Search for New High-Mass Particles Decaying to Lepton Pairs in <em>pp</em> Collisions at s√=1.96  TeV

Article

Reference

Search for New High-Mass Particles Decaying to Lepton Pairs in pp Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

A search for new particles (X) that decay to electron or muon pairs has been performed using approximately 200  pb−1 of pp collision data at s√=1.96  TeV collected by the CDF II experiment at the Fermilab Tevatron. Limits on σ(pp→X)BR(X→ℓℓ) are presented as a function of dilepton invariant mass mℓℓ>150  GeV/c2, for different spin hypotheses (0, 1, or 2). The limits are approximately 25 fb for mℓℓ>600  GeV/c2. Lower mass bounds for X from representative models beyond the standard model including heavy neutral gauge bosons are presented.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for New High-Mass Particles Decaying to Lepton Pairs in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2005, vol.

95, no. 25, p. 252001

DOI : 10.1103/PhysRevLett.95.252001

Available at:

http://archive-ouverte.unige.ch/unige:38289

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

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Search for New High-Mass Particles Decaying to Lepton Pairs in p p Collisions at p s

1:96 TeV

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K. Sumorok,³¹H. Sun,⁵⁴T. Suzuki,⁵³A. Taffard,²³R. Tafirout,³²R. Takashima,³⁹Y. Takeuchi,⁵³K. Takikawa,⁵³ M. Tanaka,²R. Tanaka,³⁹M. Tecchio,³³P. K. Teng,¹K. Terashi,⁴⁸S. Tether,³¹J. Thom,¹⁶A. S. Thompson,²⁰ E. Thomson,⁴³P. Tipton,⁴⁷V. Tiwari,¹²S. Tkaczyk,¹⁶D. Toback,⁵¹K. Tollefson,³⁴T. Tomura,⁵³D. Tonelli,⁴⁴ M. To¨nnesmann,³⁴S. Torre,⁴⁴D. Torretta,¹⁶S. Tourneur,¹⁶W. Trischuk,³²R. Tsuchiya,⁵⁵S. Tsuno,³⁹N. Turini,⁴⁴ F. Ukegawa,⁵³T. Unverhau,²⁰S. Uozumi,⁵³D. Usynin,⁴³L. Vacavant,²⁸A. Vaiciulis,⁴⁷S. Vallecorsa,¹⁹A. Varganov,³³

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T. Yoshida,⁴⁰I. Yu,²⁷S. S. Yu,⁴³J. C. Yun,¹⁶L. Zanello,⁴⁹A. Zanetti,⁵²I. Zaw,²¹F. Zetti,⁴⁴X. Zhang,²³ J. Zhou,⁵⁰and S. Zucchelli⁵

(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

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

14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

15Duke University, Durham, North Carolina 27708

16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

17University of Florida, Gainesville, Florida 32611, USA

18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

19University of Geneva, CH-1211 Geneva 4, Switzerland

20Glasgow University, Glasgow G12 8QQ, United Kingdom

21Harvard University, Cambridge, Massachusetts 02138, USA

252001-2

22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

23University of Illinois, Urbana, Illinois 61801, USA

24The Johns Hopkins University, Baltimore, Maryland 21218, USA

25Institutfu¨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 H3A 2T8, Canada and University of Toronto, Toronto M5S 1A7, Canada

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 Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

46Purdue University, West Lafayette, Indiana 47907, USA

47University of Rochester, Rochester, New York 14627, USA

48The Rockefeller University, New York, New York 10021, USA

49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza’’, I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, USA

51Texas A&M University, College Station, Texas 77843, USA

52Istituto Nazionale di Fisica Nucleare, University of Trieste, Udine, Italy

53University of Tsukuba, Tsukuba, Ibaraki 305, Japan

54Tufts University, Medford, Massachusetts 02155, USA

55Waseda University, Tokyo 169, Japan

56Wayne State University, Detroit, Michigan 48201, USA

57University of Wisconsin, Madison, Wisconsin 53706, USA

58Yale University, New Haven, Connecticut 06520, USA (Received 25 July 2005; published 12 December 2005)

A search for new particles (X) that decay to electron or muon pairs has been performed using approximately200 pb¹ ofpp collision data at

ps

1:96 TeVcollected by the CDF II experiment at the Fermilab Tevatron. Limits on pp!XBRX!‘‘ are presented as a function of dilepton invariant massm‘‘>150 GeV=c², for different spin hypotheses (0, 1, or 2). The limits are approximately 25 fb form_‘‘>600 GeV=c². Lower mass bounds forXfrom representative models beyond the standard model including heavy neutral gauge bosons are presented.

