Search for new particles leading to Z+jets final states in $p\bar{p}$ collisions at $\sqrt{s}$=1.96 TeV

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p

p collisions at

¯

√

s=1.96 TeV

T. Aaltonen, A. Abulencia, J. Adelman, T. Affolder, T. Akimoto, M. G.

Albrow, S. Amerio, D. Amidei, A. Anastassov, K. Anikeev, et al.

To cite this version:

T. Aaltonen, A. Abulencia, J. Adelman, T. Affolder, T. Akimoto, et al.. Search for new particles

leading to Z+jets final states in p¯

p

collisions at

√

s=1.96 TeV. Physical Review D, American Physical

Search for New Particles Leading to Z+jets Final States in p¯

p Collisions at

s = 1.96

TeV

T. Aaltonen,23 _{A. Abulencia,}24 _{J. Adelman,}13 _{T. Affolder,}10 _{T. Akimoto,}55 _{M.G. Albrow,}17 _{S. Amerio,}43

D. Amidei,35 _{A. Anastassov,}52 _{K. Anikeev,}17_{A. Annovi,}19 _{J. Antos,}14 _{M. Aoki,}55_{G. Apollinari,}17 _{T. Arisawa,}57

A. Artikov,15 _{W. Ashmanskas,}17 _{A. Attal,}3 _{A. Aurisano,}53 _{F. Azfar,}42 _{P. Azzi-Bacchetta,}43 _{P. Azzurri,}46

N. Bacchetta,43 W. Badgett,17 A. Barbaro-Galtieri,29 V.E. Barnes,48B.A. Barnett,25 S. Baroiant,7 V. Bartsch,31

G. Bauer,33 _{P.-H. Beauchemin,}34 _{F. Bedeschi,}46 _{S. Behari,}25 _{G. Bellettini,}46 _{J. Bellinger,}59 _{A. Belloni,}33

D. Benjamin,16_{A. Beretvas,}17 _{J. Beringer,}29 _{T. Berry,}30 _{A. Bhatti,}50_{M. Binkley,}17 _{D. Bisello,}43 _{I. Bizjak,}31

R.E. Blair,2 C. Blocker,6 B. Blumenfeld,25 A. Bocci,16 A. Bodek,49 V. Boisvert,49 G. Bolla,48 A. Bolshov,33

D. Bortoletto,48 _{J. Boudreau,}47 _{A. Boveia,}10 _{B. Brau,}10 _{L. Brigliadori,}5 _{C. Bromberg,}36 _{E. Brubaker,}13

J. Budagov,15 _{H.S. Budd,}49 _{S. Budd,}24 _{K. Burkett,}17 _{G. Busetto,}43 _{P. Bussey,}21 _{A. Buzatu,}34 _{K. L. Byrum,}2

S. Cabreraq,16 M. Campanelli,20 M. Campbell,35 F. Canelli,17 A. Canepa,45 S. Carrilloi,18 D. Carlsmith,59

R. Carosi,46 _{S. Carron,}34 _{B. Casal,}11 _{M. Casarsa,}54 _{A. Castro,}5_{P. Catastini,}46 _{D. Cauz,}54 _{M. Cavalli-Sforza,}3

A. Cerri,29_{L. Cerrito}m_,31_{S.H. Chang,}28_{Y.C. Chen,}1 _{M. Chertok,}7 _{G. Chiarelli,}46 _{G. Chlachidze,}17_{F. Chlebana,}17

I. Cho,28 K. Cho,28 D. Chokheli,15J.P. Chou,22 G. Choudalakis,33S.H. Chuang,52 K. Chung,12 W.H. Chung,59

Y.S. Chung,49 _{M. Cilijak,}46 _{C.I. Ciobanu,}24 _{M.A. Ciocci,}46 _{A. Clark,}20 _{D. Clark,}6_{M. Coca,}16 _{G. Compostella,}43

M.E. Convery,50 _{J. Conway,}7 _{B. Cooper,}31 _{K. Copic,}35 _{M. Cordelli,}19 _{G. Cortiana,}43 _{F. Crescioli,}46

C. Cuenca Almenarq,7 J. Cuevasl,11R. Culbertson,17 J.C. Cully,35 S. DaRonco,43 M. Datta,17 S. D’Auria,21

T. Davies,21 _{D. Dagenhart,}17 _{P. de Barbaro,}49 _{S. De Cecco,}51 _{A. Deisher,}29 _{G. De Lentdecker}c_,49 _{G. De Lorenzo,}3

M. Dell’Orso,46 _{F. Delli Paoli,}43 _{L. Demortier,}50 _{J. Deng,}16 _{M. Deninno,}5 _{D. De Pedis,}51 _{P.F. Derwent,}17

G.P. Di Giovanni,44 C. Dionisi,51 B. Di Ruzza,54 J.R. Dittmann,4 M. D’Onofrio,3 C. D¨orr,26 S. Donati,46

P. Dong,8 _{J. Donini,}43 _{T. Dorigo,}43_{S. Dube,}52 _{J. Efron,}39_{R. Erbacher,}7_{D. Errede,}24 _{S. Errede,}24 _{R. Eusebi,}17

H.C. Fang,29 _{S. Farrington,}30_{I. Fedorko,}46_{W.T. Fedorko,}13 _{R.G. Feild,}60 _{M. Feindt,}26 _{J.P. Fernandez,}32_{R. Field,}18

G. Flanagan,48 R. Forrest,7 S. Forrester,7 M. Franklin,22 J.C. Freeman,29I. Furic,13M. Gallinaro,50 J. Galyardt,12

J.E. Garcia,46 _{F. Garberson,}10 _{A.F. Garfinkel,}48 _{C. Gay,}60 _{H. Gerberich,}24_{D. Gerdes,}35 _{S. Giagu,}51_{P. Giannetti,}46

K. Gibson,47 _{J.L. Gimmell,}49 _{C. Ginsburg,}17 _{N. Giokaris}a_,15 _{M. Giordani,}54 _{P. Giromini,}19 _{M. Giunta,}46

G. Giurgiu,25V. Glagolev,15 D. Glenzinski,17 M. Gold,37 N. Goldschmidt,18 J. Goldsteinb,42 A. Golossanov,17

G. Gomez,11 _{G. Gomez-Ceballos,}33 _{M. Goncharov,}53_{O. Gonz´alez,}32 _{I. Gorelov,}37_{A.T. Goshaw,}16 _{K. Goulianos,}50

A. Gresele,43 _{S. Grinstein,}22 _{C. Grosso-Pilcher,}13 _{R.C. Group,}17 _{U. Grundler,}24 _{J. Guimaraes da Costa,}22

Z. Gunay-Unalan,36_{C. Haber,}29_{K. Hahn,}33_{S.R. Hahn,}17_{E. Halkiadakis,}52 _{A. Hamilton,}20_{B.-Y. Han,}49_{J.Y. Han,}49

R. Handler,59 F. Happacher,19 K. Hara,55 D. Hare,52 M. Hare,56 S. Harper,42 R.F. Harr,58 R.M. Harris,17

M. Hartz,47 _{K. Hatakeyama,}50_{J. Hauser,}8 _{C. Hays,}42 _{M. Heck,}26 _{A. Heijboer,}45 _{B. Heinemann,}29 _{J. Heinrich,}45

C. Henderson,33 _{M. Herndon,}59 _{J. Heuser,}26 _{D. Hidas,}16_{C.S. Hill}b_,10_{D. Hirschbuehl,}26_{A. Hocker,}17 _{A. Holloway,}22

S. Hou,1 M. Houlden,30S.-C. Hsu,9 B.T. Huffman,42 R.E. Hughes,39U. Husemann,60J. Huston,36 J. Incandela,10

G. Introzzi,46_{M. Iori,}51 _{A. Ivanov,}7 _{B. Iyutin,}33_{E. James,}17 _{D. Jang,}52 _{B. Jayatilaka,}16 _{D. Jeans,}51_{E.J. Jeon,}28

