First Search for Multijet Resonances in √s=1.96  TeV <em>pp</em> Collisions

Article

Reference

First Search for Multijet Resonances in √s=1.96  TeV pp Collisions

CDF Collaboration

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

Abstract

We present the first model independent search for three-jet hadronic resonances within multijet events in s√=1.96  TeV pp collisions at the Fermilab Tevatron using the CDF II detector. Pair production of supersymmetric gluinos and squarks with hadronic R-parity violating decays is employed as an example of a new physics benchmark for this signature.

Selection criteria based on the kinematic properties of an ensemble of jet combinations within each event help to extract signal from copious QCD background. No significant excess outside the top quark mass window is observed in data with an integrated luminosity of 3.2   fb−1. We place 95% confidence level limits on the production cross section σ(pp→XX′)×BR(g˜g˜→3  jet+3  jet) where X, X′=g˜, q˜, or q˜, with q˜, q˜→g˜+jet, as a function of gluino mass, in the range of 77  GeV/c2 to 240  GeV/c2.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . First Search for Multijet Resonances in √s=1.96  TeV pp Collisions. Physical Review Letters , 2011, vol. 107, no. 04, p. 042001

DOI : 10.1103/PhysRevLett.107.042001

Available at:

http://archive-ouverte.unige.ch/unige:38687

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

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First Search for Multijet Resonances in ffiffiffi p s

¼ 1:96 TeV p p Collisions

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S. S. Yu,¹⁵J. C. Yun,¹⁵A. Zanetti,^52aY. Zeng,¹⁴and S. Zucchelli^6b,6a (CDF Collaboration)

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

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3University of Athens, 157 71 Athens, Greece

4Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA

6aIstituto Nazionale di Fisica Nucleare Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7University of California, Davis, Davis, California 95616, USA

8University of California, Los Angeles, Los Angeles, California 90024, USA

9Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

10Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

11Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

12Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

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

20Harvard University, Cambridge, Massachusetts 02138, USA

21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

22University of Illinois, Urbana, Illinois 61801, USA

23The Johns Hopkins University, Baltimore, Maryland 21218, USA

24Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany

25Center 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; Chonbuk National University, Jeonju 561-756, Korea

26Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

27University of Liverpool, Liverpool L69 7ZE, United Kingdom

28University College London, London WC1E 6BT, United Kingdom

29Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

30Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

31Institute of Particle Physics: McGill University, Montre´al, Que´bec, Canada H3A 2T8; Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6; University of Toronto, Toronto, Ontario, Canada M5S 1A7;

and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3

042001-2

32University of Michigan, Ann Arbor, Michigan 48109, USA

33Michigan State University, East Lansing, Michigan 48824, USA

34Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

35University of New Mexico, Albuquerque, New Mexico 87131, USA

36Northwestern University, Evanston, Illinois 60208, USA

37The Ohio State University, Columbus, Ohio 43210, USA

38Okayama University, Okayama 700-8530, Japan

39Osaka City University, Osaka 588, Japan

40University of Oxford, Oxford OX1 3RH, United Kingdom

41aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, Italy

41bUniversity of Padova, I-35131 Padova, Italy

42LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

44aIstituto Nazionale di Fisica Nucleare Pisa, Italy

44bUniversity of Pisa, Italy

44cUniversity of Siena, Italy

44dScuola 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 10065, USA

49aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, Italy

49bSapienza Universita` di Roma, I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, USA

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

52aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy

52bUniversity of Trieste/Udine, I-33100 Udine, Italy

53University of Tsukuba, Tsukuba, Ibaraki 305, Japan

54Tufts University, Medford, Massachusetts 02155, USA

55University of Virginia, Charlottesville, Virginia 22906, 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 16 May 2011; published 22 July 2011)

We present the first model independent search for three-jet hadronic resonances within multijet events inpffiffiffis

¼1:96 TeVpp collisions at the Fermilab Tevatron using the CDF II detector. Pair production of supersymmetric gluinos and squarks with hadronicR-parity violating decays is employed as an example of a new physics benchmark for this signature. Selection criteria based on the kinematic properties of an ensemble of jet combinations within each event help to extract signal from copious QCD background. No significant excess outside the top quark mass window is observed in data with an integrated luminosity of 3:2 fb¹. We place 95% confidence level limits on the production cross section ðpp!XX⁰Þ BRð~gg~!3jetþ3jetÞwhereX,X⁰¼g,~ q, or~ ~q, with q,~ ~q!g~þjet, as a function of gluino mass, in the range of77 GeV=c²to240 GeV=c².

