Search for Quark-Lepton Compositeness and a Heavy <em>W′</em> Boson Using the <em>eν</em> Channel in <em>pp</em> Collisions at s√=1.8TeV

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Reference

Search for Quark-Lepton Compositeness and a Heavy W′ Boson Using the eν Channel in pp Collisions at s√=1.8TeV

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

We present searches for quark-lepton compositeness and a heavy W′ boson at high electron-neutrino transverse mass. We use ∼110pb−1 of data collected in pp¯ collisions at s√=1.8TeV by the CDF Collaboration during 1992–1995. The data are consistent with standard model expectations. Limits are set on the quark-lepton compositeness scale Λ, the ratio of partial cross sections σ(W′→eν)/σ(W→eν), and the mass of a W′ boson with standard model couplings. We exclude Λ

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Quark-Lepton

Compositeness and a Heavy W′ Boson Using the eν Channel in pp Collisions at s√=1.8TeV.

Physical Review Letters , 2001, vol. 87, no. 23, p. 231803

DOI : 10.1103/PhysRevLett.87.231803

Available at:

http://archive-ouverte.unige.ch/unige:37991

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

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Search for Quark-Lepton Compositeness and a Heavy W

⁰

Boson Using the en Channel in p p ¯ Collisions at p p p

s 5 1.8 TeV

T. Affolder,²³ H. Akimoto,⁴⁵ A. Akopian,³⁷M. G. Albrow,¹¹ P. Amaral,⁸D. Amidei,²⁵ K. Anikeev,²⁴ J. Antos,¹ G. Apollinari,¹¹T. Arisawa,⁴⁵ A. Artikov,⁹T. Asakawa,⁴³W. Ashmanskas,⁸ F. Azfar,³⁰ P. Azzi-Bacchetta,³¹ N. Bacchetta,³¹ H. Bachacou,²³S. Bailey,¹⁶ P. de Barbaro,³⁶ A. Barbaro-Galtieri,²³ V. E. Barnes,³⁵ B. A. Barnett,¹⁹

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I. Yu,²¹ S. Yu,³² Z. Yu,⁴⁷ A. Zanetti,⁴² F. Zetti,²³ and S. Zucchelli³ (CDF Collaboration)

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

2Argonne National Laboratory, Argonne, Illinois 60439

3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

4Brandeis University, Waltham, Massachusetts 02254

5University of California at Davis, Davis, California 95616

6University of California at Los Angeles, Los Angeles, California 90024

7Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

8Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637

9Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

10Duke University, Durham, North Carolina 27708

11Fermi National Accelerator Laboratory, Batavia, Illinois 60510

12University of Florida, Gainesville, Florida 32611

13Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

14University of Geneva, CH-1211 Geneva 4, Switzerland

15Glasgow University, Glasgow G12 8QQ, United Kingdom

16Harvard University, Cambridge, Massachusetts 02138

17Hiroshima University, Higashi-Hiroshima 724, Japan

18University of Illinois, Urbana, Illinois 61801

19The Johns Hopkins University, Baltimore, Maryland 21218

20Institut für Experimentelle Kernphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany

21Center 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

22High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

23Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720

24Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

25University of Michigan, Ann Arbor, Michigan 48109

26Michigan State University, East Lansing, Michigan 48824

27University of New Mexico, Albuquerque, New Mexico 87131

28The Ohio State University, Columbus, Ohio 43210

29Osaka City University, Osaka 588, Japan

30University of Oxford, Oxford OX1 3RH, United Kingdom

31Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy

32University of Pennsylvania, Philadelphia, Pennsylvania 19104

33Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy

34University of Pittsburgh, Pittsburgh, Pennsylvania 15260

35Purdue University, West Lafayette, Indiana 47907

36University of Rochester, Rochester, New York 14627

37Rockefeller University, New York, New York 10021

38Rutgers University, Piscataway, New Jersey 08855

39Texas A&M University, College Station, Texas 77843

40Texas Tech University, Lubbock, Texas 79409

41Institute of Particle Physics, University of Toronto, Toronto M5S 1A7, Canada

42Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

43University of Tsukuba, Tsukuba, Ibaraki 305, Japan

44Tufts University, Medford, Massachusetts 02155

231803-2 231803-2

45Waseda University, Tokyo 169, Japan

46University of Wisconsin, Madison, Wisconsin 53706

47Yale University, New Haven, Connecticut 06520 (Received 5 July 2001; published 15 November 2001)

