Search for chargino-neutralino production in p-pbar collisions at 1.96 TeV with high pT leptons

0.66 MB·English

by  CDF Collaboration

Search for chargino-neutralino production in pp¯ collisions at √s = 1.96 TeV
with high-p leptons
T
T. Aaltonen,23 J. Adelman,13 T. Akimoto,54 M.G. Albrow,17 B. A´lvarez Gonz´alez,11 S. Amerio,42
D. Amidei,34 A. Anastassov,51 A. Annovi,19 J. Antos,14 M. Aoki,24 G. Apollinari,17 A. Apresyan,47
T. Arisawa,56 A. Artikov,15 W. Ashmanskas,17 A. Attal,3 A. Aurisano,52 F. Azfar,41 P. Azzi-Bacchetta,42
8 P. Azzurri,45 N. Bacchetta,42 W. Badgett,17 A. Barbaro-Galtieri,28 V.E. Barnes,47 B.A. Barnett,25
0 S. Baroiant,7 V. Bartsch,30 G. Bauer,32 P.-H. Beauchemin,33 F. Bedeschi,45 P. Bednar,14 S. Behari,25
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5 K. Burkett,17 G. Busetto,42 P. Bussey,21 A. Buzatu,33 K. L. Byrum,2 S. Cabrerar,16 M. Campanelli,35
1 M. Campbell,34 F. Canelli,17 A. Canepa,44 D. Carlsmith,58 R. Carosi,45 S. Carrillol,18 S. Carron,33
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6 R. Erbacher,7 D. Errede,24 S. Errede,24 R. Eusebi,17 H.C. Fang,28 S. Farrington,29 W.T. Fedorko,13
1 R.G. Feild,59 M. Feindt,26 J.P. Fernandez,31 C. Ferrazza,45 R. Field,18 G. Flanagan,47 R. Forrest,7
3 S. Forrester,7 M. Franklin,22 J.C. Freeman,28 I. Furic,18 M. Gallinaro,49 J. Galyardt,12 F. Garberson,10
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B. Heinemann,28 J. Heinrich,44 C. Henderson,32 M. Herndon,58 J. Heuser,26 S. Hewamanage,4 D. Hidas,16
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B. Iyutin,32 E. James,17 B. Jayatilaka,16 D. Jeans,50 E.J. Jeon,27 S. Jindariani,18 W. Johnson,7 M. Jones,47
K.K. Joo,27 S.Y. Jun,12 J.E. Jung,27 T.R. Junk,24 T. Kamon,52 D. Kar,18 P.E. Karchin,57 Y. Kato,40
R. Kephart,17 U. Kerzel,26 V. Khotilovich,52 B. Kilminster,38 D.H. Kim,27 H.S. Kim,27 J.E. Kim,27
M.J. Kim,17 S.B. Kim,27 S.H. Kim,54 Y.K. Kim,13 N. Kimura,54 L. Kirsch,6 S. Klimenko,18 M. Klute,32
B. Knuteson,32 B.R. Ko,16 S.A. Koay,10 K. Kondo,56 D.J. Kong,27 J. Konigsberg,18 A. Korytov,18
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S. Levy,13 J.D. Lewis,17 C. Lin,59 C.S. Lin,28 J. Linacre,41 M. Lindgren,17 E. Lipeles,9 A. Lister,7
D.O. Litvintsev,17 T. Liu,17 N.S. Lockyer,44 A. Loginov,59 M. Loreti,42 L. Lovas,14 R.-S. Lu,1 D. Lucchesi,42
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A. Messina,35 T. Miao,17 N. Miladinovic,6 J. Miles,32 R. Miller,35 C. Mills,22 M. Milnik,26 A. Mitra,1
G. Mitselmakher,18 H. Miyake,54 S. Moed,22 N. Moggi,5 C.S. Moon,27 R. Moore,17 M. Morello,45
P. Movilla Fernandez,28 J. Mu¨lmensta¨dt,28 A. Mukherjee,17 Th. Muller,26 R. Mumford,25 P. Murat,17
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K. Osterberg,23 S. PaganGriso,42 C. Pagliarone,45 E. Palencia,17 V. Papadimitriou,17 A. Papaikonomou,26
A.A. Paramonov,13 B. Parks,38 S. Pashapour,33 J. Patrick,17 G. Pauletta,53 M. Paulini,12 C. Paus,32
D.E. Pellett,7 A. Penzo,53 T.J. Phillips,16 G. Piacentino,45 J. Piedra,43 L. Pinera,18 K. Pitts,24 C. Plager,8
L. Pondrom,58 X. Portell,3 O. Poukhov,15 N. Pounder,41 F. Prakoshyn,15 A. Pronko,17 J. Proudfoot,2
F. Ptohosh,17 G. Punzi,45 J. Pursley,58 J. Rademackerc,41 A. Rahaman,46 V. Ramakrishnan,58 N. Ranjan,47
I. Redondo,31 B. Reisert,17 V. Rekovic,36 P. Renton,41 M. Rescigno,50 S. Richter,26 F. Rimondi,5
L. Ristori,45 A. Robson,21 T. Rodrigo,11 E. Rogers,24 S. Rolli,55 R. Roser,17 M. Rossi,53 R. Rossin,10
P. Roy,33 A. Ruiz,11 J. Russ,12 V. Rusu,17 H. Saarikko,23 A. Safonov,52 W.K. Sakumoto,48 G. Salamanna,50
O. Salt´o,3 L. Santi,53 S. Sarkar,50 L. Sartori,45 K. Sato,17 A. Savoy-Navarro,43 T. Scheidle,26 P. Schlabach,17
E.E. Schmidt,17 M.A. Schmidt,13 M.P. Schmidt,59 M. Schmitt,37 T. Schwarz,7 L. Scodellaro,11 A.L. Scott,10
A. Scribano,45 F. Scuri,45 A. Sedov,47 S. Seidel,36 Y. Seiya,40 A. Semenov,15 L. Sexton-Kennedy,17
A. Sfyria,20 S.Z. Shalhout,57 M.D. Shapiro,28 T. Shears,29 P.F. Shepard,46 D. Sherman,22 M. Shimojiman,54
M. Shochet,13 Y. Shon,58 I. Shreyber,20 A. Sidoti,45 P. Sinervo,33 A. Sisakyan,15 A.J. Slaughter,17
J. Slaunwhite,38 K. Sliwa,55 J.R. Smith,7 F.D. Snider,17 R. Snihur,33 M. Soderberg,34 A. Soha,7
S. Somalwar,51 V. Sorin,35 J. Spalding,17 F. Spinella,45 T. Spreitzer,33 P. Squillacioti,45 M. Stanitzki,59
