Search for new heavy particles decaying to Z⁰Z⁰ -&gt; <em>eeee</em> in <em>pp</em> collisions at s√=1.96  TeV

Article

Reference

Search for new heavy particles decaying to Z

Z

-> eeee in pp collisions at s√=1.96  TeV

CDF Collaboration

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

Abstract

We report the results of a search for the anomalous production of a massive particle decaying to four electrons via two Z0 bosons in 1.1  fb−1 of pp collisions at s√=1.96  TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range 500–1000  GeV/c2 to be 0.028±0.009(stat)±0.011(syst) events. We observe zero events in this search region. Assuming a Randall-Sundrum graviton production model, we set 95% C.L.

limits on σ×BF(G→Z0Z0)

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for new heavy particles decaying to Z

Z

-> eeee in pp collisions at s√=1.96  TeV. Physical Review. D , 2008, vol. 78, no. 01, p. 012008

DOI : 10.1103/PhysRevD.78.012008

Available at:

http://archive-ouverte.unige.ch/unige:38533

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

Search for new heavy particles decaying to Z

Z

! eeee in p p collisions at ffiffiffi p s

¼ 1:96 TeV

T. Aaltonen,²³J. Adelman,¹³T. Akimoto,⁵⁴M. G. Albrow,¹⁷B. A´ lvarez Gonza´lez,¹¹S. Amerio,⁴²D. Amidei,³⁴ A. Anastassov,⁵¹A. Annovi,¹⁹J. Antos,¹⁴M. Aoki,²⁴G. Apollinari,¹⁷A. Apresyan,⁴⁷T. Arisawa,⁵⁶A. Artikov,¹⁵ W. Ashmanskas,¹⁷A. Attal,³A. Aurisano,⁵²F. Azfar,⁴¹P. Azzi-Bacchetta,⁴²P. Azzurri,⁴⁵N. Bacchetta,⁴²W. Badgett,¹⁷

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Y. Zheng,^8,band 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

14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

16Duke University, Durham, North Carolina 27708, USA

17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

18University of Florida, Gainesville, Florida 32611, USA

19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

20University of Geneva, CH-1211 Geneva 4, Switzerland

21Glasgow University, Glasgow G12 8QQ, United Kingdom

22Harvard University, Cambridge, Massachusetts 02138, USA

23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

24University of Illinois, Urbana, Illinois 61801, USA

25The Johns Hopkins University, Baltimore, Maryland 21218, USA

26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

27Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

Sungkyunkwan University, Suwon 440-746, Korea;

Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;

Chonnam National University, Gwangju, 500-757, Korea

28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

29University of Liverpool, Liverpool L69 7ZE, United Kingdom

30University College London, London WC1E 6BT, United Kingdom

31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;

and University of Toronto, Toronto, Canada M5S 1A7

34University of Michigan, Ann Arbor, Michigan 48109, USA

35Michigan State University, East Lansing, Michigan 48824, USA

36University of New Mexico, Albuquerque, New Mexico 87131, USA

37Northwestern University, Evanston, Illinois 60208, USA

38The Ohio State University, Columbus, Ohio 43210, USA

39Okayama University, Okayama 700-8530, Japan

40Osaka City University, Osaka 588, Japan

41University of Oxford, Oxford OX1 3RH, United Kingdom

42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

45Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

47Purdue University, West Lafayette, Indiana 47907, USA

48University of Rochester, Rochester, New York 14627, USA

49The Rockefeller University, New York, New York 10021, USA

50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

51Rutgers University, Piscataway, New Jersey 08855, USA

52Texas A&M University, College Station, Texas 77843, USA

53Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

54University of Tsukuba, Tsukuba, Ibaraki 305, Japan

55Tufts University, Medford, Massachusetts 02155, USA

56Waseda University, Tokyo 169, Japan

57Wayne State University, Detroit, Michigan 48201, USA

58University of Wisconsin, Madison, Wisconsin 53706, USA

59Yale University, New Haven, Connecticut 06520, USA (Received 9 January 2008; published 29 July 2008)