S. Jindariani,18 _{W. Johnson,}7 _{M. Jones,}48 _{K.K. Joo,}28 _{S.Y. Jun,}12 _{J.E. Jung,}28 _{T.R. Junk,}24 _{T. Kamon,}53

P.E. Karchin,58 Y. Kato,41Y. Kemp,26 R. Kephart,17 U. Kerzel,26 V. Khotilovich,53 B. Kilminster,39 D.H. Kim,28

H.S. Kim,28 _{J.E. Kim,}28_{M.J. Kim,}17 _{S.B. Kim,}28 _{S.H. Kim,}55_{Y.K. Kim,}13_{N. Kimura,}55 _{L. Kirsch,}6 _{S. Klimenko,}18

M. Klute,33 _{B. Knuteson,}33_{B.R. Ko,}16_{K. Kondo,}57 _{D.J. Kong,}28_{J. Konigsberg,}18 _{A. Korytov,}18 _{A.V. Kotwal,}16

A.C. Kraan,45 J. Kraus,24 M. Kreps,26 J. Kroll,45 N. Krumnack,4 M. Kruse,16 V. Krutelyov,10 T. Kubo,55

S. E. Kuhlmann,2 _{T. Kuhr,}26 _{N.P. Kulkarni,}58 _{Y. Kusakabe,}57_{S. Kwang,}13 _{A.T. Laasanen,}48 _{S. Lai,}34 _{S. Lami,}46

S. Lammel,17 _{M. Lancaster,}31 _{R.L. Lander,}7 _{K. Lannon,}39_{A. Lath,}52 _{G. Latino,}46 _{I. Lazzizzera,}43 _{T. LeCompte,}2

J. Lee,49 J. Lee,28 Y.J. Lee,28S.W. Leeo,53 R. Lef`evre,20 N. Leonardo,33 S. Leone,46S. Levy,13 J.D. Lewis,17

C. Lin,60 _{C.S. Lin,}17 _{M. Lindgren,}17 _{E. Lipeles,}9 _{A. Lister,}7 _{D.O. Litvintsev,}17 _{T. Liu,}17 _{N.S. Lockyer,}45

A. Loginov,60 _{M. Loreti,}43 _{R.-S. Lu,}1_{D. Lucchesi,}43 _{P. Lujan,}29 _{P. Lukens,}17 _{G. Lungu,}18 _{L. Lyons,}42 _{J. Lys,}29

R. Lysak,14 E. Lytken,48 P. Mack,26D. MacQueen,34 R. Madrak,17 K. Maeshima,17 K. Makhoul,33 T. Maki,23

P. Maksimovic,25 _{S. Malde,}42 _{S. Malik,}31 _{G. Manca,}30 _{A. Manousakis}a_,15 _{F. Margaroli,}5 _{R. Marginean,}17

C. Marino,26 _{C.P. Marino,}24 _{A. Martin,}60 _{M. Martin,}25 _{V. Martin}g_,21 _{M. Mart´ınez,}3 _{R. Mart´ınez-Ballar´ın,}32

T. Maruyama,55 P. Mastrandrea,51 T. Masubuchi,55H. Matsunaga,55M.E. Mattson,58 R. Mazini,34P. Mazzanti,5

K.S. McFarland,49 _{P. McIntyre,}53 _{R. McNulty}f_,30 _{A. Mehta,}30 _{P. Mehtala,}23 _{S. Menzemer}h_,11 _{A. Menzione,}46

P. Merkel,48 _{C. Mesropian,}50 _{A. Messina,}36 _{T. Miao,}17 _{N. Miladinovic,}6 _{J. Miles,}33 _{R. Miller,}36 _{C. Mills,}10

R. Moore,17M. Morello,46 P. Movilla Fernandez,29 J. M¨ulmenst¨adt,29 A. Mukherjee,17Th. Muller,26 R. Mumford,25

P. Murat,17 _{M. Mussini,}5 _{J. Nachtman,}17_{A. Nagano,}55 _{J. Naganoma,}57 _{K. Nakamura,}55_{I. Nakano,}40 _{A. Napier,}56

V. Necula,16 _{C. Neu,}45 _{M.S. Neubauer,}9_{J. Nielsen}n_,29 _{L. Nodulman,}2 _{O. Norniella,}3 _{E. Nurse,}31 _{S.H. Oh,}16

Y.D. Oh,28 I. Oksuzian,18 T. Okusawa,41 R. Oldeman,30 R. Orava,23 K. Osterberg,23 C. Pagliarone,46

E. Palencia,11 _{V. Papadimitriou,}17 _{A. Papaikonomou,}26 _{A.A. Paramonov,}13 _{B. Parks,}39 _{S. Pashapour,}34

J. Patrick,17 _{G. Pauletta,}54 _{M. Paulini,}12 _{C. Paus,}33 _{D.E. Pellett,}7 _{A. Penzo,}54_{T.J. Phillips,}16 _{G. Piacentino,}46

J. Piedra,44 L. Pinera,18 K. Pitts,24 C. Plager,8 L. Pondrom,59 X. Portell,3 O. Poukhov,15 N. Pounder,42

F. Prakoshyn,15 _{A. Pronko,}17 _{J. Proudfoot,}2 _{F. Ptohos}e_,19 _{G. Punzi,}46 _{J. Pursley,}25 _{J. Rademacker}b_,42

A. Rahaman,47 _{V. Ramakrishnan,}59 _{N. Ranjan,}48 _{I. Redondo,}32 _{B. Reisert,}17 _{V. Rekovic,}37 _{P. Renton,}42

M. Rescigno,51 S. Richter,26 F. Rimondi,5 L. Ristori,46 A. Robson,21 T. Rodrigo,11E. Rogers,24 S. Rolli,56

R. Roser,17 _{M. Rossi,}54 _{R. Rossin,}10 _{P. Roy,}34 _{A. Ruiz,}11 _{J. Russ,}12 _{V. Rusu,}13 _{H. Saarikko,}23 _{A. Safonov,}53

W.K. Sakumoto,49 _{G. Salamanna,}51 _{O. Salt´o,}3 _{L. Santi,}54 _{S. Sarkar,}51 _{L. Sartori,}46 _{K. Sato,}17 _{P. Savard,}34

A. Savoy-Navarro,44 _{T. Scheidle,}26_{P. Schlabach,}17 _{E.E. Schmidt,}17_{M.P. Schmidt,}60 _{M. Schmitt,}38 _{T. Schwarz,}7

L. Scodellaro,11 A.L. Scott,10 A. Scribano,46 F. Scuri,46 A. Sedov,48 S. Seidel,37 Y. Seiya,41 A. Semenov,15

L. Sexton-Kennedy,17 _{A. Sfyrla,}20_{S.Z. Shalhout,}58 _{M.D. Shapiro,}29 _{T. Shears,}30 _{P.F. Shepard,}47 _{D. Sherman,}22

M. Shimojimak_,55 _{M. Shochet,}13_{Y. Shon,}59_{I. Shreyber,}20_{A. Sidoti,}46_{P. Sinervo,}34 _{A. Sisakyan,}15_{A.J. Slaughter,}17

J. Slaunwhite,39K. Sliwa,56 J.R. Smith,7 F.D. Snider,17 R. Snihur,34 M. Soderberg,35 A. Soha,7 S. Somalwar,52

V. Sorin,36 _{J. Spalding,}17 _{F. Spinella,}46 _{T. Spreitzer,}34 _{P. Squillacioti,}46 _{M. Stanitzki,}60 _{A. Staveris-Polykalas,}46

R. St. Denis,21_{B. Stelzer,}8 _{O. Stelzer-Chilton,}42 _{D. Stentz,}38 _{J. Strologas,}37_{D. Stuart,}10 _{J.S. Suh,}28 _{A. Sukhanov,}18

