Article
Reference
Measurement of Wγ and Zγ Production in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
The standard model predictions for Wγ and Zγ production are tested using an integrated luminosity of 200 pb−1 of pp¯ collision data collected at the Collider Detector at Fermilab. The cross sections are measured by selecting leptonic decays of the W and Z bosons, and photons with transverse energy ET>7 GeV that are well separated from leptons. The production cross sections and kinematic distributions for the Wγ and Zγ data are compared to SM predictions.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of Wγ and Zγ
Production in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2005, vol. 94, no. 04, p.
041803
DOI : 10.1103/PhysRevLett.94.041803
Available at:
http://archive-ouverte.unige.ch/unige:38083
Disclaimer: layout of this document may differ from the published version.
1 / 1
Measurement of W and Z Production in p p 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
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
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23University of Illinois, Urbana, Illinois 61801, USA
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, Korea; 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; and 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 4 October 2004; published 2 February 2005)
The standard model predictions forWandZproduction are tested using an integrated luminosity of 200 pb1 of pp collision data collected at the Collider Detector at Fermilab. The cross sections are measured by selecting leptonic decays of theWandZbosons, and photons with transverse energyET>
7 GeVthat are well separated from leptons. The production cross sections and kinematic distributions for theWandZdata are compared to SM predictions.
DOI: 10.1103/PhysRevLett.94.041803 PACS numbers: 13.85.Qk, 12.15.Ji, 14.60.Fg, 14.60.Hi
The study of the characteristics ofWandZproduc- tion is an important test of the standard model (SM) description of gauge boson interactions and is sensitive to physics beyond the SM. TheWandZcross sections are directly sensitive to the trilinear gauge couplings which are uniquely predicted by the non-Abelian gauge group of the SM electroweak sectorSU2LU1Y.W produc- tion can be used to study the WWvertex, and Zpro- duction can be used to constrain theZZandZvertices which vanish in the SM [1– 3]. Physics beyond the SM (e.g., compositeness models or excited W or Z bosons) could alter the cross sections and the production kinemat- ics.WandZproductions are also important background
contributions to searches for new physics, e.g., in gauge mediated supersymmetry breaking models [4].
This Letter presents measurements of pp !lX andpp !llXproduction at
ps
1:96 TeVat the Tevatron accelerator using data obtained with the upgraded Collider Detector at Fermilab (CDF). In the SM the l andllfinal states occur due toW!landZ! ll production, as well as via lepton bremsstrahlung:
W !l!landZ!ll!ll. Throughout this Letter the notation ‘‘Z’’ is used to specifyZ= production via the Drell-Yan process. The notationWandZis used to denote the landllfinal states.
The data are taken at higher center of mass energy and constitute a larger data sample by at least a factor of 2 than previous measurements [5– 9]. They were collected be- tween March 2002 and September 2003, and correspond to an integrated luminosity of about 200 pb1. W and Z bosons are selected in their electron and muon decay modes. Additionally, a photon with transverse energy above 7 GeV is selected. The production properties of theWandZevents are compared to the SM predictions.
The CDF detector is described in detail elsewhere [10].
Transverse momenta of charged particles (pT) are mea- sured by an eight-layer silicon strip detector [11] and a 96- layer drift chamber (COT) [12] inside a 1.4 T magnetic field. [We use a cylindrical coordinate system about the beam pipe in whichis the polar angle,is the azimuthal angle, and lntan=2. ET Esin and pT psinwhereEis the energy measured by the calorimeter and p the momentum measured in the tracking system.
6E~T P
iEiTn~iwheren~iis a unit vector that points from the interaction vertex to the ith calorimeter tower in the transverse plane.6ET is the magnitude of6E~T. If muons are identified in the event, 6ET is corrected for the muon mo- menta.] The COT provides coverage with high efficiency forjj<1. At higher jjthe silicon detector is used for measuring charged particles. Electromagnetic and had- ronic calorimeters surround the tracking system. They are segmented in a projective tower geometry and measure energies Eof charged and neutral particles in the central (jj<1:1) and forward (1:1<jj<3:6) regions. Each calorimeter has an electromagnetic shower profile detector positioned at the shower maximum. Located at the inner face of the central calorimeter, the central preradiator chambers use the solenoid coil as a radiator to measure the shower development. These two detectors are used for the photon identification and background determination.