rWith visitors from IFIC (CSIC-Universitat de Valencia), 46071 Valencia, Spain,

qWith visitors from Texas Tech University, Lubbock, TX 79409, USA

pWith visitors from Queen Mary, University of London, London, E1 4NS, England,

oWith visitors from the University de Oviedo, E-33007 Oviedo, Spain,

nWith visitors from Nagasaki Institute of Applied Science, Nagasaki, Japan,

mWith visitors from the University of Manchester, Manchester M13 9PL, England,

lWith visitors from Universidad Iberoamericana, Mexico D.F., Mexico,

kWith visitors from the University of Heidelberg, D-69120 Heidelberg, Germany,

jWith visitors from the University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom,

iWith visitors from the University College Dublin, Dublin 4, Ireland,

hWith visitors from the University of Cyprus, Nicosia CY- 1678, Cyprus,

gWith visitors from Cornell University, Ithaca, New York 14853, USA

fWith visitors from the University of California Santa Cruz, Santa Cruz, California 95064, USA

eWith visitors from the University of California Irvine, Irvine, California 92697, USA

dWith visitors from the University Libre de Bruxelles, B-1050 Brussels, Belgium,

cWith visitors from the University of Bristol, Bristol BS8 1TL, United Kingdom,

bWith visitors from the Chinese Academy of Sciences, Beijing 100864, China,

aWith visitors from the University of Athens, 15784 Athens, Greece,

We report the results of a search for the anomalous production of a massive particle decaying to four electrons via twoZ⁰ bosons in 1:1 fb¹ of pp collisions at ffiffiffi

ps

¼1:96 TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range500–1000 GeV=c² to be0:028 0:009ðstatÞ 0:011ðsystÞ events. We observe zero events in this search region. Assuming a Randall- Sundrum graviton production model, we set 95% C.L. limits onBFðG!Z⁰Z⁰Þ<4–6 pb, depend- ing on the graviton mass.

DOI:10.1103/PhysRevD.78.012008 PACS numbers: 12.60.Cn, 13.85.Rm, 14.70.Hp

I. INTRODUCTION

We present a search for new heavy particles ‘‘G’’ in the decay modeG!Z⁰Z⁰!eeeein pp collisions at ffiffiffi

ps 1:96 TeV performed with the CDF II detector at the¼ Fermilab Tevatron. Previous analyses of double gauge boson production at the Tevatron have focused on standard model production [1–4]. Here, we present for the first time a search for massive particles G which decay to Z⁰Z⁰ which could indicate physics beyond the standard model.

The goal of this search is to be sensitive to production of any massive particle which could decay toZ⁰Z⁰, that is, to avoid focusing on any one specific model; however, for the purpose of quantifying acceptance for this search, we consider the virtual production of gravitons in a simple Randall-Sundrum (RS1) scenario [5,6]. In this model, the geometry consists of two three-branes which confine the standard model sector separated from each other by a single extra dimension. One can look for evidence of the extra dimension at particle colliders in the form of a Kaluza-Klein tower of discrete, massive gravitons. In the RS1 scenario, the gravitons predominantly decay to jets, and the remaining modes are W^þW (10%), Z⁰Z⁰ (5%), (5%), and ll (2% per lepton)[7].

Searches for the decays of such particles to photons and electrons have been performed [8–10]. If the couplings to leptons and photons are suppressed relative to the cou- plings to gauge bosons [11], such a particle might escape detection in these searches. Here, we have searched for massive particles in their decays toZ⁰ bosons.

In the leptonic final states of Z⁰ decay, the expected signal from the model described above is small, as are the backgrounds. In order to maximize acceptance and effi- ciency for the four-electron signature, we use optimized calorimetric electron identification criteria, select electron candidates identified as isolated tracks where there is no calorimeter coverage, and use low electron energy thresholds.

The organization of this article is as follows: first, we describe the components of the CDF II detector relevant to this search and summarize the data sample and event selection criteria. Then we describe the background esti- mation, report the results of the search, and interpret the results in the context of the lightest massive RS1 graviton.

II. THE CDF II DETECTOR

This analysis uses1:1 fb¹ofppcollisions collected by the CDF II detector, a general purpose magnetic spec- trometer. We briefly describe the components of the detec- tor relevant to this search here. A complete description can be found elsewhere [12].

