Measurement of the <em>W⁺W⁻</em> Production Cross Section in <em>pp</em> Collisions at s√=1.96  TeV using Dilepton Events

Article

Reference

Measurement of the W

W

Production Cross Section in pp Collisions at s√=1.96  TeV using Dilepton Events

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We present a measurement of the W+W− production cross section using 184  pb−1 of pp collisions at a center-of-mass energy of 1.96 TeV collected with the Collider Detector at Fermilab. Using the dilepton decay channel W+W−→ℓ+νℓ−ν¯, where the charged leptons can be either electrons or muons, we find 17 candidate events compared to an expected background of 5.0+2.2−0.8 events. The resulting W+W− production cross-section measurement of σ(pp→W+W−)=14.6+5.8−5.1(stat)+1.8−3.0(syst)±0.9(lum)  pb agrees well with the standard model expectation.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the W

W

Production Cross Section in pp Collisions at s√=1.96  TeV using Dilepton Events. Physical Review Letters , 2005, vol. 94, no. 21, p. 211801

DOI : 10.1103/PhysRevLett.94.211801

Available at:

http://archive-ouverte.unige.ch/unige:38095

Disclaimer: layout of this document may differ from the published version.

1 / 1

Measurement of the W

W

Production Cross Section in p p Collisions at

p s

1:96 TeV using Dilepton Events

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A. Zsenei,¹⁸and S. Zucchelli⁴

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

5Brandeis University, Waltham, Massachusetts 02254, USA

6University of California at Davis, Davis, California 95616, USA

7University of California at Los Angeles, Los Angeles, California 90024, USA

8University of California at San Diego, La Jolla, California 92093, USA

9University of California at Santa Barbara, Santa Barbara, California 93106, USA

10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

14Duke University, Durham, North Carolina 27708, USA

15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

16University of Florida, Gainesville, Florida 32611, USA

17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

18University of Geneva, CH-1211 Geneva 4, Switzerland

19Glasgow University, Glasgow G12 8QQ, United Kingdom

211801-2

20Harvard University, Cambridge, Massachusetts 02138, USA

21The Helsinki Group: Helsinki Institute of Physics, and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00044, Helsinki, Finland

22Hiroshima University, Higashi-Hiroshima 724, Japan

23University of Illinois, Urbana, Illinois 61801, USA

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 21 January 2005; published 2 June 2005)

We present a measurement of theWWproduction cross section using184 pb¹ofppcollisions at a center-of-mass energy of 1.96 TeV collected with the Collider Detector at Fermilab. Using the dilepton decay channelWW!‘‘, where the charged leptons can be either electrons or muons, we find 17 candidate events compared to an expected background of 5:0^2:2_0:8 events. The resulting WW production cross-section measurement of pp !WW 14:6^5:8_5:1stat^1:8_3:0syst 0:9lumpb agrees well with the standard model expectation.

DOI: 10.1103/PhysRevLett.94.211801 PACS numbers: 13.38.Be, 14.70.Fm

The measurement of theWpair production cross section inpp collisions at

ps

1:96 TeVprovides an important test of the standard model. AnomalousWW andWWZ triple gauge boson couplings [1], as well as the decays of new particles such as Higgs bosons [2], could result in a rate ofW pair production that is larger than the standard

model cross section of 12:40:8 pb [3]. The first evi- dence forWpair production was found inppcollisions by the Collider Detector at Fermilab (CDF) Collaboration at ps

1:8 TeV [4]. The properties of W pair production have been extensively studied by the CERN LEP Collaborations in ee collisions up to

ps

209 GeV

[5], and have been shown to be in good agreement with the standard model. The D0 experiment has recently reported a measurement of the W pair production cross section at Run II of the Fermilab Tevatron [6].

In this Letter we describe a measurement of theWW production cross section in the dilepton decay channel WW!‘‘ (‘e; ), and compare the event kinematics with standard model predictions. The signature forWW!‘‘ events is two high-P_T leptons and missing transverse energy, 6E_T, from the undetected neu- trinos [7]. Jets from the hadronization of additional partons in the event due to initial-state radiation may be present.

This analysis is based on18411 pb¹of data collected by the upgraded CDF during the Tevatron Run II period.

For details of the 6% luminosity uncertainty see [8].

