Search for a Higgs boson produced in association with a Z boson in ppbar
  collisions
FERMILAB-PUB-07/076-E
Search for a Higgs boson produced in association with a Z boson in pp collisions
V.M. Abazov,35 B. Abbott,75 M. Abolins,65 B.S. Acharya,28 M. Adams,51 T. Adams,49 E. Aguilo,5 S.H. Ahn,30
M. Ahsan,59 G.D. Alexeev,35 G. Alkhazov,39 A. Alton,64,∗ G. Alverson,63 G.A. Alves,2 M. Anastasoaie,34
L.S. Ancu,34 T. Andeen,53 S. Anderson,45 B. Andrieu,16 M.S. Anzelc,53 Y. Arnoud,13 M. Arov,60 M. Arthaud,17
A. Askew,49 B. Åsman,40 A.C.S. Assis Jesus,3 O. Atramentov,49 C. Autermann,20 C. Avila,7 C. Ay,23 F. Badaud,12
A. Baden,61 L. Bagby,52 B. Baldin,50 D.V. Bandurin,59 P. Banerjee,28 S. Banerjee,28 E. Barberis,63 A.-F. Barfuss,14
P. Bargassa,80 P. Baringer,58 J. Barreto,2 J.F. Bartlett,50 U. Bassler,16 D. Bauer,43 S. Beale,5 A. Bean,58
M. Begalli,3 M. Begel,71 C. Belanger-Champagne,40 L. Bellantoni,50 A. Bellavance,50 J.A. Benitez,65 S.B. Beri,26
G. Bernardi,16 R. Bernhard,22 L. Berntzon,14 I. Bertram,42 M. Besançon,17 R. Beuselinck,43 V.A. Bezzubov,38
P.C. Bhat,50 V. Bhatnagar,26 C. Biscarat,19 G. Blazey,52 F. Blekman,43 S. Blessing,49 D. Bloch,18 K. Bloom,67
A. Boehnlein,50 D. Boline,62 T.A. Bolton,59 G. Borissov,42 K. Bos,33 T. Bose,77 A. Brandt,78 R. Brock,65
G. Brooijmans,70 A. Bross,50 D. Brown,78 N.J. Buchanan,49 D. Buchholz,53 M. Buehler,81 V. Buescher,21
S. Burdin,42,¶ S. Burke,45 T.H. Burnett,82 C.P. Buszello,43 J.M. Butler,62 P. Calfayan,24 S. Calvet,14 J. Cammin,71
S. Caron,33 W. Carvalho,3 B.C.K. Casey,77 N.M. Cason,55 H. Castilla-Valdez,32 S. Chakrabarti,17
D. Chakraborty,52 K. Chan,5 K.M. Chan,55 A. Chandra,48 F. Charles,18 E. Cheu,45 F. Chevallier,13 D.K. Cho,62
S. Choi,31 B. Choudhary,27 L. Christofek,77 T. Christoudias,43 S. Cihangir,50 D. Claes,67 B. Clément,18
C. Clément,40 Y. Coadou,5 M. Cooke,80 W.E. Cooper,50 M. Corcoran,80 F. Couderc,17 M.-C. Cousinou,14
S. Crépé-Renaudin,13 D. Cutts,77 M. Ćwiok,29 H. da Motta,2 A. Das,62 G. Davies,43 K. De,78 P. de Jong,33
S.J. de Jong,34 E. De La Cruz-Burelo,64 C. De Oliveira Martins,3 J.D. Degenhardt,64 F. Déliot,17 M. Demarteau,50
R. Demina,71 D. Denisov,50 S.P. Denisov,38 S. Desai,50 H.T. Diehl,50 M. Diesburg,50 A. Dominguez,67 H. Dong,72
L.V. Dudko,37 L. Duflot,15 S.R. Dugad,28 D. Duggan,49 A. Duperrin,14 J. Dyer,65 A. Dyshkant,52 M. Eads,67
D. Edmunds,65 J. Ellison,48 V.D. Elvira,50 Y. Enari,77 S. Eno,61 P. Ermolov,37 H. Evans,54 A. Evdokimov,73
V.N. Evdokimov,38 A.V. Ferapontov,59 T. Ferbel,71 F. Fiedler,24 F. Filthaut,34 W. Fisher,50 H.E. Fisk,50 M. Ford,44
M. Fortner,52 H. Fox,22 S. Fu,50 S. Fuess,50 T. Gadfort,82 C.F. Galea,34 E. Gallas,50 E. Galyaev,55 C. Garcia,71
A. Garcia-Bellido,82 V. Gavrilov,36 P. Gay,12 W. Geist,18 D. Gelé,18 C.E. Gerber,51 Y. Gershtein,49 D. Gillberg,5
G. Ginther,71 N. Gollub,40 B. Gómez,7 A. Goussiou,55 P.D. Grannis,72 H. Greenlee,50 Z.D. Greenwood,60
E.M. Gregores,4 G. Grenier,19 Ph. Gris,12 J.-F. Grivaz,15 A. Grohsjean,24 S. Grünendahl,50 M.W. Grünewald,29
F. Guo,72 J. Guo,72 G. Gutierrez,50 P. Gutierrez,75 A. Haas,70 N.J. Hadley,61 P. Haefner,24 S. Hagopian,49
J. Haley,68 I. Hall,75 R.E. Hall,47 L. Han,6 K. Hanagaki,50 P. Hansson,40 K. Harder,44 A. Harel,71
R. Harrington,63 J.M. Hauptman,57 R. Hauser,65 J. Hays,43 T. Hebbeker,20 D. Hedin,52 J.G. Hegeman,33
J.M. Heinmiller,51 A.P. Heinson,48 U. Heintz,62 C. Hensel,58 K. Herner,72 G. Hesketh,63 M.D. Hildreth,55
R. Hirosky,81 J.D. Hobbs,72 B. Hoeneisen,11 H. Hoeth,25 M. Hohlfeld,21 S.J. Hong,30 R. Hooper,77 S. Hossain,75