H. Sun,56 I. Suslov,15 T. Suzuki,55 A. Taffardp,24 R. Takashima,40 Y. Takeuchi,55 R. Tanaka,40 M. Tecchio,35

P.K. Teng,1 _{K. Terashi,}50 _{J. Thom}d_,17_{A.S. Thompson,}21 _{E. Thomson,}45 _{P. Tipton,}60 _{V. Tiwari,}12_{S. Tkaczyk,}17

D. Toback,53 _{S. Tokar,}14 _{K. Tollefson,}36_{T. Tomura,}55 _{D. Tonelli,}46 _{S. Torre,}19 _{D. Torretta,}17 _{S. Tourneur,}44

W. Trischuk,34S. Tsuno,40 Y. Tu,45 N. Turini,46F. Ukegawa,55 S. Uozumi,55 S. Vallecorsa,20 N. van Remortel,23

A. Varganov,35 _{E. Vataga,}37_{F. Vazquez}i_,18 _{G. Velev,}17 _{C. Vellidis}a_,46 _{G. Veramendi,}24_{V. Veszpremi,}48_{M. Vidal,}32

R. Vidal,17 _{I. Vila,}11 _{R. Vilar,}11 _{T. Vine,}31 _{M. Vogel,}37 _{I. Vollrath,}34 _{I. Volobouev}o_,29 _{G. Volpi,}46 _{F. W¨urthwein,}9

P. Wagner,53 R.G. Wagner,2R.L. Wagner,17 J. Wagner,26W. Wagner,26R. Wallny,8 S.M. Wang,1 A. Warburton,34

D. Waters,31 _{M. Weinberger,}53 _{W.C. Wester III,}17 _{B. Whitehouse,}56 _{D. Whiteson}p_,45 _{A.B. Wicklund,}2

E. Wicklund,17 _{G. Williams,}34 _{H.H. Williams,}45 _{P. Wilson,}17 _{B.L. Winer,}39 _{P. Wittich}d_,17 _{S. Wolbers,}17

C. Wolfe,13 T. Wright,35 X. Wu,20 S.M. Wynne,30 A. Yagil,9 K. Yamamoto,41 J. Yamaoka,52 T. Yamashita,40

C. Yang,60_{U.K. Yang}j_,13 _{Y.C. Yang,}28 _{W.M. Yao,}29 _{G.P. Yeh,}17 _{J. Yoh,}17 _{K. Yorita,}13 _{T. Yoshida,}41 _{G.B. Yu,}49

I. Yu,28 _{S.S. Yu,}17 _{J.C. Yun,}17 _{L. Zanello,}51 _{A. Zanetti,}54 _{I. Zaw,}22 _{X. Zhang,}24 _{J. Zhou,}52 _{and S. Zucchelli}5

(CDF Collaboration∗)

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

2

Argonne National Laboratory, Argonne, Illinois 60439

3

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

4

Baylor University, Waco, Texas 76798

5

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

6

Brandeis University, Waltham, Massachusetts 02254

7_{University of California, Davis, Davis, California 95616}

8_{University of California, Los Angeles, Los Angeles, California 90024}

9_{University of California, San Diego, La Jolla, California 92093}

10

University of California, Santa Barbara, Santa Barbara, California 93106

11

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

12

Carnegie Mellon University, Pittsburgh, PA 15213

13

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

14

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

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

16_{Duke University, Durham, North Carolina 27708}

17_{Fermi National Accelerator Laboratory, Batavia, Illinois 60510}

18

University of Florida, Gainesville, Florida 32611

19

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

20

University of Geneva, CH-1211 Geneva 4, Switzerland

21

Glasgow University, Glasgow G12 8QQ, United Kingdom

22

Harvard University, Cambridge, Massachusetts 02138

23_{Division of High Energy Physics, Department of Physics,}

University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

24

25

The Johns Hopkins University, Baltimore, Maryland 21218

26_{Institut f¨ur Experimentelle Kernphysik, Universit¨at Karlsruhe, 76128 Karlsruhe, Germany}

27_{High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan}

28

Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea; Seoul National University, Seoul 151-742,

Korea; SungKyunKwan University, Suwon 440-746, Korea

29

Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720

30

University of Liverpool, Liverpool L69 7ZE, United Kingdom

31_{University College London, London WC1E 6BT, United Kingdom}

32

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

33

Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

34_{Institute of Particle Physics: McGill University, Montr´eal,}

Canada H3A 2T8; and University of Toronto, Toronto, Canada M5S 1A7

35

University of Michigan, Ann Arbor, Michigan 48109

36_{Michigan State University, East Lansing, Michigan 48824}

37

University of New Mexico, Albuquerque, New Mexico 87131

38

Northwestern University, Evanston, Illinois 60208

39_{The Ohio State University, Columbus, Ohio 43210}

40_{Okayama University, Okayama 700-8530, Japan}

41

Osaka City University, Osaka 588, Japan

42

University of Oxford, Oxford OX1 3RH, United Kingdom

43_{University of Padova, Istituto Nazionale di Fisica Nucleare,}

Sezione di Padova-Trento, I-35131 Padova, Italy

44

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

45_{University of Pennsylvania, Philadelphia, Pennsylvania 19104}

46

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

47_{University of Pittsburgh, Pittsburgh, Pennsylvania 15260}

48

Purdue University, West Lafayette, Indiana 47907

49

University of Rochester, Rochester, New York 14627

50_{The Rockefeller University, New York, New York 10021}

51_{Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1,}

University of Rome “La Sapienza,” I-00185 Roma, Italy

52

Rutgers University, Piscataway, New Jersey 08855

53_{Texas A&M University, College Station, Texas 77843}

54

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

55

University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56_{Tufts University, Medford, Massachusetts 02155}

57

Waseda University, Tokyo 169, Japan

58

Wayne State University, Detroit, Michigan 48201

59_{University of Wisconsin, Madison, Wisconsin 53706}

60

Yale University, New Haven, Connecticut 06520 (Dated: June 15, 2007)

We present the results of a search for new particles that lead to a Z boson plus jets in p¯p

collisions at√s = 1.96 TeV using the Collider Detector at Fermilab (CDF II). A data sample with a

luminosity of 1.06 fb−1collected using Z boson decays to ee and µµ is used. We describe a completely

data-based method to predict the dominant background from standard-model Z+jet events. This method can be similarly applied to other analyses requiring background predictions in multi-jet environments, as shown when validating the method by predicting the background from W +jets in

t¯t production. No significant excess above the background prediction is observed, and a limit is set

using a fourth generation quark model to quantify the acceptance. Assuming BR(b0→ bZ) = 100%

and using a leading-order calculation of the b0cross section, b0 quark masses below 268 GeV/c2are

excluded at 95% confidence level.

∗_{With visitors from}a_{University of Athens, 15784 Athens, Greece,} b_{University of Bristol, Bristol BS8 1TL, United Kingdom,} c_{University Libre de Bruxelles, B-1050 Brussels, Belgium,}d_Cornell

University, Ithaca, NY 14853,eUniversity of Cyprus, Nicosia CY-1678, Cyprus, f_{University College Dublin, Dublin 4, Ireland,}

g_{University of Edinburgh, Edinburgh EH9 3JZ, United}

King-dom, h_{University of Heidelberg, D-69120 Heidelberg, Germany,} i_{Universidad Iberoamericana, Mexico D.F., Mexico,} j_University

of Manchester, Manchester M13 9PL, England,k_Nagasaki

I. INTRODUCTION

This paper presents a search for new particles

de-caying to Z gauge bosons created in p¯p collisions at

√

s = 1.96 TeV with the CDF II detector at the Fermi-lab Tevatron, extending and complementing other work with such final states [1–4]. A variety of extensions to the standard model predict new particles with couplings to Z bosons [5–9]. We wish to discover or rule out these types of models, while maintaining model independence in the search. That is, while these theories offer guidance about the possible characteristics of physics beyond the standard model, they do not necessarily correspond to what actually exists in nature, and so the analysis is not tailored to specific models.