A combination of tracking systems reconstruct the tra- jectories and measure momenta of charged particles in a 1.4 T solenoidal magnetic field. Trajectories of charged particles are reconstructed using an eight-layer silicon microstrip vertex tracker [13] at radii 1:3< r <29 cm [14], and a 96-layer open-cell drift chamber (COT) providing eight superlayers of alternating axial and stereo position measurements [15]. The COT allows track recon- struction at large radii 43< r <132 cm in the region jj<1:6, and provides full geometric coverage for jj<1:0.

Outside the tracking volume, segmented electromag- netic (EM) lead-scintillator and hadronic (HAD) iron- scintillator sampling calorimeters measure particle ener- gies [16]. In the central region (jj<1:1), the calorimeters are arranged in a projective-tower cylindrical geometry, divided azimuthally into 15 wedges which measure EM energies with a resolution of½ðEÞ=E² ¼ ð13:5%Þ²=E_Tþ ð2%Þ². In the region covered by the forward calorimeter (1:1<jj<3:6), the calorimeters are arranged in an azimuthally-symmetric disk geometry and measure EM energies with a resolution of ½ðEÞ=E² ¼ ð16:0%Þ²=Eþ ð1%Þ². Wire chambers (scintillator strips) embedded in the central (forward) EM calorimeters at the electromagnetic shower maximum (6X₀) provide position and lateral shower development measurements used to identify elec- trons by their characteristic energy-deposition distribution.

The beam luminosity is determined by measuring the inelastic pp collision rate with gas Cherenkov detectors [17], located in the region3:7<jj<4:7.

III. EVENT SELECTION

Events are selected for collection by a three-level trigger system. We search in data collected by triggering on a central high-momentum electron. Each of two trigger paths used requires an energy cluster in the central calorimeter and a track which projects to the energy cluster. The

primary trigger path requires a clustered transverse energy E_T>18 GeV, transverse momentum of the associated trackp_T>9 GeV=c, the ratio of energy measured in the hadronic to electromagnetic calorimeters, E_HAD=E_EM<

0:125and a lateral shower profile consistent with an elec- tron. The second trigger path requires a cluster withE_T>

70 GeVand an associated track withp_T>15 GeV=c. We select events containing one ‘‘seed’’ electron which satisfies trigger requirements and those listed in Table I, and three other electrons which satisfy either the central, forward, or track requirements in Table I. To maximize acceptance and efficiency for events containing four elec- trons, we select three other electron candidates using opti- mized identification and kinematic criteria in the central or forward calorimeters, and from isolated tracks pointing to uninstrumented regions of the calorimeters.

Electron candidates are formed in the central and for- ward calorimeters from isolated energy deposits withET 5 GeV. An electron is considered to be isolated in the calorimeter if the sum of the transverse energy detected within a cone R ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðÞ²þ ðÞ²

p 0:4, minus the

electron E_T, is less than 20% of the electron E_T. For clusters in the central calorimeter, where tracking effi- ciency is optimal, we require that a track reconstructed in the COT project to the cluster. Tracks must include mea- surements in at least three axial and two stereo superlayers of the COT, and the track coordinate along the beam direction, z₀, must also lie within the nominal extent of the interaction region, jz0j<60 cm. To reduce back- grounds from hadrons misidentified as electrons, central candidates must also satisfy an energy-dependent require- ment E_HAD=E_EM<0:055þ0:00045 GeV¹E_EM. Forward candidates must have EHAD=EEM<0:05and lie within1:1<jj<2:5.

The seed electron candidate must satisfy the above requirements to be reconstructed in the central calorimeter, and satisfy additional selection criteria imposed by the trigger. Specifically, the seed electron must also haveE_T 20 GeV, satisfy the same lateral shower profile require- ment as the triggering one, and have an associated track withp_T10 GeV=c.

Some calorimeter acceptance for electrons is lost in 24 1 gaps in between the central calorimeter projective wedges, a region at 0:7< <1:0 and 75< <90 which accommodates cables and cryogenic utilities for

the solenoid, a gap between central calorimeter arches at ¼0, and the gap at the junction between the central and forward calorimeters at 1:0<jj<1:2. Together, these regions add up to approximately 17% of the solid angle for jj<1:2, which for a four-lepton final state represents an acceptance loss of approximately half.