The CDF II detector [9] has undergone a major upgrade since the Run I data-taking period. The Central Outer Tracker (COT) is a large-radius cylindrical drift chamber with 96 measurement layers organized into alternating axial and 2 stereo superlayers [10], and is used to reconstruct the trajectories (tracks) of charged particles and measure their momenta. The COT coverage extends tojj 1. A silicon microstrip detector [11,12] provides precise tracking information near the beam line in the region jj<2. The entire tracking volume sits inside a 1.4 T magnetic field. Segmented calorimeters, covering the pseudorapidity region jj<3:6, surround the tracking system. The central (jj<1) and forward (jj>1) elec- tromagnetic calorimeters are lead-scintillator sampling de- vices, instrumented with proportional and scintillating strip detectors that measure the position and transverse profile of electromagnetic showers. The hadron calorimeters are iron-scintillator sampling detectors. Four layers of planar drift chambers located outside the central hadron calorim- eters (CMU) and another set behind a 60 cm thick iron shield (CMP) detect muons with jj<0:6. Additional drift chambers and scintillation counters (CMX) detect muons in the region0:6<jj<1:0. Gas Cherenkov coun- ters [13] measure the average number of inelastic pp collisions per bunch crossing and thereby determine the beam luminosity.

A trigger selects events with a central electron with E_T>18 GeV, a muon withP_T>18 GeV=c, or a forward electron withE_T>20 GeV. For forward electrons, 6E_T >

15 GeVis also required.

Off-line, electron candidates are selected in the central region by matching a well-measured track reconstructed in the fiducial region of the COT to an energy cluster with E_T>20 GeV deposited in the surrounding calorimeters with identification requirements described in detail in [8].

For forward electrons (1:2<jj<2:0), the track-energy cluster association utilizes a calorimeter seeded silicon tracking algorithm [14].

Muon candidates are selected off-line by demanding P_T>20 GeV=c, energy deposition in the calorimeter con-

sistent with that of a minimum ionizing particle, and the same requirements on the reconstructed track as for central electrons. A tightly selected muon category requires the COT track to extrapolate to track segments in either the CMU and CMP chambers or the CMX chambers; a loosely selected category requires the COT track to extrapolate to gaps in the muon chamber coverage.

Significant backgrounds to WW production in the dilepton decay channel include Drell-Yan events with large 6E_T (mismeasured in the case ofZ= !ee,or due to’s in the case ofZ= !),Wjet= events in which the jet or photon fakes a lepton,ttproduction, and heavy diboson (WZ,ZZ) production.

All lepton candidates are required to be isolated in order to suppress the background from fake leptons. To be iso- lated, the fraction of the additionalE_Tfound in a cone with

radius R

²²

p 0:4 around the electron

(muon) must be less than 10% of the electron E_T (muon P_T). The corresponding isolation requirement calculated using track momenta is also imposed.

Candidate events are required to have two well identi- fied, oppositely charged leptons (electrons or muons), and are classified as ee,, or e. An event can contain at most one loose muon. We reject events containing more than two leptons passing all the above identification and isolation criteria. We require events to contain no cone radius 0.4 jets withE_T>15 GeVandjj<2:5.

We require all candidate events to have 6E_T>25 GeV, after the 6E_T has been corrected for the escaping muon momentum when muon candidates are present. To reduce the likelihood of falsely reconstructed 6E_T due to mismeas- ured leptons, the 6E_T direction must have an azimuthal angle of at least 20 from the closest lepton if the 6E_T is less than 50 GeV. To further reduce the Drell-Yan back- ground, eeand candidates with a dilepton invariant mass in theZmass region76< M_‘‘<106 GeV=c² must pass an additional requirement of 6E^sig_T >3 GeV¹⁼². Here, missing transverse energy significance is defined as6E^sig_T 6E_T=

E_T

p whereE_T is the scalar transverse energy sum over all calorimeter towers.E_T is corrected for muons in an identical manner to the 6E_T calculation.

The signal acceptance is computed using a large sample ofWW!‘‘ events generated using thePYTHIA

Monte Carlo program [15] and passed through a detailed detector simulation. W ! decays are included, and their contribution to the total acceptance is taken into account. CTEQ5 parton distribution functions (PDF’s) [16] are used for the signal as well as the background Monte Carlo samples. The trigger and lepton identification efficiencies are measured using Z!‘‘ data [8]. The final acceptance estimate for WW events, assuming a branching ratio BRW !‘ 0:10680:0012[17], is 0:450:05%. The number ofWW events expected in the dilepton decay channels is calculated using this accep-

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tance number and a NLO (next-to-leading-order) estimate for the total WW cross section in pp collisions at 1.96 TeV of12:40:8 pb[3], using CTEQ6 PDF’s [18].