We present searches for quark-lepton compositeness and a heavyW⁰boson at high electron-neutrino transverse mass. We use

⬃110 pb

²¹of data collected inpp¯collisions atp

s

苷

1.8 TeVby the CDF Col- laboration during 1992–1995. The data are consistent with standard model expectations. Limits are set on the quark-lepton compositeness scaleL, the ratio of partial cross sectionss

共

W⁰!en

兲兾

s

共

W !en

兲

, and the mass of a W⁰ boson with standard model couplings. We exclude L ,2.81 TeV and aW⁰ boson with mass below 754 GeV兾c² at the 95%confidence level. Combining with our previously published limit obtained using the muon channel, we exclude a W⁰ boson with mass below 786 GeV兾c² at the 95%confidence level.

DOI: 10.1103/PhysRevLett.87.231803 PACS numbers: 12.60.Rc, 13.85.Qk, 12.60.Cn

The standard model (SM) gives a good description of nature in terms of the fundamental fermions and their in- teractions via gauge bosons. However, the SM is not ex- pected to be a complete theory. For example, it does not explain the number of fermion families or their mass hier- archy. It also does not provide a unified description of all gauge symmetries. Compositeness models postulate con- stituents of the SM fermions and new strong dynamics that bind these constituents [1]. Other extensions of the SM postulate larger gauge groups and therefore new forces associated with additional charged gauge bosons, which we generically call W⁰. For instance, the left-right sym- metric model [2] expands theSU共2兲L 3U共1兲electroweak group to SU共2兲L 3SU共2兲R 3 U共1兲, predicting an addi- tional right-handed charged gauge boson.

At center-of-mass energies much smaller than the com- positeness energy scaleL, interactions between composite quarks and/or leptons have been parametrized by effec- tive four-fermion contact interactions [1]. Atomic parity violation experiments have set stringent, though model- dependent limits on quark-lepton compositeness in the neutral current channel [3]. Direct searches have set limits onLin the range 2.5– 7.9 TeV [4 –11] in a broad class of neutral current models. In this Letter, we present the first results of a search for compositeness in the charged current channel共qq¯⁰en兲using theenfinal state.

Theen final state is also sensitive to the direct produc- tion and decay of aW⁰ boson. Previous indirect searches based onm decay, theKL 2KS mass difference, neutri- noless double beta decay, and studies of b particles have resulted in stringent model-dependent limits on possible W⁰ bosons [12]. Direct searches in various decay modes have produced lower limits on theW⁰mass,mW⁰. The best limit ofmW⁰ .720 GeV兾c²in theW⁰ !enchannel [13]

assumes a light and stable neutrino, standard model cou- plings for theW⁰ to fermions, and suppressedW⁰ !WZ decays, as in extended gauge models [14]. In this Letter, we set upper limits on the ratio of partial cross sections s共W⁰ !en兲兾s共W !en兲 under the same assumptions.

We use the latter to obtain the most stringent lower limit onm_W⁰. We also present the combinedW⁰mass limit with our previously published limit obtained using the muon channel [15].

We use⬃110 pb²¹of data collected inpp¯ collisions at ps 苷1.8 TeV by the Collider Detector at Fermilab [16]

during 1992–1995. The detector includes a tracking sys- tem immersed in a 1.4 T magnetic field, scintillator-based sampling electromagnetic and hadronic calorimeters, and a muon detector. For this analysis, electron candidates are accepted in the pseudorapidity range 0.05 ,jhj,1.0, whereh 苷2log关tan共u兾2兲兴, anduis the polar angle with respect to the beam axis. Electrons detected near the fiducial edges of the calorimeter are removed to ensure uniform calorimeter response. We use a combination of electron and neutrino triggers to obtain an efficiency ex- ceeding 99% for the high transverse mass en final states that pass our off-line selection criteria.

After off-line reconstruction, the electromagnetic calorimeter cluster with the highest transverse energy 共E_T ⬅Esinu兲 in the event must satisfy the following requirements: (i) the electron must deposit most [17] of its energy in the electromagnetic calorimeter, (ii) a track in the central drift chamber must match the calorime- ter cluster in position, and (iii) the electron must be isolated in a cone of radius R ⬅p

Dh² 1 Df² 苷0.4, such that the fractional excess transverse energy in the cone, 关E_T^tot共R 苷 0.4兲 2E^e_T兴兾E^e_T ,0.1, where E_T^tot and E_T^e are the total and electron transverse energies, re- spectively. The kinematic cuts used to define the data sample are E_T^e . 30 GeV, the transverse momentum 共pT兲 of the associated track p_T^e .13 GeV兾c, the miss- ing transverse energy E兾T .30 GeV, and the electron- neutrino transverse mass mT共en兲 .50 GeV兾c², where mT共en兲 苷p

2ET^eE兾T共12 cosf_en兲, and f_en is the azi- muthal angle between the electron and the E兾T direction.