The calorimeters are surrounded by muon drift chambers covering jj<1. Gas Cherenkov counters [13] measure the average number of pp inelastic collisions per bunch crossing and thereby determine the beam luminosity.
For theWandZboson selection with decays into muons or central electrons, the trigger is solely based on the identification of a high transverse momentum lepton [14].
ForW’s decaying to forward electrons, the trigger addi- tionally requires6ET >15 GeV. Off-line, a high-pT lepton (le; ) is required to fulfill tighter selection criteria [14]. Electron candidates are required to have ET >
25 GeVandjj<2:6. In the central region, a COT track withpT>10 GeV=cmust be associated with the energy deposition, while in the forward region calorimeter-seeded silicon tracking is used to associate a track with the elec- tromagnetic shower [15]. The electromagnetic shower pro- file of an electron candidate must be consistent with expectations from test beam data. Muons are selected by requiring a COT track withpT>20 GeV=cand the asso- ciated energy deposition in the calorimeter to be consistent
with that expected for a muon [14]. In addition, for at least one muon per event, the track segments in the muon chambers must match the extrapolated position of the muon track and be in the rangejj<1:0. Both electrons and muons must be isolated from other calorimeter energy depositions [14]. The selected samples correspond to an integrated luminosity of202 pb1 (168 pb1 ) for central (forward) electrons and192 pb1(175 pb1) for muons in the regionjj<0:6(0:6<jj<1:0).
For W!l candidates, we also require 6ET>25 (20) GeV in the electron (muon) channel as evidence for the neutrino. For the W ! channel, events with an additional track with pT>10 GeV=c and a calorimeter signal consistent with a muon are rejected as potential background fromZ!. For the selection ofZcan- didates, a second electron is required in the electron chan- nel and a second isolated track is consistent with a minimum ionizing particle in the muon channel.
In theZanalysis the invariant mass of the dilepton pair, Ml; l, is required to be in the range40< Ml; l<
130 GeV=c2to enhance the sensitivity to on-shellZboson production. In the W analysis the transverse mass, MTl;6ET, is required to be in the range30< MTl;6ET<
120 GeV=c2 to select on-shell W boson production. The transverse mass is used since the longitudinal component of the neutrino momentum cannot be measured:
MTl;6ET
2plT6ET1cosl;6ET
q , where l;6ET is the
difference in azimuthal angle between the lepton momen- tum and the missing transverse momentum vector.
After reconstructing a W or Z candidate, we select a photon withET >7 GeVwithinjj<1:0which is iso- lated from other particles in both the calorimeter and the tracking detectors. The transverse energy deposit around
the photon in a coneR
i2 i2 q 0:4is required to be less than10%of the photon transverse energy for ET<20 GeV and less than 20:02ET 20 GeV for ET>20 GeV. Here, i and i denote the location of the energy deposit in the ith calorimeter tower excluding those associated with the photon candidate. The total sum of the track transverse momenta in a cone of0:4 around the photon candidate is also required to be less than 2 GeV=c. To remove electron background we require there to be no track with pT>1 GeV=c pointing toward the photon candidate. The photon candidate also must have a shower shape consistent with a single particle and must be
separated from the lepton by Rl;
l2 l2
q >0:7. This last requirement is placed to suppress the contribution from bremsstrahlung photons. After all selection criteria are applied, 323 W candidates and 71Zcandidates are found.
The most important source of background to both theZ and the W analyses is the production of a real Z or W boson and a hadron which is misidentified as a photon.
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This background is determined using large event samples triggered on jets at severalET thresholds: 20, 50, 70, and 100 GeV. We measure the fraction of jets in the samples which pass all the photon selection requirements. This fraction is then corrected for prompt photon contamination within the jet samples. Two methods were used to estimate this contamination. ForET<40 GeV, the estimate of the prompt photon contamination exploits the broader shower shape of0!showers compared to promptshowers in the electromagnetic shower profile detector [16]. For ET>40 GeVhits in the central preradiator chambers are counted. In this method prompt photons are distinguished from meson decays since the probability of a photon conversion in the magnetic coil is higher for 0’s than for prompt photons [16]. The resulting fake rate for a jet to pass all photon selection cuts is about 0.3% at ET 10 GeV and decreases exponentially to about 0.07% for ET 25 GeV.