R. St. Denis,21 B. Stelzer,8 O. Stelzer-Chilton,41 D. Stentz,37 J. Strologas,36 D. Stuart,10 J.S. Suh,27
A. Sukhanov,18 H. Sun,55 I. Suslov,15 T. Suzuki,54 A. Taffarde,24 R. Takashima,39 Y. Takeuchi,54
R. Tanaka,39 M. Tecchio,34 P.K. Teng,1 K. Terashi,49 J. Thomg,17 A.S. Thompson,21 G.A. Thompson,24
E.Thomson,44 P.Tipton,59 V.Tiwari,12 S.Tkaczyk,17 D. Toback,52 S. Tokar,14 K.Tollefson,35 T.Tomura,54
D. Tonelli,17 S. Torre,19 D. Torretta,17 S. Tourneur,43 W. Trischuk,33 Y. Tu,44 N. Turini,45 F. Ukegawa,54
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W. Wagner,26 T. Wakisaka,40 R. Wallny,8 S.M. Wang,1 A. Warburton,33 D. Waters,30 M. Weinberger,52
W.C. Wester III,17 B. Whitehouse,55 D. Whitesone,44 A.B. Wicklund,2 E. Wicklund,17 G. Williams,33
H.H. Williams,44 P. Wilson,17 B.L. Winer,38 P. Wittichg,17 S. Wolbers,17 C. Wolfe,13 T. Wright,34 X. Wu,20
S.M. Wynne,29 A. Yagil,9 K. Yamamoto,40 J. Yamaoka,51 T. Yamashita,39 C. Yang,59 U.K. Yangm,13
Y.C. Yang,27 W.M. Yao,28 G.P. Yeh,17 J. Yoh,17 K. Yorita,13 T. Yoshida,40 G.B. Yu,48 I. Yu,27
S.S. Yu,17 J.C. Yun,17 L. Zanello,50 A. Zanetti,53 I. Zaw,22 X. Zhang,24 Y. Zhengb,8 and S. Zucchelli5
(CDF Collaboration∗)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254
7University of California, Davis, Davis, California 95616
8University of California, Los Angeles, Los Angeles, California 90024
9University of California, San Diego, La Jolla, California 92093
10University of California, Santa Barbara, Santa Barbara, California 93106
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, PA 15213
3
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510
18University of Florida, Gainesville, Florida 32611
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138
23Division of High Energy Physics, Department of Physics,
University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801
25The Johns Hopkins University, Baltimore, Maryland 21218
26Institut fu¨r Experimentelle Kernphysik, Universit¨at Karlsruhe, 76128 Karlsruhe, Germany
27Center for High Energy Physics: Kyungpook National University,
Daegu 702-701, Korea; Seoul National University, Seoul 151-742,
Korea; Sungkyunkwan University, Suwon 440-746,
Korea; Korea Institute of Science and Technology Information, Daejeon,
305-806, Korea; Chonnam National University, Gwangju, 500-757, Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
33Institute of Particle Physics: McGill University, Montr´eal,
Canada H3A 2T8; and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109
35Michigan State University, East Lansing, Michigan 48824
36University of New Mexico, Albuquerque, New Mexico 87131
37Northwestern University, Evanston, Illinois 60208
38The Ohio State University, Columbus, Ohio 43210
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42University of Padova, Istituto Nazionale di Fisica Nucleare,
Sezione di Padova-Trento, I-35131 Padova, Italy
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104
45Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa,
Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
46University of Pittsburgh, Pittsburgh, Pennsylvania 15260
47Purdue University, West Lafayette, Indiana 47907
48University of Rochester, Rochester, New York 14627
49The Rockefeller University, New York, New York 10021
50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1,
University of Rome “La Sapienza,” I-00185 Roma, Italy
51Rutgers University, Piscataway, New Jersey 08855
52Texas A&M University, College Station, Texas 77843
53Istituto Nazionale di Fisica Nucleare, University of Trieste/ Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201
58University of Wisconsin, Madison, Wisconsin 53706
59Yale University, New Haven, Connecticut 06520
(Dated: February 2, 2008)
We present a search for the associated production of charginos and neutralinos in pp¯ collisions
at √s = 1.96 TeV. The data were collected at the Collider Detector at Fermilab (CDF II) and
4
correspond to integrated luminosities between 0.7 and 1.0 fb−1. We look for final states with one
high-p electron ormuon,andtwoadditionalleptons. Ourresultsareconsistentwith thestandard
T
model expectations, and we set limits on the cross section as a function of the chargino mass in
threedifferent supersymmetric scenarios. For a specific MSSM scenario with no slepton mixing we
set a 95% C.L. limit at 151 GeV/c2.