DOI:10.1103/PhysRevLett.95.252001 PACS numbers: 13.85.Rm, 12.38.Qk, 13.85.Qk, 14.70.Pw

A search for new particles (X) has been performed in the dilepton (ee and ) decay channel using pp collision data at

ps

1:96 TeVcollected by the upgraded Collider Detector at Fermilab (CDF II) at the Tevatron. The ob- served dilepton invariant mass (m_‘‘) distribution is com- pared with that expected from standard model (SM) processes for m_‘‘>150 GeV=c². Many models beyond the SM predict such particles with masses at or below the

TeV scale [1]. Generic searches for spin 0, 1, and 2 par- ticles are performed, taking into account the dependence of the experimental acceptance on the spin-dependent angular distributions of the lepton pair. While this approach pro- vides sensitivity to broad classes of new models, the spin 1 result addresses an issue of fundamental importance in particle physics: the possible existence of extra neutral gauge bosons expected in many models with a higher

gauge structure than that of the SM. A generic SM-like (sequential) Z⁰ boson (Z⁰_SM) is defined to have the same coupling strengths to fermions as those of the SMZ⁰boson and its mass bound provides a convenient reference indi- cating the experimental sensitivity. The previous bestZ⁰_SM lower mass bounds from direct searches are690 GeV=c² by the CDF collaboration [2] and670 GeV=c² by the D0 collaboration [3] at the 95% confidence level (C.L.) [4].

Increased integrated luminosity and center-of-mass energy for Run II are expected to provide a significant improve- ment over these previous results. Indirect limits on the mass ofZ⁰bosons have been set by the LEP II experiments [5]. A more detailed discussion of the LEP results and the advantages of the Tevatron search can be found in Ref. [6].

In addition toZ⁰_SM, we considerZ⁰bosons (spin 1) from the E₆model (Z,Z ,Z,Z_I) [7] and the littlest Higgs model (Z_H) [8], technicolor (TC) particles (spin 1) [9], sneutrinos (~) in anR-parity violating supersymmetric model (spin 0) [10], and gravitons in the Randall-Sundrum (RS) warped extra dimension model (spin 2) [11]. Independent of spe- cific models, the limits onX_‘‘ pp !XBRX!

‘‘presented here can be used to set lower bounds on the mass ofX (m_X) in many classes of models with a narrow width resonance. Using the spin 1 X_‘‘ limit result, bounds on the couplings in more generalized Z⁰ models [6] have been derived and are presented.

The CDF II detector is a forward-backward and azimu- thally symmetric detector with a tracking system immersed in a 1.4 T solenoidal magnetic field, calorimetry for mea- suring the energies of particles, and detectors to identify deeply penetrating muons [12]. The tracking system con- sists of an open-cell drift chamber, the central outer tracker (COT), surrounding an eight layer silicon tracker. The fiducial coverage of the COT isjj<1:0and the silicon extends this coverage forward tojj<1:8[13]. The track- ing system is surrounded by electromagnetic (EM) and hadronic calorimeters that are divided into a central calo- rimeter (jj<1:1) and two forward, or ‘‘plug,’’ calorim- eters (1:2<jj<3:6). Drift chambers, located outside the hadronic calorimeters and also outside an additional 60 cm of iron shield, detect muons havingjj<1:0.

Candidate events are selected from data collected during 2002 – 2003, corresponding to an integrated luminosity ranging from 173 to200 pb¹, depending upon the detec- tor elements required for the analysis. Dielectron events with a central candidate are collected using a single- electron trigger requiring a loosely selected electron in the central EM calorimeter with E_T >18 GeV and a matching COT track with p_T>9 GeV=c. Dielectron events without a central candidate are collected using a trigger requiring two loosely selected electron candidates in the plug EM calorimeter with E_T >18 GeV and no tracking requirement. Additional triggers with higher E_T thresholds but looser electron-selection requirements are used to ensure full efficiency for high-mass events.