We obtain the background prediction by applying this fake rate to jets inW andZevents. The background due to events where neither the leptons nor the photon are genuine is implicitly taken into account in the above estimate. In theWanalysis an additional background arises fromZ production where large 6ET is observed due to an unde- tected lepton. This background is larger in the muon than in the electron channel due to the smaller muon coverage of the CDF detector. Another source of background is !ll production. These two backgrounds are determined using the Monte Carlo generators described below. is found to be a negligible source of back- ground in both analyses.
A summary of the background contributions for theW analysis is given in Table I. For theZanalysis, the only background is due to jets misidentified as photons. For ee, the estimated background is 2:80:9 events, and, for, it is2:10:6events.
Thepp !lX andpp !llXSM signal predic- tions are determined using leading order Monte Carlo generators for all three lepton generations. The matrix element generator [2,3] includes initial and final state photon radiation and theWWvertex diagram. Initial state QCD radiation and hadronization are included using
PYTHIA[17]. The parton momentum distribution is mod- eled with CTEQ5L parton density functions (PDF’s) [18].
OsQCD corrections [19] to theWandZproduction cross sections are calculated using CTEQ5M PDF’s [18].
These corrections increase the W (Z) cross section by 33%–55%(27%–32%) forET in the range10–55 GeV.
The SM cross section forpp !lXproduction for the kinematic region ET>7 GeV and Rl; >0:7 is 19:31:4 pb for the W boson decaying into a single lepton flavor. For the same kinematic region and with the invariant mass of the dilepton pair Ml; l>
40 GeV=c2, the cross section for pp !llX produc- tion is4:50:3 pbfor theZboson decaying into a lepton pair of a single flavor. The 7% uncertainty on the cross section due to higher order contributions and uncertainties on the PDF’s is evaluated by changing the factorization scale (2%), changing the renormalization scale (3%), and comparing the predictions made with several PDF’s (5%) [20,21].
The observed (Nobs) and expected numbers of signal (Nsig) and background (Nbg) events in the W and Z analyses are given in Table II. Both the electron and muon data are in good agreement with expectations. The systematic uncertainties on these measurements include uncertainties on the event selection efficiency and accep- tance. The main contributions come from higher order QCD corrections to the acceptance and the efficiency of the photon selection. The dominant uncertainty on the background is due to the jet fake rate uncertainty. The total systematic uncertainty on the cross sections is 9% –-14%
for theWand 3% for theZcross sections. An additional uncertainty of 6% arises from the luminosity measurement.
TABLE I. Background event contributions for the e and analyses. The combined statistical and systematic uncer- tainty on the background prediction is also quoted.
e
Wjet 59:518:1 27:67:5
1:50:2 2:30:2
ll 6:30:3 17:41:0
Total background 67:318:1 47:37:6
TABLE II. Expected and observed numbers of events fore and , ee and production. The systematic uncertainties listed for the expected number of events excludes the 7% uncertainty on the theoretical cross section and the 6%
uncertainty in luminosity measurement. The product of the acceptance and efficiency,A", and the measured cross sec- tions,#land#ll, are also listed. The first uncertainty is statistical and the second is systematic. There is a separate error on the luminosity normalization of1:2 pbfor theWand 0:3 pbfor theZcross section measurements.
e
Nsig 126:85:8 95:24:9
NsigNbg 194:119:1 142:49:5
Nobs 195 128
A" 3:3% 2:4%
#l(pb) 19:42:12:9 16:32:31:8
ee
Nsig 31:31:6 33:61:5 NsigNbg 34:11:8 35:71:7
Nobs 36 35
A" 3:4% 3:7%
#ll(pb) 4:80:80:3 4:40:80:2
The cross section#l is measured in the kinematic rangeRl; >0:7andET>7 GeVwith# Nobs Nbg=A"R
Ldt NobsNbg=Nsig#SM. Here, R
Ldt is the integrated luminosity,Ais the accep- tance,"is the selection efficiency, and#SMis the SM cross section of the Monte Carlo simulation sample used for estimating the acceptance and number of expected signal events. The resulting cross sections are given in Table II.