P. Houben,33 Y. Hu,72 Z. Hubacek,9 V. Hynek,8 I. Iashvili,69 R. Illingworth,50 A.S. Ito,50 S. Jabeen,62 M. Jaffré,15
S. Jain,75 K. Jakobs,22 C. Jarvis,61 R. Jesik,43 K. Johns,45 C. Johnson,70 M. Johnson,50 A. Jonckheere,50
P. Jonsson,43 A. Juste,50 D. Käfer,20 S. Kahn,73 E. Kajfasz,14 A.M. Kalinin,35 J.M. Kalk,60 J.R. Kalk,65
S. Kappler,20 D. Karmanov,37 J. Kasper,62 P. Kasper,50 I. Katsanos,70 D. Kau,49 R. Kaur,26 V. Kaushik,78
R. Kehoe,79 S. Kermiche,14 N. Khalatyan,38 A. Khanov,76 A. Kharchilava,69 Y.M. Kharzheev,35 D. Khatidze,70
H. Kim,31 T.J. Kim,30 M.H. Kirby,34 M. Kirsch,20 B. Klima,50 J.M. Kohli,26 J.-P. Konrath,22 M. Kopal,75
V.M. Korablev,38 B. Kothari,70 A.V. Kozelov,38 D. Krop,54 A. Kryemadhi,81 T. Kuhl,23 A. Kumar,69 S. Kunori,61
A. Kupco,10 T. Kurča,19 J. Kvita,8 D. Lam,55 S. Lammers,70 G. Landsberg,77 J. Lazoflores,49 P. Lebrun,19
W.M. Lee,50 A. Leflat,37 F. Lehner,41 J. Lellouch,16 V. Lesne,12 J. Leveque,45 P. Lewis,43 J. Li,78 L. Li,48
Q.Z. Li,50 S.M. Lietti,4 J.G.R. Lima,52 D. Lincoln,50 J. Linnemann,65 V.V. Lipaev,38 R. Lipton,50 Y. Liu,6
Z. Liu,5 L. Lobo,43 A. Lobodenko,39 M. Lokajicek,10 A. Lounis,18 P. Love,42 H.J. Lubatti,82 A.L. Lyon,50
A.K.A. Maciel,2 D. Mackin,80 R.J. Madaras,46 P. Mättig,25 C. Magass,20 A. Magerkurth,64 N. Makovec,15
P.K. Mal,55 H.B. Malbouisson,3 S. Malik,67 V.L. Malyshev,35 H.S. Mao,50 Y. Maravin,59 B. Martin,13
R. McCarthy,72 A. Melnitchouk,66 A. Mendes,14 L. Mendoza,7 P.G. Mercadante,4 M. Merkin,37 K.W. Merritt,50
A. Meyer,20 J. Meyer,21 M. Michaut,17 T. Millet,19 J. Mitrevski,70 J. Molina,3 R.K. Mommsen,44 N.K. Mondal,28
R.W. Moore,5 T. Moulik,58 G.S. Muanza,19 M. Mulders,50 M. Mulhearn,70 O. Mundal,21 L. Mundim,3 E. Nagy,14
M. Naimuddin,50 M. Narain,77 N.A. Naumann,34 H.A. Neal,64 J.P. Negret,7 P. Neustroev,39 H. Nilsen,22
http://arxiv.org/abs/0704.2000v1
C. Noeding,22 A. Nomerotski,50 S.F. Novaes,4 T. Nunnemann,24 V. O’Dell,50 D.C. O’Neil,5 G. Obrant,39
C. Ochando,15 D. Onoprienko,59 N. Oshima,50 J. Osta,55 R. Otec,9 G.J. Otero y Garzón,51 M. Owen,44 P. Padley,80
M. Pangilinan,77 N. Parashar,56 S.-J. Park,71 S.K. Park,30 J. Parsons,70 R. Partridge,77 N. Parua,54 A. Patwa,73
G. Pawloski,80 P.M. Perea,48 K. Peters,44 Y. Peters,25 P. Pétroff,15 M. Petteni,43 R. Piegaia,1 J. Piper,65
M.-A. Pleier,21 P.L.M. Podesta-Lerma,32,§ V.M. Podstavkov,50 Y. Pogorelov,55 M.-E. Pol,2 A. Pompoš,75
B.G. Pope,65 A.V. Popov,38 C. Potter,5 W.L. Prado da Silva,3 H.B. Prosper,49 S. Protopopescu,73 J. Qian,64
A. Quadt,21 B. Quinn,66 A. Rakitine,42 M.S. Rangel,2 K.J. Rani,28 K. Ranjan,27 P.N. Ratoff,42 P. Renkel,79
S. Reucroft,63 P. Rich,44 M. Rijssenbeek,72 I. Ripp-Baudot,18 F. Rizatdinova,76 S. Robinson,43 R.F. Rodrigues,3
C. Royon,17 P. Rubinov,50 R. Ruchti,55 G. Safronov,36 G. Sajot,13 A. Sánchez-Hernández,32 M.P. Sanders,16
A. Santoro,3 G. Savage,50 L. Sawyer,60 T. Scanlon,43 D. Schaile,24 R.D. Schamberger,72 Y. Scheglov,39
H. Schellman,53 P. Schieferdecker,24 T. Schliephake,25 C. Schmitt,25 C. Schwanenberger,44 A. Schwartzman,68
R. Schwienhorst,65 J. Sekaric,49 S. Sengupta,49 H. Severini,75 E. Shabalina,51 M. Shamim,59 V. Shary,17
A.A. Shchukin,38 R.K. Shivpuri,27 D. Shpakov,50 V. Siccardi,18 V. Simak,9 V. Sirotenko,50 P. Skubic,75 P. Slattery,71
D. Smirnov,55 R.P. Smith,50 G.R. Snow,67 J. Snow,74 S. Snyder,73 S. Söldner-Rembold,44 L. Sonnenschein,16
A. Sopczak,42 M. Sosebee,78 K. Soustruznik,8 M. Souza,2 B. Spurlock,78 J. Stark,13 J. Steele,60 V. Stolin,36