Of course, some assumptions are necessary in choosing how to discriminate between the standard model back-ground and new signals. We examine final states with Z bosons and additional jets. In particular, we focus on final states in which there are at least 3 jets, each

with at least 30 GeV of transverse energy ET. This

as-sumption was motivated by studying the optimal kine-matic selection of a specific model, the fourth generation model [5]. In the fourth generation model, an additional pair of heavy quarks is added to the standard model’s

three. The production mechanisms of the new

down-type quark (called the b0) would be identical to that of

the top quark, with pair-production having the largest cross section. Depending on its mass, the direct tree-level

decays of the b0 could be either kinematically forbidden

or heavily Cabibbo-suppressed. These situations could

give rise to a large branching ratio of b0→ bZ via a loop

diagram. While the selection was chosen as the optimal set of kinematic cuts using this model as a signal, this analysis constrains all models with Z+3 jet final states.

The dominant background for this final state is from standard model Z production with jets from higher order QCD processes. A leading order calculation of this back-ground is insufficient. Use of higher order calculations is complicated because it involves hard-scattering matrix elements in combination with soft non-perturbative QCD processes. Recent NLO predictions [10] have been used [11] with the aid of Monte Carlo simulations to account for the non-perturbative overlap. Any such method re-quires validation with data. In this paper, we develop a different approach that uses the data as more than a val-idation tool, and uses it alone for the background estima-tion. In this approach, we extrapolate the jet transverse energy distributions from a low energy control region of the data into the high energy signal region.

E-33007 Oviedo, Spain,m_{University of London, Queen Mary}

Col-lege, London, E1 4NS, England, n_{University of California Santa}

Cruz, Santa Cruz, CA 95064, o_{Texas Tech University, Lubbock,}

TX 79409, p_{University of California, Irvine, Irvine, CA 92697,} q_{IFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain,}

This paper is organized as follows. Section II contains a brief overview of the portions of the CDF II detector relevant to this measurement. Section III lists the trig-ger requirements, and describes and motivates the signal sample selections. Section IV lists the backgrounds. Sec-tion V describes, validates, and applies the method of predicting the dominant background. In Sec. VI the pre-dictions for the remaining backgrounds are described. In Sec. VII we present the results of the search, and con-clude in Sec. VIII.

II. THE CDF II DETECTOR

The CDF II detector is described in detail elsewhere [12]; here, only the portions required for this analysis are described. We first describe the coordinate system con-ventions. In the CDF coordinate system, the origin is the center of the detector, and the z axis is along the beam axis, with positive z defined as the proton beam direction. The x axis points radially outward from the Tevatron ring, leaving the y axis direction perpendicular to the earth’s surface with positive direction upward. Spherical coordinates are used where appropriate, in which θ is the polar angle (zero in the positive z direction), φ is the az-imuthal angle (zero in the positive x direction), and the pseudorapidity η is defined by η ≡ − ln[tan(θ/2)]. At hadron colliders, transverse energies and momenta are usually the appropriate physical quantities, defined by

ET ≡ E sin θ and pT ≡ p sin θ (where E is a particle’s

energy and p is the magnitude of a particle’s momen-tum).

A tracking system is situated directly outside the beam pipe and measures the trajectories and momenta of charged particles. The innermost part of the tracking system is the silicon detector, providing position mea-surements on up to 8 layers of sensors in the radial region 1.3 < r < 28 cm and the polar region |η| . 2.5. Out-side of this detector lies the central outer tracker (COT), an open-cell drift chamber providing measurements on up to 96 layers in the radial region 40 < r < 137 cm and the polar region |η| . 1. Directly outside of the COT a solenoid provides a 1.4 T magnetic field, allow-ing particle momenta to be obtained from the trajectory measurements in this known field.

Surrounding the tracking system, segmented electro-magnetic (EM) and hadronic calorimeters measure

par-ticle energies. In the central region, the calorimeters

are arranged in a projective barrel geometry and cover the polar region |η| < 1.2. In the forward region, the calorimeters are arranged in a projective “end-plug” ge-ometry and cover the polar region 1.2 < |η| < 3.5. Two sets of drift chambers, one directly outside the hadronic calorimeter and another outside additional steel shield-ing, measure muon trajectories in the region |η| < 0.6; another set of drift chambers similarly detects muons

in the region 0.6 < |η| < 1. Muon scintillators

trig-ger purposes. A luminosity measurement is provided by Cherenkov detectors in the region 3.7 < |η| < 4.7 via a

measurement of the average number of p¯p collisions per

crossing [13].

Collision events of interest are selected for analysis of-fline using a three level trigger system, with each level accepting events for processing at the next level. At level 1, custom hardware enables fast decisions using rudimen-tary tracking information and a simple counting of recon-structed objects. At level 2, trigger processors enable de-cisions based on partial event reconstruction. At level 3, a computer farm running fast event reconstruction soft-ware makes the final decision on event storage.

III. DATA SAMPLE AND EVENT SELECTION

We first describe the baseline Z selection, and then describe the kinematic selection used to discriminate the potential signal from the standard model background. The kinematic selection is chosen and backgrounds are predicted a priori, before looking in the signal region. While remaining as data-driven as possible throughout the analysis, Monte Carlo simulation is used in some studies, consistency checks, and for illustration purposes. In all cases, the Monte Carlo events are generated with pythia [14] and the detector responses are modeled with a geant simulation [15] as described in [16].

A. Baseline Z Selection

The data sample consists of Z → ee and Z → µµ candidate events collected using single electron and muon triggers. The electron trigger requires at least one central

electromagnetic energy cluster with ET > 18 GeV and a

matching track with pT > 9 GeV/c. The muon trigger

requires at least one central track with pT > 18 GeV/c

with matching hits in the muon drift chambers. The

average integrated luminosity of these data samples is

1.06 fb−1 [17].

Z candidate events are selected offline by requiring at least one pair of electron or muon candidates both with

pT > 20 GeV/c and invariant mass in the range 81 <

M``< 101 GeV/c2. The electron and muon identification

variables are described in detail in Refs. [16, 18]. The selection is described briefly here. To increase efficiency, only one of the lepton pair has stringent identification requirements (the “tight” candidate), while on the other lepton the identification requirements are relaxed (the “loose” candidate).

“Loose” electron candidates consist of well-isolated EM calorimeter clusters with low energy in the hadronic calorimeter; in the central part of the detector (|η| < 1.2) well-measured tracks from the COT are required; in the forward parts of the detector (|η| > 1.2) no track is quired, but the shower shape in the EM calorimeter is re-quired to be consistent with that expected from electrons.

)

2

(GeV/c

ll

M

50

100

150

2

Events/GeV/c

1

10

2

10

3

10

4

10

FIG. 1: Distribution of M`` of Z → ee and Z → µµ data

(black points and errors) using the baseline Z selection de-scribed in the text. Overlaid are standard model Z → ee and Z → µµ Monte Carlo events, normalized to the num-ber of events expected with the given luminosities using the expected cross section of 250 pb.

“Tight” electron candidates have all the requirements of “loose” candidates, and are additionally required to be central (|η| < 1.2), to have a shower shape consistent with that expected from electrons, to have calorimeter posi-tion and energy measurements consistent with its match-ing track, and to have no nearby tracks consistent with that expected in electrons from photon conversions.

“Loose” muon candidates consist of well-measured tracks in the COT and well-isolated EM and hadronic calorimeter clusters with minimal energy deposits. “Tight” muon candidates have all the requirements of “loose” candidates, and are additionally required to have matching hits in the muon drift chambers.

Finally, all electron and muon pairs are required to be consistent with originating from the same z vertex and to have a time-of-flight difference (as measured by the COT) inconsistent with that expected for cosmic rays. They are also required to be separated in φ by an angle

greater than 5◦ to remove two lepton candidates

mis-reconstructed from a single lepton.

Using this selection, the distribution of M`` is plotted

and compared to standard model Z Monte Carlo simu-lation in Fig. 1.

B. Kinematic Selection

ap-plied to extrapolate the jet energies back to the parton level using a generic jet response [20]. Jets are required to have |η| < 2.