We recover acceptance by forming electron candidates from isolated tracks which project to the gaps between instrumented regions of the calorimeter. A track is defined to be isolated in the tracking chamber if the transverse momentum of the track is more than 90% of the total transverse momentum of all tracks within a cone R 0:4around the candidate track. We require track electron candidates be consistent with originating from prompt decays by requiring that they pass within 0.2 mm (2 mm) of the axial beam position for tracks with (without) posi- tion measurements in the silicon vertex tracker.

To reconstructZ⁰ candidates, we form all unique com- binations of pairs of electrons in the event. To avoid rejecting events where the charge of one electron is mis- identified, we do not impose an opposite charge require- ment on the pair. We ensure thatZ⁰ candidates are formed from distinct electron candidates by requiring the two electron candidates are isolated from each other by a separation of 0.2 in R between the two electron candi- dates in the combination. If both candidates in a pair have associated tracks, we ensure they are consistent with orig- inating from the same parent by requiring their z₀ mea- surements to lie within 5 cm of each other. The invariant mass distributions for events containing just oneZ⁰ candi- date formed from a seed electron candidate together with just one other electron candidate and the subset where an isolated track is used as the second electron candidate are shown in Fig.1.

To reconstruct Z⁰ pairs, we form all unique combina- tions of all Z⁰ candidates containing a seed electron with all other Z⁰ candidates in the event and again require a separation of 0.2 inRbetween any two electrons in the four-electron combination.

The variable

² ¼ X

i¼1;2

m_im_Z⁰

i

₂

(1) quantifies consistency between a given combination and a Z⁰Z⁰ !eeee final state, where m_Z⁰ ¼91:19 GeV=c² is the nominal Z⁰ mass [18], m_i is the measured invariant mass of each candidate Z⁰ in the combination computed from the electron candidates’ four-momenta, and_iis the uncertainty on the mass of eachZ⁰candidate consisting of a contribution from the intrinsic width of theZ⁰boson and a contribution propagated from the individual electron energy or momentum measurements (typically 3:5 GeV=c²). In each event, we retain the one Z⁰Z⁰ combination with the lowest².

We fix the final event selection criteria before examining the event yield in the signal region. We define the signal TABLE I. Criteria for identifying electrons in the central and

forward calorimeters and as isolated tracks.

Criteria Central (Seed) Forward Track

E_T(GeV) 5ð20Þ 5

jz0j(cm) <60 <60

E_HAD=E_EM <0:055þ0:000 45 GeV¹E <0:05

Isolation <0:2 <0:2 >0:9

p_T (GeV=c) ð10Þ 10

region as the events containing a four-electron combination with²<50andmeeee>500 GeV=c², wheremeeeeis the four-electron invariant mass computed from the four mea- sured electron energies.

Events with m_eeee<400 GeV=c² are used as a low- mass control region to estimate backgrounds, as described in Sec. IV. We find 12 events containing four-electron candidates in this control region.

Although we do not focus on one specific model, for the purpose of quantifying acceptance for this signature, we consider a graviton-production scenario implemented in the HERWIG [19] Monte Carlo generator which is treated in a model-independent way, assuming only that there is a universal coupling of the graviton to standard model par- ticles. For comparison, we interpret the couplings in the context of the RS1 model. We determine geometric and kinematic acceptance and reconstruction efficiency for this model using Monte Carlo calculations followed by a

GEANT-based simulation of the CDF II detector [20]. We consider graviton production followed by decay to twoZ⁰ bosons followed by decay into four electrons. We use a leading-order calculation implemented inHERWIGto esti- mate acceptance times efficiency for the model. For a RS1 graviton with massM_G¼500 GeV=c² and ratio of warp factor to Planck mass, k=M_Pl¼0:1, Fig. 2 shows the distribution of reconstructed ² and m_eeee, the invariant mass of the Z⁰Z⁰ combination with the lowest ² com- puted from the four momenta of the twoZ⁰candidates. As expected for events containing two realZ⁰bosons, the² distribution peaks near zero, and the total invariant mass of selected combinations is centered on the generated gravi- ton mass ð500 GeV=c²Þ. Events which contain mismeas- ured electrons contribute to the population with large² values. The width of them_eeee distribution,15 GeV=c², is dominated by the detector resolutions of the constituent electron candidates. We find the geometric and kinematic

FIG. 2. Distribution of ² for simulated Randall-Sundrum signal scenario (m_G ¼500 GeV=c²) (top). Four-electron invari- ant mass distribution for events satisfying ²<50 (gray) and for events which fail this requirement (black) (bottom).