A. Stone,51 D.A. Stoyanova,38 J. Strandberg,64 S. Strandberg,40 M.A. Strang,69 M. Strauss,75 R. Ströhmer,24
D. Strom,53 M. Strovink,46 L. Stutte,50 S. Sumowidagdo,49 P. Svoisky,55 A. Sznajder,3 M. Talby,14 P. Tamburello,45
A. Tanasijczuk,1 W. Taylor,5 P. Telford,44 J. Temple,45 B. Tiller,24 F. Tissandier,12 M. Titov,17 V.V. Tokmenin,35
M. Tomoto,50 T. Toole,61 I. Torchiani,22 T. Trefzger,23 D. Tsybychev,72 B. Tuchming,17 C. Tully,68 P.M. Tuts,70
R. Unalan,65 L. Uvarov,39 S. Uvarov,39 S. Uzunyan,52 B. Vachon,5 P.J. van den Berg,33 B. van Eijk,35
R. Van Kooten,54 W.M. van Leeuwen,33 N. Varelas,51 E.W. Varnes,45 A. Vartapetian,78 I.A. Vasilyev,38
M. Vaupel,25 P. Verdier,19 L.S. Vertogradov,35 M. Verzocchi,50 F. Villeneuve-Seguier,43 P. Vint,43 E. Von Toerne,59
M. Voutilainen,67,‡ M. Vreeswijk,33 R. Wagner,68 H.D. Wahl,49 L. Wang,61 M.H.L.S Wang,50 J. Warchol,55
G. Watts,82 M. Wayne,55 G. Weber,23 M. Weber,50 H. Weerts,65 A. Wenger,22,# N. Wermes,21 M. Wetstein,61
A. White,78 D. Wicke,25 G.W. Wilson,58 S.J. Wimpenny,48 M. Wobisch,60 D.R. Wood,63 T.R. Wyatt,44
Y. Xie,77 S. Yacoob,53 R. Yamada,50 M. Yan,61 T. Yasuda,50 Y.A. Yatsunenko,35 K. Yip,73 H.D. Yoo,77
S.W. Youn,53 C. Yu,13 J. Yu,78 A. Yurkewicz,72 A. Zatserklyaniy,52 C. Zeitnitz,25 D. Zhang,50 T. Zhao,82
B. Zhou,64 J. Zhu,72 M. Zielinski,71 D. Zieminska,54 A. Zieminski,54 L. Zivkovic,70 V. Zutshi,52 and E.G. Zverev37
(DØ Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina
2LAFEX, Centro Brasileiro de Pesquisas F́ısicas, Rio de Janeiro, Brazil
3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4Instituto de F́ısica Teórica, Universidade Estadual Paulista, São Paulo, Brazil
5University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada,
York University, Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada
6University of Science and Technology of China, Hefei, People’s Republic of China
7Universidad de los Andes, Bogotá, Colombia
8Center for Particle Physics, Charles University, Prague, Czech Republic
9Czech Technical University, Prague, Czech Republic
10Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
11Universidad San Francisco de Quito, Quito, Ecuador
12Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Université Blaise Pascal, Clermont-Ferrand, France
13Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
14CPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, France
15Laboratoire de l’Accélérateur Linéaire, IN2P3-CNRS et Université Paris-Sud, Orsay, France
16LPNHE, IN2P3-CNRS, Universités Paris VI and VII, Paris, France
17DAPNIA/Service de Physique des Particules, CEA, Saclay, France
18IPHC, Université Louis Pasteur et Université de Haute Alsace, CNRS, IN2P3, Strasbourg, France
19IPNL, Université Lyon 1, CNRS/IN2P3, Villeurbanne, France and Université de Lyon, Lyon, France
20III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany
21Physikalisches Institut, Universität Bonn, Bonn, Germany
22Physikalisches Institut, Universität Freiburg, Freiburg, Germany
23Institut für Physik, Universität Mainz, Mainz, Germany
24Ludwig-Maximilians-Universität München, München, Germany
25Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
26Panjab University, Chandigarh, India
27Delhi University, Delhi, India
28Tata Institute of Fundamental Research, Mumbai, India
29University College Dublin, Dublin, Ireland
30Korea Detector Laboratory, Korea University, Seoul, Korea
31SungKyunKwan University, Suwon, Korea
32CINVESTAV, Mexico City, Mexico
33FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
34Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
35Joint Institute for Nuclear Research, Dubna, Russia
36Institute for Theoretical and Experimental Physics, Moscow, Russia
37Moscow State University, Moscow, Russia
38Institute for High Energy Physics, Protvino, Russia
39Petersburg Nuclear Physics Institute, St. Petersburg, Russia
40Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden, and
Uppsala University, Uppsala, Sweden
41Physik Institut der Universität Zürich, Zürich, Switzerland
42Lancaster University, Lancaster, United Kingdom
43Imperial College, London, United Kingdom
44University of Manchester, Manchester, United Kingdom
45University of Arizona, Tucson, Arizona 85721, USA
46Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
47California State University, Fresno, California 93740, USA
48University of California, Riverside, California 92521, USA
49Florida State University, Tallahassee, Florida 32306, USA
50Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
51University of Illinois at Chicago, Chicago, Illinois 60607, USA
52Northern Illinois University, DeKalb, Illinois 60115, USA
53Northwestern University, Evanston, Illinois 60208, USA
54Indiana University, Bloomington, Indiana 47405, USA
55University of Notre Dame, Notre Dame, Indiana 46556, USA
56Purdue University Calumet, Hammond, Indiana 46323, USA
57Iowa State University, Ames, Iowa 50011, USA
58University of Kansas, Lawrence, Kansas 66045, USA
59Kansas State University, Manhattan, Kansas 66506, USA
60Louisiana Tech University, Ruston, Louisiana 71272, USA
61University of Maryland, College Park, Maryland 20742, USA
62Boston University, Boston, Massachusetts 02215, USA
63Northeastern University, Boston, Massachusetts 02115, USA
64University of Michigan, Ann Arbor, Michigan 48109, USA
65Michigan State University, East Lansing, Michigan 48824, USA
66University of Mississippi, University, Mississippi 38677, USA
67University of Nebraska, Lincoln, Nebraska 68588, USA
68Princeton University, Princeton, New Jersey 08544, USA
69State University of New York, Buffalo, New York 14260, USA
70Columbia University, New York, New York 10027, USA
71University of Rochester, Rochester, New York 14627, USA
72State University of New York, Stony Brook, New York 11794, USA
73Brookhaven National Laboratory, Upton, New York 11973, USA
74Langston University, Langston, Oklahoma 73050, USA
75University of Oklahoma, Norman, Oklahoma 73019, USA
76Oklahoma State University, Stillwater, Oklahoma 74078, USA
77Brown University, Providence, Rhode Island 02912, USA
78University of Texas, Arlington, Texas 76019, USA
79Southern Methodist University, Dallas, Texas 75275, USA
80Rice University, Houston, Texas 77005, USA
81University of Virginia, Charlottesville, Virginia 22901, USA
82University of Washington, Seattle, Washington 98195, USA
(Dated: April 16, 2007)
We describe a search for the standard model Higgs boson with a mass of 105 GeV/c2 to 145 GeV/c2
in data corresponding to an integrated luminosity of approximately 450 pb−1 collected with the D0
detector at the Fermilab Tevatron pp collider at a center-of-mass energy of 1.96 TeV. The Higgs
boson is required to be produced in association with a Z boson, and the Z boson is required to
decay to either electrons or muons with the Higgs boson decaying to a bb pair. The data are well
described by the expected background, leading to 95% confidence level cross section upper limits
σ(pp → ZH)×B(H → bb) in the range of 3.1 pb to 4.4 pb.