The neutrino transverse momentum is identified with E兾T by requiring transverse momentum balance in the event. Electron identification cuts based on E兾p (ratio of calorimeter energy to matched track momentum) and calorimeter energy profiles, which are imposed for E_T^e ,50 GeV to suppress jet misidentification backgrounds, are released for E_T^e .50 GeV to ensure maximum signal efficiency. A total of 31 436 events pass our selection criteria.

We use the PYTHIA [18] program to compute the compositeness and W⁰ signal processes. The contact

interaction Lagrangian is weak-isoscalar and helicity-non- conserving. It is universal with respect to quark and lepton flavor, and conserves lepton flavor and lepton number.

The contact interaction couplings are all set tog苷 p 4p. The detector response is simulated using a parametrized Monte Carlo program. The electromagnetic calorimeter sampling term is derived from test beam data. The under- lying event contribution to the electron energy resolution is derived fromW !encollider data. The constant term in the electromagnetic resolution is tuned to reproduce the observed width of the Z !ee mass peak. The electro- magnetic energy scale is set so that the reconstructed Z boson mass agrees with the world-average Z mass [19].

The hadronic response and resolution are tuned by study- ing thep_T balance inZ! eeevents.

In this analysis we normalize the number of SM back- ground Monte Carlo events after detector simulation to the large inclusive W boson sample in the data. Thus we are analyzing the shape of theentransverse mass distribution, and are insensitive to the uncertainty in the integrated lu- minosity of the data and to the overall efficiency. The effi- ciency of the additional electron identification cuts applied forE^e_T ,50 GeVis determined usingZ !eedata where one of the electrons is tagged. The second electron then provides an unbiased sample with which to measure the efficiencies. Background subtraction is performed using the sidebands of theZboson mass distribution. The com- bined efficiency of these additional cuts is共95.8 60.3兲%, relative to the full efficiency at highET^e.

The most important sources of misidentification back- ground to pp¯ !en 1 X are (i) QCD multijet events, where a jet is misidentified as an electron and there is suffi- cient energy mismeasurement to create significantE兾T, and (ii)Z !eeevents where one electron is lost or misrecon- structed. The electromagnetic energy in a jet which has been misidentified as an electron is likely to be nonisolated.

We select a representative sample of misidentified elec- trons by making the electron identification cuts on the base sample without the isolation cut, and then selecting noniso- lated candidates. The relative normalization of this sample to the jet background in the signal sample is obtained from a “pure-jet” sample. The pure-jet sample is obtained in the same way as the signal sample exceptE兾T ,10 GeV, which excludes almost all W events. We make the non- isolation and isolation cuts on the pure-jet sample, and the ratio of the respective number of events accepted pro- vides the normalization factor for the background. This technique assumes that the isolation for a jet is indepen- dent of E兾T. The systematic uncertainty of 30% on the jet misidentification background is estimated by studying the correlation between isolation and E兾T. The Z !ee background is estimated using a Monte Carlo sample of Z !eeevents, passed through a full detector simulation and reconstructed like the data. The sytematic uncertainty of 23% on theZ !eebackground is estimated by varying the detector response to electrons near the fiducial edges of the calorimeter. Other systematic uncertainties, indi-

TABLE I. Systematic uncertainties on the SM background and the signal due to the parton distribution functions (PDFs), the K factor, and the detector model.

SM background (%) Signal (%)

PDFs 10 10

K factor 4 4

Hadronic resolution 0.1 2

Vertexz width 0.5 1.8

Hadronic scale 0.2 1.6

EM resolution 0.1 1.5

Electron efficiency 1.0 1.0

EM scale 0.2 0.9

Total 11 12

cated in Table I, are derived by varying the parameters in the Monte Carlo simulation. In our publishedW⁰ !mn analysis [15], the maximum parton-distribution-function (PDF) uncertainty was found to be 10% at high mT共en兲. This estimate is consistent with the PDF analysis discussed in [20]. We conservatively take the PDF uncertainty to be 10% in all our highmT共en兲search bins.