Together, these triggers are essentially 100% efficient for the eedecay mode form_‘‘>150 GeV=c². Dimuon can- didate events are collected with single-muon triggers which require a muon-chamber track with a matching track measured by the COT with p_T>18 GeV=c. The overall trigger efficiency for thedecay mode is above 90%.

The dilepton event selection requires at least two elec- tron or two muon candidates with no charge requirement.

Both electron and muon candidates are required to be isolated with a cut on the energy found within a cone of

angular radiusR

^{2 2}

p 0:4around the lep-

ton candidate. Electron candidates require an EM cluster withE_T>25 GeVand longitudinal and transverse shower profiles consistent with electrons [14]. At least one of the two electrons is required to have a matching track, except

2

Events / 5 GeV/c

10

10

1 10 10

10

10

Drell - Yan QCD Background

t

0, t

±Z

-, W

+W

-, W τ τ+

ee

2

Events / 5 GeV/c

10

10

1 10 10

10

10

2) Dielectron Mass (GeV/c 200 300 400 500 600 2Events / 5 GeV/c

0 5 10 15 20 25

2) Dielectron Mass (GeV/c 200 300 400 500 600 2Events / 5 GeV/c

0 5 10 15 20 25

2

) Dilepton Mass (GeV/c

200 400 600

2

Events / 5 GeV/c

10

10

1 10 10

10

10

Drell - Yan QCD + Cosmic

t

0, t

±Z

-, W

+W

-, W τ τ+

µ µ

2

) Dilepton Mass (GeV/c

200 400 600

2

Events / 5 GeV/c

10

10

1 10 10

10

10

2) Dimuon Mass (GeV/c 200 300 400 500 600 2Events / 5 GeV/c

0 2 4 6 8 10

2) Dimuon Mass (GeV/c 200 300 400 500 600 2Events / 5 GeV/c

0 2 4 6 8 10

FIG. 1 (color online). The ee (top panel) and (bottom panel) invariant mass distributions of the observed data (points) with the background prediction (solid line). The background is corrected for acceptance and efficiency. The insets show the data with a fixed bin width of5 GeV=c²form‘‘>150 GeV=c².

252001-4

for events with two central electrons, which both require matching tracks. The inclusion of events with two forward electrons is possible due to a calorimeter-seeded forward tracking algorithm [15]. Events with a significant amount of 6E_T are rejected to remove Wjets and others back- grounds with unreconstructed particles. All muon candi- dates are required to have a COT track with p_T>

20 GeV=c and calorimeter energy deposition consistent with a minimum-ionizing particle signal, where at least one candidate must also have a matching track in the muon chambers. To reject cosmic-ray events, muon candidates are required to have COT hit timing consistent with outward-moving particles [16].

The selected data contain 14 799 ee and 7775 candidate events with the dilepton invariant mass distri- butions shown in Fig. 1. These samples are dominated by events in the Z⁰ peak. In this region the dielectron sample has a larger acceptance; however, in the high- mass search region the two channels have similar sensitiv- ity. The lepton identification efficiencies are estimated using a purified sample of dilepton events fromZ⁰ decays [2]. Since leptons from the decay of high-mass objects typically have higher p_T than this sample, the lepton identification efficiency is studied as a function of p_T,

and the selection criteria are chosen to ensure high effi- ciencies throughout the relevant p_T range [17,18]. The geometric and kinematic acceptance as a function of reso- nance mass is estimated using Monte Carlo (MC) samples:

the PYTHIA event generator [19] with CTEQ5L parton distribution functions (PDFs) [20] and the CDF II detector simulation are used except as noted. Signal samples for the heavy Higgs (spin 0), Z⁰_SM (spin 1), and RS Graviton (spin 2) are generated to model each spin hypothesis.

The product of acceptance and selection efficiency is ap- proximately 50% form_X>400 GeV=c²foreeandfor all spins.