PACS numbers: 13.85.Ni, 13.85.Qk, 13.85Rm
Over the past two decades, increasingly precise exper-
imental results have repeatedly validated the standard
model (SM) and the relationship between gauge invari-
ance and the embedded coupling strengths. For massive
W and Z bosons, gauge invariance of the Lagrangian is
preserved through the Higgs mechanism, but the Higgs
boson (H) has yet to be observed. The current lower
bound on the mass of the Higgs boson from direct ex-
perimental searches is MH = 114.4 GeV/c
2 at the 95%
confidence level [1]. Searches for pp → WH → e(µ)νbb,
pp → WH → WWW ∗, and pp → ZH → ννbb have
been recently reported [2, 3, 4]. The CDF collabora-
tion has reported results in the pp → WH → ℓν and
pp → ZH → ℓ+ℓ−bb (ℓ = e, µ) channels with signifi-
cantly smaller data sets [5, 6, 7]. This Letter provides
the first results from the D0 experiment of searches for
a Higgs boson produced in association with a Z boson,
which then decays either to an electron pair or to a muon
pair. The Higgs is assumed to decay to a bb pair with a
branching fraction given by the SM. The Z(→ ℓ+ℓ−)H
channels reported in this letter comprise major compo-
nents of the search for a Higgs boson at the Tevatron
collider.
Z bosons are reconstructed and identified through
pairs of isolated, electrons or muons with large momen-
tum components transverse to the beam direction (pT )
having invariant mass consistent with that of the Z bo-
son. Events are required to have exactly two jets iden-
tified as arising from b quarks (b jets). The resulting
data are examined for the presence of a (H → bb̄) sig-
nal in the b-tagged dijet mass distribution. An efficient
b-identification algorithm with low misidentification rate
and good dijet mass resolution are essential to enhance
signal relative to the backgrounds. The analysis of the di-
electron [8] (dimuon [9]) channel is based on 450±27 pb−1
(370± 23 pb−1) of data recorded by the D0 experiment
between 2002 and 2004.
The D0 detector [10, 11] has a central-tracking sys-
tem consisting of a silicon microstrip tracker (SMT)
and a central fiber tracker (CFT), both located within
a ≈ 2 T superconducting solenoidal magnet, with de-
signs optimized for tracking and vertexing covering
pseudorapidities |η| < 3 and |η| < 2.5, respectively
(η = − ln[tan(θ/2)], with θ the polar angle relative to
the direction of the proton beam). Central and for-
ward preshower detectors are positioned just outside of
the superconducting coil. A liquid-argon and uranium
calorimeter has a central section (CC) covering pseudo-
rapidities up to |η| ≈ 1.1 and two end calorimeters (EC)
that extend coverage to |η| ≈ 4.2, with all three housed
in separate cryostats [11]. An outer muon system, cover-
ing |η| < 2, consists of a layer of tracking detectors and
scintillation trigger counters in front of 1.8 T toroids,
followed by two similar layers behind the toroids [12].
Luminosity is measured using plastic scintillator arrays
placed in front of the EC cryostats [13].
The primary background to the Higgs signal is the as-
sociated production of a Z boson with jets arising from
gluon radiation, among which Z+bb̄ production is an irre-
ducible background. The other background sources are
tt̄ production, diboson (ZZ and WZ) production, and
events from multijet production that are misidentified
as containing Z bosons. The backgrounds are grouped
into two categories with the first category, called physics
backgrounds, containing events with Z or W bosons aris-
ing from SM processes: inclusive Z + bb̄ production, in-
clusive Z + jj production in which j is a jet without
b flavor, tt̄, ZZ, and WZ events. This background is
estimated from simulation as described below. The sec-
ond category, called instrumental background, contains
those events from multijet production that have two jets
misidentified as isolated electrons or muons which ap-
pear to arise from the Z boson decay. This background
is modeled using control data samples and the procedure
described below.