The following discriminators are used:

N_jetX = Number of jets in the event with

ET > X GeV

J_TX = Scalar sum of ET of jets in the

event with ET > X GeV

The thresholds X as well as the cut values on these vari-ables are determined by optimization [21]. In the

opti-mization we use the figure of merit S/(1.5 +√B) (where

S is the expected number of signal events and B is the expected number of background events) to quantify the sensitivity as a compromise between best discovery and best limit potential [22, 23]. In the low background region (B  1), maximizing this figure of merit is equivalent to maximizing the signal efficiency. In the high background region (B  1), this figure of merit has the same behavior

as S/√B. For the optimization study, p¯p → b0b¯0 _Monte

Carlo simulations with a range of masses are used as the signal S. Standard model Z Monte Carlo simulations are used for the background B.

In order to be sensitive to a range of masses, we must take into account the generic behavior of new signals: as mass increases the cross section decreases while the transverse energy spectra become harder. Therefore, to be optimally sensitive to higher mass signals, we cut at

larger values of Njetand JT thus removing more of the

background to give sensitivity to the lower cross sections. For the sake of simplicity, we desire that our selection

only changes gradually with mass and uses the same ET

threshold on all jets. With a simple selection, the data-based background prediction method becomes easier. To confirm that this desire for simplicity does not consid-erably reduce the search sensitivity, and to understand what cut values and thresholds to use, we first establish a “target” selection. The “target” selection is defined as the selection with the highest sensitivity when placing

cuts on the individual jet ET’s and JT. This is found

by scanning through all possible cuts on J_T10(that is, JT

is calculated with a 10 GeV threshold on the jets) and

all possible ET thresholds for up to 4 jets (ordered by

ET), and finding the point with the optimal sensitivity.

In this scan, step sizes of 10 GeV are used for the jet

ET thresholds, and a step size of 50 GeV is used for JT10.

This scan is done independently for b0masses in the range

100 ≤ mb0 ≤ 350 GeV/c2with a step size of 50 GeV/c2.

The optimal points found by this scan for a b0 mass

of 150 GeV/c2 _{are shown in column 2 of Table I. These}

cut values give the best possible sensitivity at this mass

point when placing cuts on the individual jet ET’s and

J10

T . Again, we wish to choose a simple selection that

gradually changes as a function of mass, and use the tar-get sensitivities at all mass points for comparison. Based

on the optimal target points for b0 masses in the range

100 ≤ mb0 ≤ 350 GeV/c2, we choose the simpler

require-ments of N30

jet ≥ 3 and JT10 > mb0c2. The sensitivity of

Values Values of simple

Variable from scan selection

E_Tjet 1thresh.: 50 30 ETjet 2thresh.: 30 30 E_Tjet 3thresh.: 30 30 E_Tjet 4thresh.: 20 0 JT10cut: 0 150 Nsig: 48.5 75.5 Nbkg: 2.60 13.8 S/(1.5 +√B): 15.6 14.5

TABLE I: Optimal point compared with the simple selection

of Njet30 ≥ 3 and JT10> 150, for the mb0 = 150 GeV/c2 mass

point. Here, Nsig is the number of signal events expected in

1fb−1 after the given selection using b0 Monte Carlo

simula-tions. Nbkg is the number of background events expected in

1fb−1after the given selection using standard model Z Monte

Carlo simulations. In this optimization study, 2.7 × 105

stan-dard model Z events were used; 1500 signal events were used (both counted before jet selection).

the simple requirements is compared to the target

sen-sitivity in column 3 of Table I for the 150 GeV/c2 _mass

point.

From the table it is apparent that, for mb0 =

150 GeV/c2_{, the sensitivity of the simple cuts is only}

negligibly less than the target sensitivity. We find the same to be true for all mass points studied, except for

the mb0 = 100 GeV/c2 mass point. In that case,

how-ever, the sensitivity of the simple cuts is still adequate be-cause of the larger cross sections for lower mass particles

[24]. In addition, low masses near 100 GeV/c2 _{are less}

interesting as they are already more tightly excluded [25].

Thus, we conclude that the simpler selection of N_jet30≥ 3

and J_T10> mb0c2 is nearly optimal for the mass range of

interest.

In the above, JT was calculated using a 10 GeV ET

threshold on the jets. For the purposes of the background

estimation, it is simpler to use the same ET threshold on

JT as one uses on the Njetvariable. Therefore, a 30 GeV

threshold is used when calculating JT. This was found to

give a small decrease in sensitivity in the b0 _{model with}

the benefit of a gain in simplicity.

The kinematic jet selection was found to be optimal when using the fourth generation model as the signal.

When optimizing using the figure of merit S/(1.5 +√B),

the optimal point is independent of the normalization of the signal. That is, any model with a different cross section but the same kinematic distributions will give the same optimal point. In addition, the shape of the

kinematic distributions are mostly determined by the b0

mass. We therefore expect that this selection is nearly optimal for all models with heavy particles produced in pairs and decaying to Z+jet. In general, this selection

is sensitive to any model with high ET jets in the final

class of Z+high ET jet models is not possible.

In this optimization, we assumed new signals would lead to final states consisting of a Z boson and many high

ET jets. Of course, some assumption about signal

char-acteristics must be made in order to understand how to separate signal from background. These assumptions will naturally reduce the model independence of the search. There is a trade-off between the specificity of these as-sumptions and the sensitivity to a particular model. For example, in nearly all new physics models with Z bo-son final states, the transverse momentum spectrum of the Z is harder than for standard model Z production. This is because, in these models, the Z is usually a decay product of a massive particle. One would conclude that the Z transverse momentum is a very model-independent variable, and therefore well-motivated. However, we find,

in the b0 model sensitivity study, that the jet kinematic

requirements have much higher sensitivity than the Z transverse momentum. The cost of this sensitivity is a loss of generality: with this assumption we are no longer

sensitive to Z final states without high ET jets. The

sensitivity of the b0 model can be further enhanced by

requiring b jets using displaced vertices (because of the

b0 → bZ decay), again with a cost to generality. In our

analysis, as a compromise between model independence and sensitivity, we choose to require additional jets in the event.

To summarize, after selecting Z → ee and Z → µµ events, the kinematic selection is:

• N30

jet≥ 3, and

• J30

T > mb0c2.

That is, Z events with N30

jet ≥ 3 are selected, and the

J_T30distribution is scanned for an excess. Step sizes of 50

GeV are used.

IV. BACKGROUNDS

In the signal region described above, there are poten-tial backgrounds from the following sources:

• single-Z production in conjunction with jets, • multi-jet events, where two jets fake leptons, • cosmic rays coincident with multi-jet events, • W Z+jets, where the W decays to jets, • ZZ+jets, where one of the Z’s decays to jets, • W W +jets, where both W ’s decay to leptons, and

• t¯t+jets, where both W ’s decay to leptons.

The dominant background is from standard model single-Z production in conjunction with jets. Since be-yond leading-log order diagrams make potentially large

contributions to events with N_jet30≥ 3, calculation of this

background from theoretical first principles is extremely difficult, and therefore would require careful validation with data. Rather than using data as merely a validation tool we take a different approach, and instead measure the background directly from data, and with data alone. The following section is devoted to describing this predic-tion technique for the dominant background from Z+jet. As this technique has not been applied previously, it is explained thoroughly, with careful validation studies

de-scribed. The remaining backgrounds are estimated in

Sec. VI.

V. DATA-BASED Z+JET BACKGROUND

PREDICTION TECHNIQUE

Given the above selection, there are two tasks: the

total number of background events with N30

jet ≥ 3 must

be predicted, and the shape of the J_T30 distribution

af-ter this cut must be predicted. When combined, these

two components give the full normalized J30

T distribution

prediction. The background for events with N30

jet ≥ 3

and any J30

T cut can be obtained from this distribution.

The method for predicting each of the two components is described separately in the following two sections.