Other high pT processes also contribute to en final states. Using PYTHIA, we evaluate the following back- ground processes: W ! en (dominant), W ! tn! enX,t¯t !enX,WW !enX,WZ !enX,ZZ !enX, and g^ⴱ兾Z !tt !enX. We pass these Monte Carlo events through the parametrized detector simulation to estimate their contribution. These physics backgrounds dominate over the misidentification backgrounds at high entransverse mass, due to the presence of real neutrino(s) producing large E兾T. For example, the jet and Z ! ee misidentification background fractions amount to 25%

and 3%, respectively, formT共en兲 .150 GeV兾c². Figure 1 shows the transverse mass distribution of the data events normalized to the bin width. Also shown is the expectation based on SM processes and detector back- grounds. We apply a mass-dependent K factor [defined as the ratio of the next-to-next-to leading order (NNLO) and the leading-order (LO) Drell-Yan cross section calcu- lations from Ref. [21] ] to the LOPYTHIAcalculation. The K factor varies between 1.24 at 80 GeV兾c² and 1.65 at 800 GeV兾c². The effects of the detector acceptance and response have been folded into the theoretical prediction.

Table II shows the expected and the observed number of events in the high transverse mass bins. There is good agreement between the data and the expectation. Also shown are all backgrounds excluding the dominant SM W !en process, and the expectation of the composite- ness model withL 苷2 TeV.

To set a limit on the compositeness scale L, we gen- erate Monte Carlo events for the compositeness process using PYTHIA, corrected with the K factor. We perform a Bayesian analysis [19] of the shape of the m_T distribu- tion of events. The expected number of events in thekth transverse mass bin is denoted by N_L^k 苷b^k 1Le^ks_L^k, wheres_L^k is the predicted cross section for a given scaleL,

231803-4 231803-4

∫ L dt ∼ 110 / pb 1992-95 CDF data SM W→eν + backgrounds non-W→eν backgrounds Λ = 2 TeV + backgrounds

m

_T

(e ν ) (GeV/c

²

)

events / 10 GeV/c

2

10^-4 10^-2 1 10² 10⁴

0 200 400 600 800 1000

FIG. 1. The event yield from the data as a function of the en transverse mass, normalized to the bin width. Also shown are the SM prediction including backgrounds, all backgrounds excluding the dominant SMW !enprocess, and the prediction of the compositeness process with energy scale L

苷

2 TeV.

The simulation of the physics processes includes the effects of detector acceptance and response.

ande^k andb^k denote the total acceptance and remaining backgrounds in thekth bin. The prediction for the number of events, including all backgrounds, is normalized to the observed number of events for m_T共en兲 ,150 GeV兾c². Given the data 共D兲, we compute the posterior probabil- ity distribution forLaccording to

P共LjD兲苷 1 A

Z db de

Yn k苷1

"

e^2NLkN^kN

ok

L

N_o^k! P共b^k,e^k兲

# P共L兲. N_o^k denotes the observed number of events. We take the prior distribution P共b^k,e^k兲 of the nuisance parameters b ande to be Gaussian with the rms given by their total un- certainties. The bin-to-bin correlations in the uncertainty on the acceptance and background are taken into account.

We make the conventional choice for the prior distribution TABLE II. The observed number of events and the total ex- pected number of events from SM and detector background sources, in transverse mass bins.

mT bin

共GeV兾c

²

兲

Nobserved Nexpected

150 – 200 70 62.268.5

200 – 250 18 18.363.4

250 – 300 5 4.0160.44

300 – 350 2 1.6160.18

350 – 400 0 0.7260.08

400 – 500 1 0.4960.06

500 – 600 0 0.1160.02

600 –1000 0 0.0560.01

P共L兲 to be uniform in 1兾L². The 95% confidence level (C.L.) lower limit is defined byR`

LP共L⁰jD兲dL⁰ 苷0.95, yielding L . 2.81 TeV. The expected limit, obtained when the observed number of events is set equal to the expected number, is L . 2.70 TeV. Varying the choice of the prior distributionP共L兲changes the limit by 10%.