The measured cross sections are determined for the fullW decay phase space, transverse mass range, and photon range using extrapolations based upon the SM expectation [19]. Combining the electron and muon channels, assum- ing lepton universality, and taking into account correlations of the systematic uncertainties yields #l 18:13:1 pb. The theoretical prediction for this cross section is19:31:4 pb.
The cross section#llis measured in the kinematic range Rl; > 0.7, ET>7 GeV, and Ml; l> 40 GeV/c2. We follow the same procedure as for the W
analysis and obtain the cross section listed in Table II.
The measured cross sections are determined for the full Zdecay phase space, dilepton mass rangeMl; l>40 GeV/c2, and photon range using extrapolations based upon the SM expectation [19]. The combined electron and muon result is #ll 4:60:6 pb. The theoretical prediction for this cross section is4:50:3 pb.
In addition to these cross section measurements, we compare the SM predictions for several kinematic varia- bles with the data forET>7 GeVandRl; >0:7. We choose the ET and final state mass spectra since these are sensitive tests of SM predictions. The transverse energy of the photon inW andZproduction is shown in Fig. 1.
Figure 2 shows the cluster transverse mass [22],MTl; , for W events and the invariant mass of the (l; l; ) system,Ml; l; , forZevents. The data are in good agreement with the SM expectations for both processes.
The event with the highest ET photon, observed in the
(GeV)
γ
ET
0 10 20 30 40 50 60
Number of Events/(7 GeV)
1 10 102
a)
Data
γ ν l
ν +jet l
γ l
-l
+γ ν τ
0 10 20 30 40 50 60 1
10 102
>63 GeVTIntegral: E←
(GeV)
γ
ET
0 10 20 30 40 50 60
Number of Events/(7 GeV)
1 10
Data γ l-
l+ -+jet
+l l
b)
0 10 20 30 40 50 60 1
10
>63 GeVTIntegral: E←
FIG. 1 (color online). Photon transverse energy spectrum,ET, for (a)Wand (b)Zcandidates selected in the leptonic decay channel. The data are compared with the SM expectations for signal and background with the histograms added cumulatively.
In both figures the last bin contains all events with ET>
63 GeV.
2) ) (GeV/c ν
γ,
T(l M
50 100 150 200
)2Number of Events/(10 GeV/c
1 10
102 a)
Data
γ ν l
ν +jet l
γ l
-l
+γ ν τ
50 100 150 200
1 10 102
2 >220 GeV/cTIntegral: M←
2) ) (GeV/c γ
-,
+,l M(l
50 100 150 200
)2 Number of Events/(10 GeV/c
1 10
Data γ l-
l+ -+jet
+l l
50 100 150 200
1 10
b)
2 Integral: M>220 GeV/c←
FIG. 2 (color online). (a) The cluster transverse mass of the lepton-photon-missingETsystem forWcandidates, and (b) the invariant mass of the lepton-lepton-photon system forZ can- didates. The data are compared with the SM expectations for signal and background with the histograms added cumulatively.
In both figures the last bin contains all events with masses above 220 GeV=c2.
041803-6
eechannel withET 141 GeVandMe; e; 382 GeV=c2, is consistent with the rate expected from SM predictions.
In summary, we have measuredWandZproduction in pp collisions at ps
1:96 TeV using data from the CDF experiment. The cross sections, measured to a preci- sion of 15%, are compared to electroweak predictions having an estimated uncertainty of 7%. For ET above 7 GeV and Rl; >0:7, the production cross sections, and the photon andW=Zboson production kinematics, are found to agree with SM predictions.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
We also thank Ulrich Baur for his help and input toward the understanding of theoretical aspects of this analysis. 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 Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; and in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT-20002, Probe for New Physics.
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