FIG. 1. Distribution of m_eefor events with a single Z⁰ candidate formed from a seed electron candidate together with a second electron candidate (a), and the subset of Z⁰candidates formed from a seed electron candidate and an isolated track (b).

acceptance times efficiency for this model to be 65%, not including the fraction for Z⁰ decays to electrons. Of the events reconstructed, 93% yield a four-electron combina- tion with²<50.

The total acceptance versusmeeeeis shown in Fig.3. At very high graviton mass, the momentum of the daughter Z⁰s becomes significant, which can cause the electrons to have a small opening angle and fail the isolation requirement.

IV. BACKGROUND ESTIMATION

In studies using Monte Carlo simulation to estimate the main sources of backgrounds, we find that the dominant background consists of events in which one or more had- rons satisfy the electron ID requirements in the four- electron combination.

We use control samples in the data to obtain the shape and normalization of this background in the signal region.

Background-dominated (hadron-enriched) control samples are selected from the data. We form hadron candidates,h, from calorimeter clusters in a manner identical to the central and forward electron candidates, with two excep- tions. The hadron candidate must fail the relevant E_HAD=E_EMcriterion, and to increase the size of the control samples, we do not impose any isolation requirements. In Fig. 4we show the invariant mass of all pairings of one seed electron candidate with one hadron candidate. The absence of a significant peak at theZ⁰ mass indicates that contamination from electrons in the hadron candidates is small.¹

We obtain five control samples, namely, the four- electron sample which has m_eeee<400 GeV=c² intro- duced above, and additional control samples having one, two, three, or four hadron candidates by repeating theZ⁰Z⁰ selection procedure, forming combinations using one or more hadron candidates with electron candidates and re- taining the minimum² combination for each event. The distributions of the minimum²versusmeeee for samples with different numbers of hadron candidates are shown in Fig.5. For reconstructed masses smaller than twice theZ⁰ mass, there is a correlation between²and mass caused by a kinematic threshold effect. At higher masses, the two variables are much less correlated.

The single probability density function,

fð²; m_eeeeÞ ¼Cmeeeee²; (2) whereCis a normalization constant, provides an empirical description of the²vsmeeee distributions for each of the four hadron-enriched control samples. We obtain the pa- rameters ¼ 4:570:06 and ¼ 0:003 19 0:000 07from a two-dimensional unbinned maximum like- lihood fit to the low-mass four-electron control region and the hadron-enriched control samples simultaneously, using events with invariant mass above 185 GeV=c² (2 m_Z⁰). The control samples containing mostly hadron can- didates dominate the fit. Figure6shows the projections of the fit result in the invariant mass dimension along with the data for each hadron-enriched sample. The low-mass con- trol region (m_eeee<400 GeV=c²) contains five events with meeee>185 GeV=c² and serves to normalize the prediction of background in the high-mass search region.

We integrate the fit result above500 GeV=c²and²<50 to extract an estimate of the total background for the high- mass region. Using this method, we estimate 0:020 0:009ðstatÞ 0:007ðsystÞbackground events from hadrons FIG. 3. Acceptance for Randall-Sundrum graviton decaying to

Z⁰Z⁰ versus its mass. FIG. 4. Invariant mass distribution of one central electron satisfying trigger requirements and one hadronic candidate in data.

1The presence of a small amount of electron contamination in the hadron candidate sample has a negligible effect on the estimate of the background at high mass, and is included in the systematic uncertainty we assign to the background estima- tion method.

misidentified as electrons in the search region. The system- atic uncertainty on the background estimate is obtained by varying the functional form of the probability density function fitted to them_eeeespectrum.