The primary and irreducible SM background results from Drell-Yan production of ee and pairs. It is estimated using MC simulation normalized to fit to the data in the Z⁰ peak, after the other background contribu- tions have been subtracted. This reduces the effect of the luminosity uncertainty on the background estimate. The other contributions such as tt (generated with HERWIG

[21]), ,WW, andWZ⁰ are estimated using MC simulation. Some accepted eeevents come from nondie- lectron sources, predominantly misidentified QCD dijet events. This background is estimated by extrapolating from events where the leptons are not isolated. The QCD background in the channel is estimated using same- sign events that pass the selection criteria and is found to be small. The cosmic-ray background in the channel is estimated by applying the signal selection criteria to a sample of cosmic-ray data collected by the CDF II detector and is non-negligible at high mass (m_‘‘>400 GeV=c²).

Figure 1 compares the estimated background distributions to theeeanddata. Table I shows the integrated number of events observed and expected above a givenm_‘‘.

Systematic uncertainties on the acceptance, efficiency, and luminosity result in a relative uncertainty on the scale ofX_‘‘of approximately 10%. The largest contributions TABLE I. Integrated number of events above a givenm_‘‘for

the observed data and estimated background.

m_‘‘ ee

(GeV=c²) Observed Expected Observed Expected

>150 205 212:999:3 58 55:32:5

>200 84 78:233:4 18 20:91:0

>300 22 13:64:4 6 5:20:3

>400 5 2:90:7 1 2:30:2

>500 2 0:80:1 1 1:20:1

200 400 600

ll) (pb)→BR(X⋅X)→ p(pσ

10-2

10-1

1

ll) NLO

→ ν∼

⋅BR(

σ

BR=0.01) 2×

λ τ ( ν∼ RPV '

a) spin-0

2) X mass (GeV/c

200 400 600 800

×1.3 ll) LO SM→

⋅BR(Z σ

×1.3 ll) LO η→

⋅BR(Z σ

'

b) spin-1

200 400 600 800

×1.3 ll) LO

→ BR(G*

⋅ σ

=0.1) RS Graviton (k/MPl

95% CL limits µ) µ

→

⋅BR(X σ

→ ee)

⋅BR(X σ

→ ll)

⋅BR(X σ

c) spin-2

FIG. 2 (color online). TheX_‘‘limits fromee,, and the combined channels as a function ofm_Xfor spin 0 (a), spin 1 (b), and spin 2 (c). For the combined channel,BRX!ee BRX![BRX!‘‘] is assumed. Also shown are theoretical cross- section predictions of some representative models [22].

are from the uncertainties on luminosity, energy and mo- mentum scales and resolutions, and the choice of PDF as estimated by comparison of different PDF parametriza- tions. Background uncertainty in the ee channel ranging from 40 –80% due to misidentified jets results in absolute uncertainties on values ofX_‘‘that are large form_‘‘<

350 GeV=c² but negligible at the higher mass region.

Background uncertainties in the channel are30%

and20%due to fake muons and cosmic rays, respec- tively. The relative uncertainty with respect to the scale of X_‘‘ on the electroweak backgrounds is 5% in both channels.

Since no significant excess of events is observed, limits on X_‘‘ are extracted using a Bayesian, binned like- lihood method. For combined dilepton results assuming BRX!ee BRX!, a joint likelihood is formed from the product of the individual-channel likelihoods accounting for the correlations among systematic uncer- tainties. When the nuisance parameters are integrated out, uncertainties on PDF, luminosity and common selection efficiencies are taken as 100% correlated among the differ- ent components of the acceptance. This joint likelihood is converted to a posterior density in the signal cross section and numerically integrated to obtain the 95% C.L. limits on X_‘‘. Figure 2 and Table II show theX_‘‘limits as a function ofm_X with spins 0, 1, and 2. At high mass (m_X>

600 GeV=c²) the limits are approximately 25 fb for all spins (but best for spin 0) and are consistent with expected limits in the absence of signal. The corresponding CDF Run I limit was 40 fb [2]. The sensitivity of these searches is enhanced compared to the Run I searches by the addition of the plug-plug dielectrons (10% relative gain ineeac- ceptance), an increase in muon trigger coverage and the use of muons without muon-chamber tracks (50% relative gain in acceptance). Figure 2 also shows the predic- tions from some representative models [22] with higher order corrections [23]. The particleXis assumed to decay only to the known fermions in the mass range examined.