In each of the prediction methods, fits to various jet ET

distributions are used. A parameterization that describes

the shapes of these jet ET distributions well is therefore

required. The parameterization used is:

f (ET) = p0

e−ET/p1

(ET)p2

, (1)

where the pi are fitted parameters. This

parameteriza-tion was motivated by observaparameteriza-tions in Monte Carlo sim-ulations, control regions of data, and phenomenological

studies that: at low ET, the jet ET shape follows a power

law function; at high ET, it follows an exponential

de-cay function. The above parameterization satisfies these limiting behaviors. With the above convention, the

pa-rameter p1 has dimensions of energy, the parameter p2

is dimensionless, and both parameters are positive. Fur-ther discussion and motivation for this parameterization is provided in [18].

A. Number of Events with Njet30≥ 3

In order to predict the total number of events with

N_jet30 ≥ 3, we use the jet ET distributions in the Njet30 ≤

2 control regions. Since jets are counted above an ET

threshold (in this case 30 GeV), the Njet distribution

is completely determined from the jet ET distributions.

(GeV) T highest jet E rd 3 0 20 40 60 80 100 Events/GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 2 ≤ 30 jet N 30≥ 3 jet N

FIG. 2: ET distribution of the third highest ET jet in

stan-dard model Z → µµ Monte Carlo simulations. Events with

N30

jet ≤ 2 have ET < 30 GeV; events with Njet30 ≥ 3 have

ET > 30 GeV.

In Fig. 2, the ET distribution of the third highest jet

is shown. By construction, a cut on N30

jet ≤ 2 separates

this distribution into two regions. This distribution can

be fit in the ET < 30 GeV region and extrapolated to

the ET > 30 GeV region to get the expected number of

background events with N_jet30≥ 3.

We fit the parameterization from Eq. (1) to the jet ET

distribution of Fig. 2, and show the results in Fig. 3 [26]. The fit matches well the broad features of the distribution

above 30 GeV. The number of events with N30

jet ≥ 3 is

then predicted by integrating the fitted distribution from 30 GeV to infinity. The fit prediction obtained with this method (with its uncertainty from fit parameter error

propagation described in Sec. V C) is 116+10₋₁₃events (with

the number of generated Monte Carlo events having an

equivalent luminosity of 7 fb−1). The number of events

observed in the simulated data with N30

jet ≥ 3 is 152.

In this case, the extrapolation predicts the background

to within 31 ± 16%. The level of consistency will be

evaluated further in the validation studies with data in Sec. V D.

This method, using the jet ET distributions to

pre-dict integrals of the Njet distribution, can clearly be

ex-tended to other analyses as well. For illustration pur-poses only we describe other examples here, still using standard model Z → µµ Monte Carlo simulation.

Con-sider predicting the total number of events with N_jet80≥ 1

(that is, we require at least one jet with an ET

thresh-old of 80 GeV). In this case, a fit to the highest ET jet

distribution below 80 GeV can be extrapolated to above

that threshold, as in Fig. 4. (Note that the highest ET

distribution in this figure is harder than the third

high-est ET jet distribution, as one expects when ordering the

jets by ET). It is clear that the extrapolation describes

the distribution reasonably well.

If we instead wish to predict the number of events with

(GeV) T highest jet E rd 3 0 20 40 60 80 100 Events/GeV -2 10 -1 10 1 10 2 10 3 10 4 10 /dof: 14.0/12 2 χ

FIG. 3: ET distribution of the third highest ET jet in

stan-dard model Z → µµ Monte Carlo events. The distribution is

fit to Eq. (1) in the range 15 < ET < 30 GeV, and

extrapo-lated to the ET > 30 GeV region.

N40

jet ≥ 1, we must fit the same ET distribution below

40 GeV and extrapolate it to above that threshold, also shown in Fig. 4. It is clear that the extrapolation does

not describe the high ET portion of the distribution well.

There is a large systematic uncertainty present in extrap-olations that use such a small portion of the distribution that the shape can not be reliably obtained. This can be

mitigated by raising the ET threshold, unless the shape of

the jet ET distribution at high ET can be otherwise

con-strained. In the case examined in this analysis, we fit the

third highest ET jet (which has a softer ET distribution

than the highest ET jet) in the region ET < 30 GeV. We

have checked that the data in this region constrains the shape sufficiently with validation studies using control samples of data and Monte Carlo simulations, described later in Sec. V D.

From the above, it is apparent that one can estimate

the background for events with NX

jet ≥ n by fitting the

ET distribution of the nth highest ET jet in the region

ET < X and extrapolating the fit to the region ET > X,

as long as the fit region ET < X constrains the shape

sufficiently.

B. JT Shape Determination

We now describe the method used to determine the

shape of the J30

T distribution of events with Njet30≥ 3.

Af-ter finding the shape, it is then normalized to the number

of events with N_jet30≥ 3 found by the above method. We

again use standard model Z → µµ Monte Carlo events to explain the method, and later will apply it to data.

Since J_T30is simply the sum of the individual jet

trans-verse energies above 30 GeV, if the ET distributions of

jets for events with N30

(GeV)

T

Highest jet E

0

100

200

300

Events/GeV

-2

10

-1

10

1

10

2

10

3

10

4

10

5

10

< 80 GeV region T Fit in 20 < E < 40 GeV region T Fit in 20 < E

FIG. 4: ET of the highest ET jet in standard model Z → µµ

Monte Carlo events. The distribution is fit to Eq. (1) in the

region 20 < ET < 80 GeV (dotted line), and again in the

region 20 < ET < 40 GeV (solid line).

bution can be predicted for these events. We extrapolate

the shape of these jet ET distributions from the jet ET

distributions of N30

jet ≤ 2 events. In order to do such an

extrapolation, we must understand the variation of the

jet ET distribution as a function of Njet30.

The ET distributions of all jets in events with Njet30= 1

and 2, normalized to have equal area, is shown in Fig. 5 using Z → `` data. The general shape is similar, though

jets in N_jet30= 2 events have a slightly harder tail at high

ET. We model this by fitting to each jet ET distribution

(using Eq. (1)) and extrapolating the fit parameters to

N30

jet ≥ 3 events. To avoid simultaneously extrapolating

two fit parameters we only extrapolate the exponential

parameter (p1), as this parameter governs the high ET

behavior in our parameterization. In order to extrapolate

only this parameter, we fit the N30

jet= 1 ET spectrum

al-lowing both parameters to float freely, then fix the power

law parameter (p2) in the fit to the Njet30 = 2 ET

spec-trum. We then extrapolate the p1 parameter of Eq. (1)

linearly as a function of N30

jet, from their fitted values at

N30

jet= 1 and Njet30= 2 into the region Njet30≥ 3.

Figures 6 and 7 show the fits of the spectra for events with 1 and 2 jets. Figure 8 shows the linear extrapo-lation of the exponential parameters. For illustration,

the exponential parameter obtained from a fit to the ET

distribution in N_jet30 = 3 events (again fixing the power

law parameter to that found in the N30

jet = 1 events) is

shown on the same figure. The extrapolation reasonably

predicts the parameter for events with N_jet30= 3 [27].

This dependence of the jet ET spectra on Njet30is

mod-eled as described by our parameter extrapolation,

allow-ing us to predict the shapes of the jet ET spectra for

events with N_jet30 ≥ 3. The J30

T distribution is now

al-most completely determined. Only an estimate for the

(GeV)

T jet

E

50 100 150 200 250 300

Arbitrary normalization

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

1

= 1 bin 30 jet

All jets in the N = 2 bin

30 jet

All jets in the N

FIG. 5: ET distribution of jets in Njet30 = 1 events (open

squares) and in Njet30= 2 events (solid circles) in Z → `` data.

Events with higher Njet30 have harder ET spectra.