Physics backgrounds are simulated using the leading
order alpgen [14] and pythia [15] event generators,
with the leading order cteq5l [16] used as parton distri-
bution functions. The decay and fragmentation of heavy
flavor hadrons is done via evtgen [17]. The simulated
events are passed through a detailed D0 detector simula-
tion program based on geant [18] and are reconstructed
using the same software program used to reconstruct the
collider data. The ZH signal, for a range of Higgs masses,
is also simulated using pythia with the same processing
as applied to data. Determination of the instrumental
background and the normalization of the physics back-
grounds are discussed below.
Candidate Z → ee events are selected using a combi-
nation of single-electron triggers. Accepted events must
have two isolated electromagnetic (EM) clusters recon-
structed offline in the calorimeter. Isolation is defined as
I = (E
(0.4)
total − E
(0.2)
(0.2)
in which E
(0.4)
total is the total
calorimeter energy within ∆R < 0.4 of the electron direc-
tion and E
(0.2)
is the energy in the electromagnetic por-
tion of the calorimeter within ∆R < 0.2 of the electron
direction. Candidate electrons must satisfy I < 0.15.
Each EM cluster must have pT > 20 GeV/c and either
|ηdet| < 1.1 or 1.5 < |ηdet| < 2.5, where ηdet is the pseu-
dorapidity measured relative to the center of the detector,
with at least one cluster satisfying |ηdet| < 1.1. In ad-
dition, the lateral and longitudinal shower shape of the
energy cluster must be consistent with that expected of
electrons. At least one of the two EM clusters is also
required to have a reconstructed track matching the po-
sition of the EM cluster energy. Events with a dielectron
mass of 75 < Mee < 105 GeV/c
2 form the Z boson can-
didate sample in the dielectron channel.
Candidate Z → µ+µ− events are selected using a set
of single-muon triggers. Accepted events must have two
isolated muons reconstructed offline. The muons must
have opposite charge, pT > 15 GeV/c, and |η| < 2.0
with muon trajectories matched to tracks in the central
tracking system (i.e., the SMT and the CFT), where the
central track must contain at least one SMT measure-
ment. In addition, the central tracks are required to have
a distance of closest approach to the interaction vertex
in the transverse plane smaller than 0.25 cm. Muon iso-
lation is based on the sum of the energy measured in the
calorimeter around the muon candidate and the sum of
the pT of tracks within ∆R =
(∆φ)2 + (∆η)2 = 0.5 of
the muon candidate normalized by the muon momentum.
The distribution of this variable in background multi-
hadron events is converted to a probability distribution
such that a low probability corresponds to an isolated
muon. The product of the probabilities for both muons
in an event is computed, and the event is retained if the
product is less than 0.02. Accepted Z boson candidates
must have the opening angle of the dimuon system in
the transverse plane (azimuth) of ∆φ > 0.4, and invari-
ant mass 65 GeV/c2 < Mµµ < 115 GeV/c
2. This mass
range differs from that of the dielectrons because of the
difference in resolutions of electron energies and muon
momenta.
After selecting the Z candidate events, we define a
Z+dijet sample which, in addition to satisfying the Z
candidate selection requirements, has at least two jets
in each event. Jets are reconstructed from energy in
calorimeter towers using the Run II cone algorithm with
∆R = 0.5 [19] with towers defined as non-overlapping,
adjacent regions of the calorimeter ∆η ×∆φ = 0.1× 0.1
in size. The transverse momentum of each jet is cor-
rected for multiple pp interactions, calorimeter noise, out-
of-cone showering in the calorimeter, and energy response
of the calorimeter as determined from the transverse mo-
mentum imbalance in photon+jet events [20, 21]. Only
jets that pass standard quality requirements and satisfy
pT > 20 GeV/c and |η| < 2.5 are used in this analy-
sis. The quality requirements are based on the pattern
of energy deposition within a jet and consistency with
the energy deposition measured by the trigger system.
For the Z → ee channel, the normalizations of the
smaller tt, WZ and ZZ backgrounds are computed using
simulated events and next-to-leading-order (NLO) cross
sections. Trigger efficiency, electron identification (ID)
efficiency and resolution correction factors are derived
from comparisons of data control samples and simulated
events. The background contributions from Z+jj, Z+bj
and Z + bb processes are normalized to the observed
Z+dijet data yield reduced by the expected contribu-
tions from the smaller physics and instrumental back-
grounds. The relative fractions of the Z + jj, Z + bj and
Z+bb backgrounds in the Z+dijet sample are determined
from the acceptance and selection efficiencies multiplied
by the ratios of the NLO cross sections for these processes
computed using the mcfm [22] program and the next-to-
leading order cteq6m [23] parton distribution functions.
For the Z → µ+µ− channel, all physics backgrounds are
determined using simulated events with NLO cross sec-
tions applied. Trigger efficiency, muon ID efficiency, and
resolution correction factors are derived from comparison
of data control samples and the simulated events.