We have performed several cross-checks to ensure that the fit provides a reasonable estimate of the background rate at high mass. In particular, we have performed the fit allowing the power-law parameter to vary independently for each of the categories. The parameters resulting from this fit along with the number of events observed in each sample are shown in Table II. The fitted values forare consistent within errors across categories and with the nominal result. We have verified that the background shape inm_eeee is independent of²in subsets of data in bins of ² and have checked that the projected fit result is con- sistent with data.

Standard model production of Z⁰Z⁰ [21] is the only background to this search with two real Z⁰ bosons and possibly four electrons in the final state. While we use data

to estimate the total background from misidentified elec- trons in the search region, we have studied the background from this source at high mass with simulated events. We determine geometric and kinematic acceptance using Monte Carlo events generated by PYTHIA [22], followed by aGEANT-based simulation of the CDF II detector. The expected number of events from this background compo- nent is determined as the product of the cross section, the luminosity of the sample, and the acceptance of the detector. We estimate a total of 0:540:04events in the four-electron sample. In the invariant mass region above 500 GeV=c², we expect0:0080:006 events. We inter- pret this statistical uncertainty on the background prediction from simulated events as a systematic uncer- tainty on the total background prediction. We estimate the total background from production of standard model Z⁰Z⁰ events and events in which hadrons are mis- identified as electrons is 0:0280:009ðstatÞ 0:011ðsystÞevents.

FIG. 5. Distribution of ²vs m_eeeefor control samples containing one, two, three, or four hadron candidates with the number of events in the plot increasing with the number of hadron candidates used in the combination.

V. OTHER SYSTEMATIC UNCERTAINTIES When setting the cross section limit, we have considered other systematic uncertainties from several sources. The dominant source of these uncertainties is from the mea- sured luminosity (5.9%) [23]. Other sources include parton distribution function uncertainties (0.4%), signal Monte Carlo statistics (1.3%), initial state radiation (1.0%), and the difference between electron identification

efficiency in data and simulation (1.0% per electron). The total systematic uncertainty from all these sources is 7.3%.

VI. RESULTS

The distribution of data events surviving all require- ments is shown in Fig. 7. We observe no events in the high-mass signal region. There is one event in the low- mass region with very small ² consistent with standard model Z⁰Z⁰ production, while we expect 0.54 standard model Z⁰Z⁰ events over the entire mass range. In the observed event, the total invariant mass is 190 GeV=c², and the two Z⁰ candidates in the lowest ² combination have measured masses of 91 and92 GeV=c².

We have set limits onðpp !GÞ BFðG!Z⁰Z⁰Þin the context of a RS1 graviton scenario. We use a Bayesian binned maximum likelihood method to extract 95% con- fidence level limits in100 GeV=c²wide windows centered on each mass. We incorporate the effects of uncertainty on FIG. 6. Projections of fit to invariant mass in control samples of varying number of electron and hadronic candidates with same ordering as in Fig.5. Data are shown with the fit projection overlaid.

TABLE II. Result of fit with floating independently for each control sample.

Sample Events

eeee 5 5:902:14

eeeh 52 4:850:55

eehh 323 4:280:19

ehhh 1208 4:430:10

hhhh 1927 4:710:09

the background and signal acceptance with a flat prior for the signal rate and Gaussian priors for the acceptance and expected background. The limits including systematic un- certainties on the acceptance onBFðG!Z⁰Z⁰Þrange from 4–6 pb, depending on the graviton mass, and are shown in Fig.8 along with the prediction from the RS1 model fork=M_Pl ¼0:1. In the future, the sensitivity of this search will improve with more data as well as additional acceptance from otherZ⁰decays.

VII. CONCLUSIONS

We have searched for production of particles decaying to a pair ofZ⁰ bosons. We have estimated backgrounds from misidentified electrons using a data-based technique, and the background from standard model processes involving four electrons with simulations. Using an optimized electron selection, we expect 0:0280:009ðstatÞ 0:011ðsystÞ total background events with ²<50 above 500 GeV=c² in 1:10 fb¹, and observe no events. In the

absence of evidence for a signal, we have set limits on ðpp !Gð1:96 GeVÞÞ BFðG!Z⁰Z⁰Þ.

ACKNOWLEDGMENTS

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

This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Science and Technology Facilities Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme; the Slovak R&D Agency; and the Academy of Finland.

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