Thepvalue for theZZ!‘‘alone is 0.12 and the combinedpvalue is 5:1106 corresponding to a significance equivalent to 4.4 standard deviations. This is the first evidence of ZZ production at a hadron collider with a significance exceed- ing 4 standard deviations. We determine the ZZ cross section by fitting the data for the fraction of the expected SM yield in the full acceptance and scaling the zero-width Zboson approximation cross section by that fraction. The measured cross section ispp !ZZ 1:40:70:6 pb, con- sistent with the SM expectation. This is the first measure- ment of theZZ cross section in hadron collisions.
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 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 Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme; the Slovak R&D Agency; and the Academy of Finland.
aVisitor from University of Athens, 15784 Athens, Greece.
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dVisitor from University Libre de Bruxelles, B-1050 Brussels, Belgium.
eVisitor from University of California Irvine, Irvine, CA 92697, USA.
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