Instrumental backgrounds in both channels are deter-
mined by fitting the dilepton invariant mass distributions
to a sum of non-Z and Z boson contributions. The Z
boson lineshape is modeled using a Breit-Wigner distri-
bution convoluted with a Gaussian representing detector
resolution. The non-Z background, consisting of a sum
of events from Drell-Yan production and instrumental
background, is modeled using exponentials. The ratio of
Z boson to non-resonant Drell-Yan production is fixed
by the standard model.
The (two) jets arising from Higgs boson decay should
contain b-flavored quarks (b jets), whereas background
from Z+jets has relatively few events with b jets. To im-
prove the signal-to-background ratio, two of the jets in
the events from the Z+dijet sample are required to ex-
hibit properties consistent with those of jets containing
b quarks. The same b-jet identification algorithm [24] is
used for the dielectron and dimuon samples. It is based
on the finite lifetime of b hadrons giving a low probabil-
ity that these tracks appear to arise from the primary
vertex and considers all central tracks associated with a
jet. A small probability corresponds to jets with tracks
with large impact parameter, as expected in b hadron
decays. The efficiency for tagging a b jet from Higgs de-
cay is approximately 50%, determined as described in the
next paragraph. The probability of misidentifying a jet
arising from a charm quark as a b jet is roughly 20%.
The probability to misidentify a jet arising from a light
quark (u, d, s) or gluon as a b jet is roughly 4%. This
choice of efficiency and purity optimizes the sensitivity
of the analysis. The relatively large per-jet light-flavor
misidentification rate can be accommodated because two
tagged jets are required in each event.
For background yields determined from simulated
events, the probability as a function of jet pT and η that
a jet of a given flavor would be identified (tagged) as a
b jet is applied to each jet in an event. The probability
functions are derived from control data samples. For jets
in the simulated events, the flavor is determined from a
priori knowledge of the parton that gives rise to the jet.
The probability of having two b-tagged jets is defined by
convoluting the per-jet probabilities assuming there are
)2Dijet Mass (GeV/c
0 50 100 150 200 250
Background
Signal x10
-1, 370-450 pbOD
FIG. 1: The dijet invariant mass distribution in double–
tagged Z+dijet events. The Higgs signal corresponds to
MH = 115 GeV/c
2. (The uncertainties are statistical only.)
no jet-to-jet correlations introduced by the b-tag require-
ment. The observed number of Z+2 b-jet events and the
predicted background levels are shown in Table I.
The invariant mass of the two b jets in the Z + 2 b jet
sample is shown in Fig. 1. This distribution is searched
for an excess of events. The peak position in the dijet
mass spectrum is expected to be at a lower value than
the hypothetical Higgs mass because the jet energy is
corrected to reflect the energy of particles in the jet cone
without a general correction for the lower b jet response
compared with light jets. If a muon is within ∆R <
0.5 of the jet, then twice the muon momentum is added
to the jet momentum. This is an approximation to the
energy of both the muon and the accompanying neutrino.
The expected contribution from Higgs boson production
shown in Fig. 1 corresponds to MH = 115 GeV/c
Systematic uncertainties for signal and background
arise from a variety of sources, including uncertainties on
the trigger efficiency, on the corrections for differences
between data and simulation for lepton reconstruction
and identification efficiencies, lepton energy resolution,
jet reconstruction efficiencies and energy determination,
b-identification efficiency, uncertainties from theory and
parton distribution functions for cross sections used for
simulated events and uncertainties on the method used
for instrumental background estimates. The uncertain-
ties from these sources are shown in Table II. These are
evaluated by varying each of the corrections by ±1σ, by
comparing different methods (for the instrumental back-
grounds), and by varying the parton distribution func-
tions among the 20 error sets provided as part of the
cteq6l library. The variations seen for different pro-
cesses for a given uncertainty arise because of differences
among the various background processes and because of
intrinsic differences in the kinematic spectra from differ-
ent Higgs mass hypotheses.
The observed yield is consistent with background pre-
dictions. Upper limits on the ZH production cross sec-
tion are derived at 95% confidence level using the CLs
method [25], a modified frequentist procedure, with a log-
likelihood ratio classifier. The shapes of dijet invariant-
mass spectra of the signal and background are used to
produce likelihoods that the data are consistent with the
background-only hypothesis or with a background plus
signal hypothesis. Systematic uncertainties are folded
into the likelihoods via Gaussian distribution, with corre-
lations maintained throughout. The data yield, predicted
backgrounds and expected and observed limits are shown
in Table III for five hypothetical Higgs masses. The limits
are also shown in Fig. 2.
In summary, we have carried out a search for associated
ZH production in events having two high-pT electrons or
muons and two jets identified as arising from b quarks.