From the spin 0X_‘‘limit shown in Fig. 2(a), the lower mass bounds of 680, 620, and460 GeV=c² fromeechan- nel and 665, 590, and450 GeV=c² fromchannel are obtained for ~ for the coupling strength squared times branching fraction ⁰² Br 0:01, 0.005, and 0.001, re- spectively. For spin 1 [Fig. 2(b)] the following mass bounds are obtained from the combined channel: 825, 690, 675, 720, and 615 GeV=c² for Z⁰_SM, Z, Z , Z, and Z_I, re- spectively, and 885, 860, 805, and725 GeV=c²forZ_Hwith

the mixing parameter cot_H 1:0, 0.9, 0.7, and 0.5, re- spectively. Similarly, the lower mass limits of280 GeV=c² (270 GeV=c²) are set for_TCand!_TCin the TC model [9]

with corresponding values of Technicolor-scale mass pa- rameters M_V M_A of500 GeV=c² (400 GeV=c²). From the spin 2X_‘‘limit shown in Fig. 2(c), the lower mass bounds of 710, 510, and170 GeV=c²are obtained for the first excited state of the RS graviton for dimensionless coupling parameter (k=M_PL) 0.1, 0.05, and 0.01, respec- tively, where k is the relative strength of the warped dimension’s curvature scale and M_PL is the effective Planck scale. A method of factorizing the couplings, charges, and 1=s dependence of Z⁰ cross sections from kinematic factors that depend upon PDF parametrizations allows more general constraints on possibleZ⁰models [6].

In this formalism, a genericZ⁰is described by two parame- TABLE II. 95% C.L. upper limits onpp!XBRX!‘‘(in fb) for a givenm_X(inGeV=c²). Spin 1 limits are computed to 900 GeV=c²to accommodateZ⁰ models with large predicted cross sections.

Spinnm_X 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900

Spin 0 340 200 83 74 91 48 31 29 26 22 19 18 18 17

Spin 1 490 290 120 110 120 72 42 38 32 25 24 23 22 21 21 24

Spin 2 390 210 98 67 100 56 37 37 33 25 24 22 24 24

10^-6 10^-5 10^-4 10^-3 10^-2 10^-1

10^-6 10^-5 10^-4 10^-3 10^-2 10^-1

c

c

M = 900 GeV/c² 800 GeV/c²

600 GeV/c² 400 GeV/c² 200 GeV/c² 150 GeV/c²

The region above each curve is excluded.

B-xL q+xu (upper) q+xu (low

er) 10+x5

Z_η Z_ψ

Z_χ

Z_I

FIG. 3 (color online). Limit contours in the (c_d,c_u) plane [6]

for a given Z⁰ mass derived from the spin 1 X‘‘ limit. All possible models for theU1BxLgroup are on the diagonal solid line, and those for theU1_10x5group are below the dotted line.

The two dashed lines show the range between which the values for the U1qxu group must fall. The values for the U1dxu

group may fall anywhere on the plane. The parameters of the E6-modelZ⁰bosons are indicated.

252001-6

ters,c_d andc_u, that define the coupling of down and up- type quarks to the resonance. Figure 3 shows the bounds set by the spin 1 limits in the (c_d, c_u) plane along with the parameters describing the fourE₆-modelZ⁰bosons.

We thank D. Choudhury, A. Daleo, H. Logan, and S.

Mrenna for their useful contributions. We thank the Fermilab staff and the technical staffs of the participat- ing 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, Ger- many; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK; the Russian Foundation for Basic Research;

the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT- 2002-00292; and the Academy of Finland.

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[22] The Refs. [17,18] show all the cross-section predictions of theoretical models which are used to derive the lower mass bounds.

[23] A constantKfactor of 1.3 is used to be consistent with the previous analyses making comparison of the Z⁰ mass limits easier. The NLO calculation is used for the RPV

~

case. The dependence on the higher order corrections for theX_‘‘limits is negligible.