The fraction of WW events containing no recon- structed jets (‘‘zero-jet fraction’’) is calculated using the WWPYTHIAMonte Carlo sample and multiplied by the ratio of zero-jet fractions measured in Drell-Yan data and

PYTHIADrell-Yan Monte Carlo samples. This scale factor, 0:960:06, corrects for the underestimate of the rate of associated jet production by a leading-order matrix ele- ment Monte Carlo program such asPYTHIA. The corrected zero-jet fraction forWWevents is765%.

The systematic uncertainty on the total acceptance for WWevents in the dilepton channel is a combination of uncertainties on the zero-jet fraction (6%), choice of gen- erator and parton shower model (4%), jet energy scale (3%), lepton identification (2%), trigger efficiencies (1%), modeling of the track isolation (4%) and 6E^sig_T dis- tributions (2%), and choice of PDF (1%). We assume no correlations between these sources of uncertainty and com- bine them to give an overall 10% systematic uncertainty on theWW acceptance.

The Drell-Yan background (Z= !ee, , ) is estimated using a combination of data and Monte Carlo samples, including a large sample of

PYTHIAgenerated Drell-Yan events. The Drell-Yan back- ground estimate in theechannel is entirely Monte Carlo calculation based. The background fromZ= !in all detection channels is also based on Monte Carlo calcu- lations alone. In the like-flavor dilepton channelsee(), the background fromZ= !ee (Z= !) is estimated with a method described next that makes use of both Monte Carlo calculations and data. The background estimate outside theZmass window starts by counting the number of data events inside theZmass window that pass all the WW selection criteria applied outside. This number of events is multiplied by the ratio of the number of Drell-Yan events outside and inside theZmass window, estimated using Monte Carlo simulations after all out-of- window selection criteria have been applied. The same method is applied to estimate the background inside the Zmass window, using the ratio of events that pass both6E_T and6E^sig_T cuts to those that fail one or both of these require- ments. Monte Carlo is needed to estimate a significant contamination from non-Drell-Yan events in the data samples used in this procedure.WWevents themselves contribute to this contamination. This dependence of the Drell-Yan background estimate on the WW cross sec- tion, and vice versa, is resolved by iteratively finding a common solution to both. Statistical uncertainties on the data dominate the final systematic uncertainty on the Drell- Yan background.

We estimate the fake lepton background contribution by applying a P_T dependent lepton fake rate to ‘ 6Ed events, where d denotes any object which could fake a

lepton. Such events must pass all other WW selection criteria. The lepton fake rates are defined by the ratio N_‘=N_d. Objects that can fake leptons that are counted in the denominator (N_d) are jets with E_T>20 GeV and jj<2 for electrons and tracks with P_T >20 GeV=c andE=P <1for muons. The numerator (N_‘) is the number of objects passing all lepton identification and isolation criteria. The lepton fake rates are determined using large samples of jet triggered data with jet E_T thresholds in the range 20 –100 GeV, correcting for the presence of real leptons from W andZproduction. The probability for an object to fake a lepton is of the order10⁴to10³depend- ing on the lepton type and detector region. Studies of fake rate variations between jet samples with different trigger thresholds and using various object definitions of objects that can fake leptons have been performed. The estimated systematic uncertainty on this background is 40%.

TheW background estimate is derived using a leading- order Monte Carlo generator for the process pp ! W X!‘ X [19], which has been interfaced to

PYTHIAfor the purposes of parton showering and hadroni- zation. The sample is normalized to a NLO calculation of the W cross section [20]. The W background that is double counted in the fake lepton background estimate described above is determined to be negligible.

The remaining backgrounds fromtt, WZ, andZZ pro- duction are calculated using Monte Carlo samples gener- ated withPYTHIA and normalized to NLO cross sections.

The background from top pair production in the dilepton decay channel (tt!WbWb!‘b‘b) is greatly reduced by the zero-jet requirement. The background from WZ production has two main contributions: WZ! qq⁰‘‘, which is largely rejected by the zero-jet require- ment, andWZ!‘‘‘, which is largely rejected by the veto on trilepton events. The background coming fromZZ production is predominantly due to ZZ!‘‘. The final systematic uncertainty on the total background esti- mate is approximately 45%, dominated by the uncertainty on the Drell-Yan background.

The signal and background expectations are summa- rized in Table I, together with the number of data events passing the selection criteria [21]. The measured cross section is

WW 14:6^5:8_5:1stat^1:8_3:0syst 0:9lumpb;

where the systematic uncertainty is a combination of the uncertainties on the signal acceptance and background estimates. The third uncertainty corresponds to a 6% un- certainty from the integrated luminosity measurement. The dilepton mass and lepton transverse momenta distributions are shown in Fig. 1. There is no evidence for statistically significant discrepancies in either the dilepton mass or lepton transverse momentum distributions, which could indicate the presence of poorly estimated backgrounds or physics beyond the standard model.