Consistency is found between data and background pre-
dictions. A 95% confidence level upper limit on the Higgs
boson cross section σ(pp → ZH) × B(H → bb) is set
between 4.4 pb and 3.1 pb for Higgs bosons with mass
between 105 GeV/c2 and 145 GeV/c2, respectively.
)2Higgs Mass (GeV/c
100 110 120 130 140 150
210 -1, 370-450 pbOD
95% C.L. upper limit
( --- expected limit)
standard model
FIG. 2: The expected and observed cross–section limits are
shown as a function of Higgs mass. The cross section based
on the SM is shown for comparison.
We thank the staffs at Fermilab and collaborating in-
stitutions, and acknowledge support from the DOE and
NSF (USA); CEA and CNRS/IN2P3 (France); FASI,
Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ,
FAPESP and FUNDUNESP (Brazil); DAE and DST
(India); Colciencias (Colombia); CONACyT (Mexico);
KRF and KOSEF (Korea); CONICET and UBACyT
(Argentina); FOM (The Netherlands); PPARC (United
Kingdom); MSMT and GACR (Czech Republic); CRC
Program, CFI, NSERC and WestGrid Project (Canada);
BMBF and DFG (Germany); SFI (Ireland); The Swedish
Research Council (Sweden); CAS and CNSF (China);
Alexander von Humboldt Foundation; and the Marie
TABLE I: Number of observed and expected background events.
Z+ ≥ 2 jets 2 b tags
Final state Z → ee Z → µ+µ− Z → ℓ+ℓ− Z → ee Z → µ+µ− Z → ℓ+ℓ−
Zbb 9.1 8.3 17.4 2.0 1.3 3.3
Zjj 414 437 851 1.2 2.6 3.8
tt̄ 2.7 9.6 12.3 0.80 3.1 3.9
ZZ +WZ 9.2 21.4 30.6 0.32 0.42 0.74
Instrumental 28.0 16.1 44.1 0.18 0.41 0.59
Total background 463 493 956 4.5 7.8 12.3
Observed events 463 545 1008 5 10 15
TABLE II: Systematic uncertainty in background and signal
predictions given as the fractional uncertainty on the event
totals. The ranges correspond to variations introduced by
different processes (background), the dijet mass window re-
quirement (background and signal) and intrinsic differences in
kinematics arising from different hypothesized Higgs masses
(signal).
Source Background Signal
Lepton ID Efficiencies 11% – 16% 11% – 12%
Lepton Resolution 2% 2%
Jet ID Efficiency 5% – 11% 8%
Jet Energy Reconstruction 10% 7%
b–jet ID Efficiency 10% – 12% 9%
Cross Sections 6% – 19% 7%
Trigger Efficiency 1% 1%
Instrumental Background 2% (ee)
12% (µµ)
Curie Program.
[*] Visitor from Augustana College, Sioux Falls, SD, USA.
[¶] Visitor from The University of Liverpool, Liverpool, UK.
[§] Visitor from ICN-UNAM, Mexico City, Mexico.
[‡] Visitor from Helsinki Institute of Physics, Helsinki, Fin-
land.
[#] Visitor from Universität Zürich, Zürich, Switzerland.
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TABLE III: Numbers of predicted background and signal events and the observed yield after all selection requirements, including
the addition of a dijet mass window. The mass window is centered on the mean of the reconstructed Higgs mass in simulated
ZH events and has a width of ±1.5σ in which σ is the result of a gaussian fit to the reconstructed dijet mass distribution. The
upper bounds differ slightly between the Z → ee and Z → µ+µ− events because of different resolutions. The window is applied
for illustration, showing the yields in the region of highest predicted signal-to-background ratio. Also shown are the expected
and observed upper limits on the cross section for the combined analysis at 95% confidence level computed as described in the
text (without the mass window, but weighted by the bin-to-bin signal-to-background ratio).
MH = 105 GeV/c
MH = 115 GeV/c
MH = 125 GeV/c
ee µµ ee µµ ee µµ
Mass window(GeV/c2) [65, 113] [65, 118] [72, 125] [70, 128] [75, 136] [78, 137]
Predicted signal 0.07 0.06 0.05 0.05 0.04 0.03
Background 1.4 3.1 1.3 3.1 1.4 2.8
Data 2 3 1 3 1 4
Expected σ95 4.2 pb 4.1 pb 3.4 pb
Observed σ95 4.4 pb 4.0 pb 3.3 pb
MH = 135 GeV/c
MH = 145 GeV/c
ee µµ ee µµ
Mass window(GeV/c2) [82, 143] [84, 147] [87, 156] [92, 160]
Predicted signal 0.027 0.022 0.015 0.01
Background 1.6 2.9 1.6 2.8
Data 1 5 0 6
Expected σ95 2.8 pb 2.6 pb
Observed σ95 3.1 pb 3.4 pb
	References