(GeV)

T

E

0

100 200 300 400 500

Arbitrary Normalization

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

/dof: 70.7/80 2 χ

FIG. 6: ET distribution of jets in Njet30= 1 events in standard

model Z → µµ Monte Carlo events. The distribution is fit to

Eq. (1) in the range ET> 30.

relative fractions of events with 3, 4, 5, ... jets is needed. For this, we use an exponential fit parameterization, fit

to the N30

jet distribution in the region Njet30 ≤ 2, and use

this shape in the N30

jet ≥ 3 region. This fit is shown in

Fig. 9. There is no theoretical motivation for an expo-nential shape; we merely use it as an estimate, and verify

that the J_T30 prediction does not strongly depend on the

chosen parameterization. As the total number of events

with N30

jet ≥ 3 is already constrained using the method

from Sec. V A, the dependence of the J_T30distribution on

the exponential parameterization of the N30

jetdistribution

is small.

(GeV)

T

E

0

100 200 300 400 500

Arbitrary Normalization

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

/dof: 56.9/54 2 χ

FIG. 7: ETdistribution of jets in Njet30= 2 events in standard

model Z → µµ Monte Carlo events. The distribution is fit to

Eq. (1) in the range ET > 30, with the parameter p2 fixed to

that obtained from Fig. 6.

bin

30 jet

N

1

2

3

4

(GeV)

1

p

0

10

20

30

40

50

60

70

80

90

100

FIG. 8: The extrapolation of the exponential parameter p1

vs. Njet30 in standard model Z → µµ Monte Carlo events.

make a simple Monte Carlo program that samples these

shapes to get the J30

T distribution. The steps required to

make this J_T30 prediction are:

1. For each event, generate the number of jets by

ran-domly sampling the predicted N30

jet distribution in

the range {3, 4, 5, ...}.

2. Take the appropriate jet ET distribution for this

number of jets after extrapolating the exponential

fit parameter. Independently sample this jet ET

distribution for each jet.

3. Sum these jets to obtain the J30

T . 30 jet

N

0

1

2

3

4

Arbitrary Normalization

-1

10

1

10

2

10

3

10

4

10

5

10

6

10

FIG. 9: N30

jet distribution in standard model Z → µµ Monte

Carlo events, fit to an exponential in the range Njet30≤ 2. This

shape is used to estimate the relative fractions of events with 3, 4, 5, ... jets.

The process is repeated as necessary until the J_T30shape

is obtained to the desired level of statistical precision.

On step 2, the jet ET shapes are independently

sam-pled; however, there is potentially some correlation be-tween the individual jet energies. Including this

corre-lation in the J30

T shape prediction would have the effect

of making the tail at large values of J_T30 slightly harder.

In the validation studies in Sec. V D we verify that the correlation is below the level necessary to affect the fit prediction. To understand this further, in Fig. 10, we

plot the ET of one the jets versus the other in events

with N30

jet = 2 in the Z → `` data. There is no

correla-tion evident in the plot; in the 663 events with N30

jet= 2,

only a small correlation of 25% is found, indicating that

independently sampling the ET distribution is a

reason-able approximation.

C. Uncertainties on Fit Prediction

There are two sources of uncertainty on the mean back-ground prediction: the statistical uncertainty from the finite amount of data in the fits, and the systematic un-certainty from imperfect modeling of the various shapes in the fits.

1. Statistical Uncertainty on Fit Prediction

The third highest ET jet normalization fit predicts the

total number of events with N30

jet≥ 3, using the

of jet 1 (GeV)

T

E

0

50 100 150 200 250

of jet 2 (GeV)

_T

E

0

50

100

150

200

250

1

10

2

10

FIG. 10: The ET of a random jet vs. the ET of the other,

using jets with Njet30= 2 in Z → `` data.

from its values at the minimum − log L + 1₂. Since the

total number of events with N_jet30≥ 3 is given by a single

fit, its uncertainty is easily determined with this method.

The J30

T prediction is obtained by extrapolating the

behavior of multiple distributions, and to estimate its shape uncertainty we vary each fit parameter indepen-dently within its uncertainty (output by the fit) and re-do the extrapolation procedure. The individual uncer-tainties are combined in quadrature to obtain the total uncertainty. The normalization error is then added in quadrature as well to obtain the uncertainty on the

fully-normalized J_T30 distribution.

2. Systematic Uncertainty on Fit Prediction

As the background from Z+jet events is determined from a fit to the data, the only source of systematic uncertainties is mis-parameterization of those data. If the data were poorly parameterized, fitting a subset of the data would give a large change in the background prediction. We therefore estimate the size of the mis-parameterization uncertainties by changing the range of

each fit and re-doing the fit procedure to obtain the J30

T

normalization and shape prediction. Both uncertainties,

that on the total number of events with N_jet30 ≥ 3 (from

the third highest ET jet fit), and that on the JT30shape,

are estimated in this way. The variations from each fit range change are then added in quadrature to obtain the full uncertainty. The fit range changes are summarized in Table II. The “±1σ” range changes are chosen to give sufficient coverage when observed in control samples of data.

Finally, using the technique and the uncertainties de-veloped above in the Monte Carlo simulation, we can demonstrate that the method is self-consistent by

check-Distribution nominal range “−1σ” range “+1σ” range

Third highest ET jet (15, 30) GeV (15, 26) GeV (17, 30) GeV

Njet30= 1 jet ET (30, ∞) GeV (30, 150) GeV (70, ∞) GeV

N30

jet= 2 jet ET (30, ∞) GeV (30, 80) GeV (50, ∞) GeV

N30

jetshape [0, 2] jets [0, 1] jets [1, 2] jets

TABLE II: Nominal fit ranges and the fit range changes used to estimate systematic uncertainties. The nominal fit range of each distribution is shown in the second column. The third and fourth columns show the ranges used to estimate the uncertainty from a mis-parameterization of that distribution.

(GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 3 10 (GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 3 10 _{SM Z}0_{→µµ MC}

Prediction From Fit Fit Uncertainty

FIG. 11: The prediction for the J30

T distribution (blue line)

of standard model Z Monte Carlo and its uncertainty (gray band), compared to the actual distribution (black points with errors).

ing the normalized J30

T prediction for events with N

30 jet≥

3 matches that observed in Monte Carlo events. This comparison is shown in Fig. 11. The observed distribu-tion agrees well with the predicdistribu-tion.

D. Validation of Technique

Having demonstrated and described the procedure for obtaining the Z+jet background using Monte Carlo sim-ulation, its validation, done predominantly in data, is

now described. The Z+jet data cannot be used as a

validation sample because of potential signal bias, so we must test on other data samples. We use two sets of multi-jet data as background-only validation samples, and W +jet data as a background sample containing a

real heavy quark signal from t¯t production. Finally, we

(GeV) T highest jet E rd 3 0 50 100 150 Events/GeV -2 10 -1 10 1 10 2 10 3 10 4 10 /dof: 13.4/12 2 χ

FIG. 12: ET distribution of the third highest ET jet in

“X”+jet events selected with the jet triggers as described in

the text. The distribution is fit to Eq. (1) in the 15 < ET <

30 GeV region and extrapolated to the ET> 30 GeV region.

1. Multi-Jet Data

The Z+jet background extrapolation only requires

in-formation about the jet ET distributions, and not the

Z. It should therefore perform similarly well not only for Z+jet events, but “X”+jet events, provided that the “X” has a similar transverse momentum spectrum to the

Z. For example, if the “X” has a minimum pT

thresh-old, the ET distributions of the jets will be sculpted such

that they no longer follow the power law × exponential parameterization of Eq. (1).

We first obtain “X”+jet events from multi-jet data dominated by QCD interactions using prescaled jet

trig-gers that require at least one jet with ET > 20 GeV [28].

An “X” is then constructed by picking two random jets

in the event, requiring they both have ET > 20 GeV

(to match the electron and muon pT cuts), and requiring

MX > 70 GeV/c2 to remove the invariant mass turn-on.

The invariant mass is not further restricted to the region

81 < MX < 101 GeV/c2 to maximize statistics; in any

case the J30

T distribution is observed to not depend on

MX in this sample.