We have performed an alternative measurement of the WW production cross section, which tests the robust- ness of our result in a sample with different signal and background composition. The event selection is based on the ‘‘leptontrack’’ analysis used for our measurement of thettproduction cross section in the dilepton channel [22].

There are two important differences between the leptontrack analysis and our main analysis. First, one of the two lepton candidates is required only to be an isolated track. Second, all events must pass a6E^sig_T require- ment of 6E^sig_T >5:5 GeV¹⁼² where, here, the E_T sum is made over all jets with E_T>5 GeV. The candidate iso- lated track must haveP_T>20 GeV=cand be in the range jj<1. Again, only events with no jets are considered.

The overall acceptance is0:42%0:05%, similar to the acceptance for the main analysis. The increased acceptance for dilepton events where electrons or muons pass through gaps in the calorimetry or muon system and for single prong hadronic decays of the lepton from W! is offset by the more restrictive6E^sig_T cut required to control the larger backgrounds.

The numbers of observed events, the expected standard model backgrounds, and the predictedWW signal are compared for both analyses in Table II. The higher back- ground rates for the leptontrack analysis are mainly due to the fake lepton background contribution coming from the isolated track. The resulting cross-section measurement using the leptontrack selection is

WW 24:26:9stat^5:2_5:7syst 1:5lum pb:

The two measurements are statistically compatible with one another given an estimated 43% overlap in signal acceptance. Since combining the results of these two analyses does not result in a significant reduction of the uncertainty, we quote as the final result of this measure-

ment the analysis with the besta priorisensitivity, which is the analysis summarized in Table I.

In summary, we have measured theWWcross section inpp collisions atps

1:96 TeVto be14:6^6:1_6:0 pb. This is based on the observation of 17 events consistent with originating fromW pair production and subsequent decay to two charged leptons, compared to a total estimated background of5:0^2:2_0:8 events. The measured cross section is consistent with a NLO standard model prediction and is corroborated by an independent leptontrack analysis.

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

We also thank John Campbell and Keith Ellis for many useful discussions. This work was supported by the U.S.

Department of Energy and National Science Foundation;

TABLE I. Estimated backgrounds, WW signal, and the observed number of events in184 pb¹for each dilepton cate- gory. TheWW expectation assumes a total cross section of 12.4 pb. Systematic uncertainties are included.

ee e

Z= !‘‘ 0:21^1:29_0:16 0:43^1:56_0:38 0:430:14 WZ 0:290:03 0:330:03 0:150:02 ZZ 0:350:04 0:340:04 0:0110:002

W 0:480:13 — 0:570:13

tt 0:0210:011 0:0120:007 0:0460:018

Fake 0:520:19 0:170:16 0:650:37

Background 1:9^1:3_0:3 1:3^1:6_0:4 1:90:4 ExpectedWW 2:60:3 2:50:3 5:10:6 Total expected 4:5^1:4_0:5 3:8^1:6_0:5 7:00:8

Observed 6 6 5

2) Dilepton Invariant Mass (GeV/c

0 20 40 60 80 100 120 140 160 180 200

2 Events/20 GeV/c

0 1 2 3 4 5 6 7

8 WW+Bkgnd

Bkgnd Data L = 184 pb-1

(GeV/c) Lepton PT

0 20 40 60 80 100 120 140 160 180 200

Leptons/10 GeV/c

0 2 4 6 8 10 12

WW+Bkgnd Bkgnd Data L = 184 pb-1

FIG. 1 (color online). The dilepton mass (top) and lepton transverse momentum distribution (bottom) for the candidate events in comparison with the standard model expectation.

Kolmogorov-Smirnov tests of these distributions yieldpvalues of 13% (top) and 78% (bottom).

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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 fuer 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-2002-00292, Probe for New Physics.

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TABLE II. Estimated backgrounds, WW signal, and the observed number of events for both the main (MAIN) analysis using184 pb¹ and the leptontrack (LTRK) analysis using 197 pb¹. The signal expectation assumes a totalWWcross section of 12.4 pb. Systematic uncertainties are included.

MAIN LTRK

Drell-Yan (Z= !‘‘) 1:06^2:03_0:44 1:81^2:36_1:38

WZ 0:760:06 1:010:24

ZZ 0:700:07 0:760:18

W 1:060:19 0:330:13

tt 0:0780:023 0:180:04

Fake 1:340:66 7:963:47

Background 5:0^2:2_0:8 12:1^4:2_3:8

ExpectedWW 10:201:19 10:231:37

Total expected 15:2^2:5_1:5 23:0^4:4_4:0

Observed 17 32