To set a limit on the mass of a W⁰ boson, we com- pute the Poisson probability for the observed number of events given Nexpected 苷Nbackground 1NW⁰. The Pois- son probability is computed separately in three search windows: 0.5MW⁰ ,mT , 0.65MW⁰,0.65MW⁰ , mT , 0.8MW⁰, and0.8MW⁰ ,mT , 1.1MW⁰, and then the prob- abilities are combined. The use of three windows allows us to exploit the difference in the shape of theW⁰ signal and background m_T distributions. Uncertainties in the back- grounds and signal acceptance are incorporated by convo- luting the probabilityP共N_W⁰兲over Gaussian fluctuations in these parameters, taking correlations across bins into ac- count. The 95% C.L. upper limit on the number ofW⁰sig- nal events,N_W⁹⁵⁰, is defined byR^N_W⁹⁵0

0 P共N_W⁰兲d共N_W⁰兲 苷0.95.

The limitN_W⁹⁵⁰ may be expressed as a 95% C.L. limit on the ratiosB共W⁰ !en兲兾sB共W !en兲using

µsB共W⁰ ! en兲 sB共W !en兲

∂

95 苷 N_W⁹⁵⁰A_W AW⁰NW

,

where NW is the observed number of SMW events and AW⁰共AW兲is the total acceptance forW⁰ ! en 共W !en兲 decays. The 95% C.L. upper limit on sB共W⁰ !en兲兾 sB共W ! en兲 is plotted as a function of MW⁰ in Fig. 2 together with the theory curve fromPYTHIA6.129, assum- ing standard model couplings and including theK factor.

From the intersection of the two curves, aW⁰ boson with mass m_W⁰ ,754 GeV兾c² is excluded at 95% C.L. The expected limit in this case is 748 GeV兾c². We combine this result with our previously published result on a W⁰ boson using themn final state [15]. Taking the PDF un- certainty to be fully correlated between the two analyses and with the same model assumptions, we obtain the com- bined limit excludingmW⁰ ,786 GeV兾c²at the 95% C.L.

In conclusion, we find no significant deviation between the measuredentransverse mass distribution at high trans- verse mass and the SM prediction. We have used the data to exclude the quark-lepton compositeness scaleL , 2.81 TeV, in the context of an effective contact interac- tion. We set limits on the ratio of the cross section times branching ratio toenof a W⁰ boson to a standard model Wboson. We use the latter to exclude aW⁰boson with SM couplings and massm_W⁰ ,754 GeV兾c². Combining with our muon channel result, we excludemW⁰ , 786 GeV兾c² at the 95% C.L.

We thank T. Sjöstrand and K. Lane for discussions re- garding PYTHIAand W. L. Van Neerven for providing the code to compute the NNLO SM Drell-Yan cross section.

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

∫ L dt ∼ 110 / pb PYTHIA 6.129

m

_W'

(GeV/c

²

)

σ (W' → l ν ) / σ (W → l ν )

eν data 95% C.L. upper limit eν & µν 95% C.L. upper limit

754 GeV/c²

786 GeV/c² 10^-5

10^-4 10^-3 10^-2

200 400 600 800 1000

FIG. 2. The 95% C.L. upper limit on the ratio of partial cross sectionss共W⁰!ᐉn兲兾s共W !ᐉn兲, for theedata and the com- bined e1 m data. Also shown is the SM prediction for this ratio, and themW⁰ limits obtained from the intersection of the experimental and theory curves.

National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Science, Sports and Culture of Japan; the Natural Sciences and En- gineering Research Council of Canada; the National Sci- ence Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bun- desministerium fuer Bildung und Forschung, Germany; the Korea Science and Engineering Foundation (KoSEF); the Korea Research Foundation; and the Comision Interminis- terial de Ciencia y Tecnologia, Spain.

*Now at Northwestern University, Evanston, IL 60208.

†Now at Carnegie Mellon University, Pittsburgh, PA 15213.

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[17] For electron candidates we require E_had

兾

E_em,0.0551 0.0453

共E

^e

兾100 GeV兲

, where Ehad and Eem are the hadronic and electromagnetic energies, respectively.

[18] PYTHIA version 6.129, T. Sjöstrand, Comput. Phys. Com- mun.82,74 (1994).

[19] Particle Data Group, D. E. Groomet al., Eur. Phys. J. C 15,1 (2000).

[20] S. Kuhlmannet al.,Phys. Lett. B476,291 (2000).

[21] R. Hamberg, W. L. Van Neerven, and T. Matsuura, Nucl.

Phys. B359,343 (1991).

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