DOI:10.1103/PhysRevLett.107.042001 PACS numbers: 13.85.t, 11.30.Pb

Most searches for new physics at high energy hadron colliders use signatures that require leptons, photons, or missing transverse energy (E6 _T) [1] in order to suppress backgrounds from QCD. Final states with multijets and

6

E_T have also been explored [2,3].

In this Letter, we present a first new physics search in an entirely hadronic channel with no E6 _T signature using data collected with the Collider Detector at Fermilab (CDF). This data set corresponds to an integrated luminos- ity of 3:2 fb¹ of pp collisions at ffiffiffi

ps

¼1:96 TeVat the Tevatron collider. The search utilizes a novel approach

[4,5]: an ensemble of all possible jet triplets within an event consisting of at least six jets is used to extract a signal from the multijet QCD backgrounds. We model the possible new physics origin for this signature with pair production ofSUð3ÞCadjoint Majorana fermions each one decaying into three quarks [6,7]. This search is sensitive to models such as hadronicR-parity violating supersymmetry (RPV SUSY) [4] with a gluino, chargino, or neutralino lightest superpartner, as well as the hadronic decay modes of pairs of top quarks or fourth generation quarks and it complements existing di-jet resonances searches at hadron PRL107,042001 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011

colliders. Moreover, it does not require any b-quark jet identification which is an important tool, often used for top quark identification.

The CDF II detector is a multipurpose particle detector consisting of tracking and calorimeter systems [8]. The data were collected using an online event selection that requires at least four calorimeter jets [9] with uncorrected transverse energy E_T>15 GeV. A jet is formed by a cluster of calorimeter towers and reconstructed with a cone algorithm using a fixed cone of R¼0:4 [10], with R¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

²þ²

p [1]. In the online selection an additional request is made for the sum of the transverse energy of all clusters to be larger than 175 GeV. At the analysis level, jet energies are corrected to account for effects such as nonlinearities in the detector response and multiplepp collisions in an event [11].

Events are selected with at least six jets with transverse momentum (p_T) greater than 15 GeV=c and jj<2:5. The scalar sum of the most energetic six jets’ p_T, Pp_T, is required to be greater than250 GeV=cand events with

6

E_T>50 GeVare removed. Multiple interactions, resulting in the reconstruction of more than one primary vertex in the same event, contribute to the multijet background. We require at least one primary vertex and discard events with more than four primary vertices. To further reduce this background, we require jets in an event to originate from near the same point on the beam line. We associate tracks with each jet where possible [12] by requiringRbetween the track and the jet to be less than 0.4. The mean zcoordinate of all tracks associated with each jet (z_j for the jth jet), and the associated standard deviation [ðz_jÞ]

are determined. Events with jets that havejz_jj>60cm are discarded. We then evaluate the standard deviation of thez_j of all jets in the event [ðz_allÞ] and select events that have at least four jets with ðz_jÞ<4 cm, and ðz_allÞ<0:5 cm, consistent with the resolution of tracks associated with jets. Once the selection is applied, pileup effects are sig- nificantly reduced. Since we select events with at least six jets, we consider an ensemble of 20 (or more) possible jet triplets. We discard those triplets that have more than one jet with nozinformation. In addition, all jets in the triplet must have ðz_jÞ<2:5 cm, and originate from within 10 cm of the primary vertex of the event.

The biggest challenge of this analysis is to reduce the large multijet QCD background. To extract signal from this background, we apply the following technique: for every accepted triplet we calculate the invariant mass,M_jjj, and scalar sump_T,P

jjjp_T. Triplets made of uncorrelated jets tend to haveM_jjjcP

jjjp_T, while signal triplets should haveM_jjjas close to the mass of the decaying particle as allowed by jet energy resolution. We then select triplets with P

jjjp_TM_jjjc >, being a diagonal offset as illustrated in Fig. 1. The diagonal offset values are opti- mized for the best signal over background ratio separately

for each hadronic resonance mass in this search. The optimized diagonal offset selection greatly reduces the QCD background and the contribution from incorrect com- binations of jets. We note that for a small fraction of events it is possible for multiple triplets to pass all selection criteria.