Given this “X” selection, the remaining jets in the event are used to validate the procedure. Figure 12 shows

the third highest ET jet distribution. We extrapolate this

distribution above 30 GeV using Eq. (1). A prediction of

97±27 (statistical uncertainty only) events with N30

jet≥ 3

is obtained. 80 events are observed. This is consistent within the uncertainties. To quantitatively evaluate the level of consistency we calculate the probability to mea-sure the observed number of events or higher given the background prediction, as well as convert this probabil-ity to units of standard deviations [29]. This calculation gives a corresponding probability of 0.73; this is a 0.6σ level of consistency.

(GeV)

T

E

0

100 200 300 400 500

Arbitrary Normalization

-3

10

-2

10

-1

10

1

10

2

10

3

10

/dof: 44.7/8 2 χ

FIG. 13: ET distribution of jets in Njet30= 1 “X”+jet events,

selected with the jet triggers as described in the text. The

distribution is fit to Eq. (1) in the ET> 30 GeV region.

We now predict the J30

T shape. Figures 13 and 14 show

the fits to the jet ET spectra for events with Njet30= 1 and

2. We extrapolate the parameter p1 using the plot in

Fig. 15 to events with N30

jet≥ 3. The Njet30shape is taken

from the fit in Fig. 16. Using these ingredients, the simple

Monte Carlo program is used to obtain the J30

T shape,

which is normalized to the prediction of 97 events with

N_jet30≥ 3. The prediction and total uncertainty is shown

overlaid with the actual distribution in “X”+jet data in Fig. 17. The distribution clearly agrees well within the uncertainty envelope.

Because the J30

T uncertainties in each bin are

corre-lated, an independent data/background comparison in each bin is not straightforward. Rather, we test the shape agreement once using the (arbitrarily chosen) region of

J_T30> 200 GeV. Above 200 GeV, 19.7+9.2_−9.0events are

ex-pected and 20 events are observed.

The background extrapolation method can accurately

predict the normalization and shape of the J30

T

distribu-tion in the jet triggered sample. However, because of the prescale, this sample has relatively low statistics despite the large cross section of QCD multi-jet processes. To obtain a higher statistics sample of multi-jet data, we can use the electron triggers, which are not prescaled. In this sample we construct an “X” by pairing the triggered electron with a “fake” electron, which is an EM calorime-ter cluscalorime-ter that is reconstructed as an electron but fails

the low hadronic energy requirement. “X” events

se-lected in this way are dominated by QCD dijet events.

Again, MX > 70 GeV/c2 is required to remove the

in-variant mass turn-on. Additionally the inin-variant mass

region 81 < MX < 101 GeV/c2 is vetoed to remove real

Z → ee events. Figure 18 shows the plot of the invariant mass before these requirements.

(GeV)

T

E

0

100 200 300 400 500

Arbitrary Normalization

-3

10

-2

10

-1

10

1

10

2

10

3

10

/dof: 20.7/13 2 χ

FIG. 14: ET distribution of jets in Njet30= 2 “X”+jet events

selected with the jet triggers as described in the text. The

distribution is fit to Eq. (1) in the ET> 30 GeV region with

the parameter p2fixed to that obtained from the fit in Fig. 13.

bin

30 jet

N

1

2

3

4

(GeV)

1

p

0

20

40

60

80

100

120

140

FIG. 15: The extrapolation of the exponential parameter p1

vs. N30

jet in “X”+jet events selected with the jet triggers as

described in the text.

the third highest ET jet distribution. We extrapolate this

distribution above 30 GeV using Eq. (1). A prediction of

4427+354₋₃₁₀(statistical uncertainty only) events with N30

jet≥

3 is obtained. 4509 events are observed. Approximating the Poisson distribution of the number of observed events as a Gaussian, this is a 0.23σ level of consistency.

The J30

T shape is predicted using the previously

de-scribed procedure of extrapolating the jet ET

distribu-tions from events with N30

jet = 1 and 2 to Njet30 ≥ 3.

The normalized prediction and its uncertainty are com-pared to the actual distribution in the data in Fig. 20. The distribution agrees well within the uncertainty

en-velope. Above 200 GeV, 1412+477₋₂₁₂ events are expected;

30 jet

N

0

1

2

3

4

5

Arbitrary Normalization

-1

10

1

10

2

10

3

10

4

10

5

10

6

10

FIG. 16: N30

jet distribution in “X”+jet events selected with

the jet triggers as described in the text. The distribution is

fit to an exponential in the range Njet30≤ 2.

(GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 (GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 Jet-triggered Data Prediction From Fit Fit Uncertainty

FIG. 17: The prediction (blue line) and uncertainty (gray

band) for the J30

T distribution of “X”+jet events selected with

the jet triggers as described in the text. The prediction is compared to the actual distribution (black points with errors). The observation agrees with the prediction.

1128 events are observed, for a −1.3σ level of consistency. The background prediction is compared to the number of

observed events as a function of the J_T30cut in Table III.

The prediction agrees well over the entire J30

T

distribu-tion.

)

2

(GeV/c

X

M

0

50

100

150

200

)

2

Events/(GeV/c

3

10

5

10

15

20

25

30

Vetoed

FIG. 18: Distribution of MXin “X”+jet events selected from

the electron triggers as described in the text. The shaded

regions are removed; that is, events with MX > 70 GeV/c2

are selected, and the 81 < MX< 101 GeV/c2region is vetoed.

(GeV) T highest jet E rd 3 0 50 100 150 Events/GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 /dof: 8.4/12 2 χ

FIG. 19: ET distribution of the third highest ET jet in

“X”+jet events selected with the electron triggers as de-scribed in the text. The distribution is fit to Eq. (1) in the

15 < ET < 30 GeV region and extrapolated to the ET > 30

GeV region.

To validate the third highest ET jet extrapolation, we

evaluate the consistency between the fit prediction and the observation in each subsample. The pull distribution from these calculations is observed to be consistent with a Gaussian with mean 0 and width of 1, indicating that the mean prediction and the uncertainties are correctly

calculated for the N30

jet≥ 3 prediction. On average, the

background prediction is 3 ± 5% low relative to the data. That is, the background prediction underestimates the background, but by an amount consistent with zero. This is consistent with the fit done in standard model Z Monte Carlo simulation in Sec. V A, in which the background

Minimum J30

T cut Total Bkg. (events) Data (events)

50 4430+1270 −600 4509 100 4380+1250−590 4463 150 2810+830−360 2602 200 1410+480−210 1128 250 667+281−133 436 300 312+172−81.8 170 350 146+106−47.4 62 400 68.7+64.8−26.2 27 450 32.8+38.9 −14.3 15 500 16.2+23.3−8.4 6 550 7.9+14.5−4.5 3 600 3.9+8.8 −2.5 0

TABLE III: The “X”+jet data (selected with the electron

triggers as described in the text) vs. J30

T , compared with the

background prediction. (GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 (GeV) 30 T J 0 200 400 600 800 1000 Events/50 GeV -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 QCD Multi-jet Data Prediction From Fit Fit Uncertainty

FIG. 20: The prediction (blue line) and uncertainty (gray

band) for the J30

T distribution of “X”+jet events selected with

the electron triggers as described in the text. The prediction is compared to the actual distribution (black points with

er-rors). The observation agrees with the prediction, with a

maximum fluctuation downward of 1.9σ. The data are below the prediction for several point because the shape uncertainty is correlated between bins.

prediction was 31 ± 16% low relative to the data.

To validate the J_T30 shape prediction, in each

subsam-ple we evaluate the consistency between the fit

predic-tion and the observapredic-tion using a cut of J30

T > 200 GeV.

In this case, the resulting pull distribution was inconsis-tent with a Gaussian with mean 0 and width 1. We find that the background prediction overestimates the num-ber of observed events, and that the uncertainty is overly conservative, after correcting for this bias. On average, the background prediction is 23 ± 7% high relative to the data. However, we find that this bias is covered by the uncertainties, with an average uncertainty on the back-ground prediction of 47%. To clarify, these biases are

only present in the J30