The QCD background is estimated from a 5-jet data sample, which is statistically independent of the signal sample of 6 jets (for brevity referred to as 6-jet). The 5-jetM_jjjdistribution is rescaled by the ratio of the 6-jet to 5-jet population in eachP

jjjp_T bin. A Landau function is chosen [4] to fit the scaled 5-jet M_jjj distribution. The Landau parameters extracted from the scaled 5-jet M_jjj distribution vary by less than2 GeV=c²from similar fits to the 6-jet sample, indicating that the scaled 5-jet sample describes the background in the 6-jet sample well. The contribution to the background fromtt pair production is estimated using the PYTHIA Monte Carlo (MC) generator [13] followed by the CDF detector simulation [14]. These events were generated assuming a top quark mass of 172:5 GeV=c² and production cross section of 7.5 pb. To ensure a proper fit to the QCD background, the fit is blinded to the mass region corresponding to the top quark, 153 GeV=c²< M_jjj<189 GeV=c². Additionally, we find that truncating the Landau fit for lower values ofgives an improved description of the QCD background. The Landau parameters extracted from the fits vary smoothly as functions of the diagonal offset value. We now have a

[GeV/c]

pT

∑

500 1000

]23 jet invariant mass [GeV/c

0 500

1 10 102

103

PYTHIA RPV Gluino m=190 GeV/c2

multiple entries per event

2] [GeV/c Mjjj

0 100 200 300 2 9 GeV/cEntries

0 10 20 30

FIG. 1 (color online). Distribution ofM_jjjversusP

jjjp_Tfor a pair-produced RPV gluino with invariant mass 190 GeV=c² generated withPYTHIAMC generator. Triplets to the right of a diagonal offset (P

jjjp_TM_jjjc¼), indicated by the dashed line, are kept. The inset shows theM_jjjdistribution for the RPV signal MC simulation and with no QCD background after a diagonal offset of 195 GeV=c along with a Gaussian plus a Landau fit; the Landau shows the combinatorial contribution within the signal jet ensemble. The QCD background distribu- tion resembles that of the combinatorial contribution, because they are both due to effectively uncorrelated triplets.

042001-4

firm prediction for the QCD background and fix the pa- rameters when we fit for signal.

The signal is modeled using thePYTHIAMC generator.

The processpp !XX⁰ whereX,X⁰ ¼g,~ q, or~ ~qis simu- lated at several gluino mass values, ranging from 74 GeV=c² to 245 GeV=c² with hadronic uds RPV SUSY decays turned on, allowing gluino decays to three light jets. Two scenarios of squark masses are considered (0:5 TeV=c²< m_q~<0:7 TeV=c²,m_q~¼m_g~þ10 GeV=c²) and were found to give equivalent acceptances.

The acceptance of the trigger, reconstruction, and selec- tion requirements for signal events is determined by fitting the pair-produced RPV gluino MC simulation with a Landau plus Gaussian function, corresponding to the com- binatorial contribution and signal peak, respectively. An example is shown in the inset of Fig.1. The Gaussian is integrated in a1range to extract the number of signal triplets. This procedure is repeated for various diagonal offset values and the optimal offset for each hadronic resonance mass is determined. The acceptance, calculated for these optimal offset values, is 510⁵, constant within 20% across all gluino mass points.

The expected sensitivity of this analysis in the absence of signal is determined with a set of background-only experiments (pseudoexperiments). A pseudoexperiment is constructed with the background modeled by a Landau function whose parameters are chosen randomly from within the range allowed by the background shape fits, with the expected amount ofttadded. Each pseudoexperi- ment is fit with the Landau background shape parameters fixed, and a signal Gaussian whose position is determined by the mass point being fit, and whose amplitude and width are allowed to vary within a range determined by the expected signal shape. The number of signal triplets al- lowed by each pseudoexperiment is extracted by integrat- ing the Gaussian in the same way as in the acceptance calculation.

Two broad categories of systematic uncertainties are accounted for in extracting a cross section: uncertainties in the shape of theM_jjjdistribution and uncertainties in the acceptance of the signal. Shape uncertainties, determined from background and signal fits, are incorporated in the pseudoexperiments themselves. Acceptance uncertainties arise from modeling the signal Monte Carlo simulation and include effects of initial and final state radiation [15]

(20%), parton distribution functions (PDFs) from CTEQ [16] (10%), jet energy scale [11] (31%), and luminosity [17] (6%) uncertainties. The overall acceptance uncer- tainty due to these sources is 38%.

We search for a hadronic resonance in the data for an invariant mass (m) 77–240 GeV=c² in 9 GeV=c² steps, consistent with jet energy resolution. For each mass, jet triplets are selected by the optimal diagonal offset value. The dataM_jjjdistribution is fit in exactly the same way as the pseudoexperiments. Figure 2 shows the M_jjj

distribution for m¼112 GeV=c² and 175 GeV=c². The latter fit shows a noticeable excess consistent in mass with the hadronic decay of the top quark. The Gaussian compo- nent of the fit integrated from165 GeV=c²to185 GeV=c², corresponding to a 1 window around the Gaussian peak, gives 115 triplets. The number of expected QCD background triplets in the same mass window from the Landau function is81. Thettcontribution to back- ground is evaluated usingPYTHIA. It is cross-checked with higher order ttMC generatorsALPGEN [18] andMC@NLO

[19], samples that varied the amount of initial and final state radiation, as well as samples that varied the PDFs within their uncertainties. These studies lead us to expect between 0.5 and 1.1 triplets from tt production in the aforementioned mass range. We note that 10% of the triplets in the top mass window originate from two or more combinations in a jet ensemble of a given event, consistent with thePYTHIAttsimulation. We evaluate the significance of the excess using the pseudoexperiment method de- scribed above, which includes systematic uncertainties on signal acceptance as well as the shape of the M_jjj distri- bution. The observed excess is 2 standard deviations (2) above the prediction. Additional cross-checks, such as requiring one of the jets to have originated from a b-quark, suggest that the excess is consistent with coming from top quarks.

We do not observe a significant deviation from standard model backgrounds anywhere in the data. A Bayesian approach is used to place 95% confidence level limits on ðpp !XX⁰Þ BRð~gg~!3jetþ3jetÞ where X, X⁰¼

~

g,q, or~ ~q, withq,~ ~q!g~þjet, versus gluino mass, shown in Fig.3. The largest excess observed is the one previously noted located near the top quark mass. We find that our

2cGeV / 9 entriesN ²⁰

40 60 80 100

CDF RUN II 3.2 fb-1

6jet Data

≥

fixed at m=112 GeV/c2

+ Gaussian fit QCD Landau prediction

(a)

2] 3 jet invariant mass [GeV/c

50 100 150 200 250

2cGeV / 9 entriesN

0 5 10 15

20 ≥ 6jet Data

fixed at m=175 GeV/c2

+ Gaussian fit QCD Landau prediction

(b)

FIG. 2. M_jjj distributions in 3:2 fb¹ data fitted to a Landau (parameters are extracted from fits to the scaled 5-jet M_jjj distribution) plus a Gaussian at (a)112 GeV=c²(optimal diago- nal offset value 155 GeV=c) and (b) 175 GeV=c² (optimal diagonal offset value of190 GeV=c). The fit function in panel (b) includes a Gaussian fixed atm¼175 GeV=c².

PRL107,042001 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011

background estimate has a 2.3% probability of producing such a deviation. Comparisons to the theoretical cross section forðpp !XX⁰Þ BRðg~g~!3jetþ3jetÞfrom

PYTHIA corrected by a next-to-leading-order (NLO) kfactor calculated using PROSPINO[20] are shown in the dashed and dash-dotted lines for two different squark mass scenarios. For a decoupled squark mass (0:5 TeV=c²<

m_q~<0:7 TeV=c²) we exclude gluinos below a mass of 144 GeV=c² (dashed line). In the case of a squark mass which is nearly degenerate with the gluino mass (m_q~ ¼ m_g~þ10 GeV=c²) we exclude gluinos below155 GeV=c² (dash-dotted line).

We have performed a first search for three-jet hadronic resonances in a six or more jet final state using a data sample with an integrated luminosity of3:2 fb¹ collected by the CDF II detector. A novel technique is introduced that exploits kinematic features within an ensemble of jet combinations that allows us to extract signal from the QCD background. We observe no significant excess in the data in an invariant mass range from77 GeV=c² to240 GeV=c² and place 95% confidence level limits on the production cross section ðpp !XX⁰Þ BRð~gg~!3 jetþ3 jetÞ where X, X⁰ ¼g,~ q, or~ ~q, with q,~ ~q!g~þjet, versus gluino mass. The results are presented as limits on RPV gluinos decaying to three jets, but are more widely appli- cable to any new particle with a three-jet decay mode. Two different squark mass scenarios have been considered:

decoupled squarks and squarks nearly degenerate in mass with the gluino. We can exclude gluinos below 144 GeV=c² and155 GeV=c², respectively.

We thank R. Essig, S. Mrenna, M. Park, and Y. Zhao for assistance with this analysis. We also 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 World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, U.K.; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.

aDeceased.

bVisitor from Universidad de Oviedo, E-33007 Oviedo, Spain.

cOn leave from J. Stefan Institute, Ljubljana, Slovenia.

dVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.

eVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.

fVisitor from CERN,CH-1211 Geneva, Switzerland.

gVisitor from Queen Mary, University of London, London, E1 4NS, U.K.

hVisitor from National Research Nuclear University, Moscow, Russia.

iVisitor from University of California Irvine, Irvine, CA 92697, USA.

jVisitor from Yarmouk University, Irbid 211-63, Jordan.

kVisitor from Muons, Inc., Batavia, IL 60510, USA.

lVisitor from Cornell University, Ithaca, NY 14853, USA.

mVisitor from Kansas State University, Manhattan, KS 66506, USA.

nVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.

oVisitor from Iowa State University, Ames, IA 50011, USA.

pVisitor from University of California Santa Barbara, Santa Barbara, CA 93106, USA.

qVisitor from University of Notre Dame, Notre Dame, IN 46556, USA.

rVisitor from Texas Tech University, Lubbock, TX 79609, USA.

sVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.

tVisitor from University College Dublin, Dublin 4, Ireland.

uVisitor from University of Iowa, Iowa City, IA 52242, USA.

2] gluino mass [GeV/c

80 100 120 140 160 180 200 220 240 3j + 3j) [pb]→ BR(pp×σ

10 102 103 104 105

CDF RUN II 3.2 fb-1

< 0.7 TeV/c2

~q

< m 0.5 TeV/c2

+ 10 GeV/c2 g~

q=m

m~ 95% C.L. limit observed

95% C.L. limit expected on expected limit σ

± 1

FIG. 3 (color online). The observed (points) and expected (solid black line) 95% confidence level limits on the production cross section ðpp!XX⁰Þ BRð~gg~!3jetþ3jetÞ where X, X⁰¼g,~ q, or~ q, including systematic uncertainties. The~ shaded bands represent the total uncertainty on the limit. Also shown is the model cross section fromPYTHIAcorrected by an NLO k factor (dash-dotted line for 0:5 TeV=c²< m_q~<

0:7 TeV=c², dashed line form_~q¼m_g~þ10 GeV=c²).

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vVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.

wVisitor from Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile.

xVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.

yVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.

zVisitor from University of Manchester, Manchester M13 9PL, U.K.

aaVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.

[1] CDF uses a (z,,) coordinate system with thezaxis in the direction of the proton beam;andare the azimu- thal and polar angle, respectively. The pseudorapidity is defined as¼ lnðtan₂Þ, and the transverse momentum and energy asP_T¼psinandE_T¼Esin, respectively.

Missing transverse energy (6ET¼ j6E~Tj) is defined asE6~T¼ P

iEⁱ_Tn^_iwheren^_iis a unit vector in the transverse plane that points from the beam line to theith calorimeter tower.

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102, 121801 (2009).

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[11] A. Bhattiet al.,Nucl. Instrum. Methods Phys. Res., Sect.

A566, 375 (2006).

[12] Note that tracking efficiency drops significantly beyond jj>1:0. Jets at largejjdo not have associated tracks and have only limitedzinformation.

[13] T. Sjo¨strandet al.,Comput. Phys. Commun.135, 238 (2001).

[14] S. Agostinelli et al. (GEANT4 Collaboration), Nucl.

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9611232.

PRL107,042001 (2011) P H Y S I C A L R E V I E W L E T T E R S 22 JULY 2011