PACSnumbers:
I. INTRODUCTION set by the LEP experiments at 103.5 GeV/c2 for
thelightestchargino,inscenarioswithlargesfermion
masses [5], and at 50.3 GeV/c2 for the lightest neu-
Supersymmetry [1, 2] (SUSY) is a proposed sym-
tralino in mSUGRA. These constraints are very ro-
metry of nature. It predicts the existence of su-
bustwithin mSUGRA-inspiredSUSYmodelsanddo
persymmetric partners for the standard model (SM)
notdependonthecharginodecaymodes,exceptfora
particles, called gauginos (higgsinos) for the gauge
few pathological cases [6]. The DO collaboration ex-
(Higgs) bosons, and squarks/sleptons for fermions. cludesthecharginomassbelow117GeV/c2 inaspe-
The lightest SUSY particle is referredto as the LSP.
cific SUSY breaking scenario described in [7], where
If SUSY is an exact symmetry, the supersymmetric
the standard mixing between the left and the right
andtheSMparticleshavethesamemass,relatedcou-
components in the third generation families is sup-
plings, and spin differing by 1/2. As a consequence
pressed.
of the non-observation of light SUSY particles, such
In this article we present a search for the associ-
as the selectron, SUSY must be a broken symme- ±
ated production of the lightest chargino χ˜ and the
1
try, if realized. Several symmetry breaking models second-lightest neutralino χ˜0 (shown in Fig. 1), per-
2
have been discussed in the past years. The gravi-
formedasacountingexperimentindatacollectedby
tational interactions are responsible for the symme-
the CDF detector. Charginos and neutralinos can
try breaking in the mSUGRA [3] scenario, whereas
the ordinary gauge interactions are the source of
SUSY breaking in the GMSB [2] model. In bro-
ken SUSY, gauginos and higgsinos combine to form
±
masseigenstatescalledcharginos(χ˜ )andneutrali-
1,2
nos(χ˜0 ). The lightestneutralino,χ˜0,canbe the
1,2,3,4 1
LSP. SUSY is one of the most promising theories of
physics beyond the SM as it can accommodate grav-
ity and unify the gauge interactions. In SUSY mod-
els where R-parity [4] is conserved,the LSP is stable FIG. 1: Leading-order Feynman diagrams for chargino
andonlyweaklyinteracting,andthusisaviabledark and neutralino associated production. The interaction is
matter candidate. mediatedthroughvirtualW(left)andsquark(q˜,right).
Experimental bounds on the gaugino masses are
be among the lightest SUSY particles in the mod-
els we explore, with associated production cross sec-
∗With visitors from aUniversity of Athens, 15784 Athens, tions within the reach of the Tevatron collider [8]. If
Greece,bChineseAcademyofSciences,Beijing100864,China, these sparticles decay leptonically within the detec-
cUniversity of Bristol, Bristol BS8 1TL, United Kingdom,
tor, the final state is characterized by the presence
dUniversity Libre de Bruxelles, B-1050 Brussels, Belgium,
eUniversityofCaliforniaIrvine,Irvine,CA92697,fUniversity ofleptons andsignificantmissingenergy[9], E/T, due
of California Santa Cruz, Santa Cruz, CA 95064, gCornell to particles escaping detection. While the process
University, Ithaca, NY 14853, hUniversity of Cyprus, Nicosia χ˜±1χ˜01 ℓνχ˜01χ˜01 results in a final state with ℓ+E/T,
CY-1678, Cyprus, iUniversity College Dublin, Dublin 4, Ire- which→has a large inclusive W background, the dis-
land, jUniversity of Edinburgh, Edinburgh EH9 3JZ, United tinct signature of χ˜±χ˜0 ℓℓℓνχ˜0 makes the search
Kingdom,kUniversityofHeidelberg,D-69120Heidelberg,Ger- 1 2 → 1 ±
many, lUniversidad Iberoamericana, Mexico D.F., Mexico, fortheassociatedproductionofcharginoχ˜1 andneu-
mUniversity of Manchester, Manchester M13 9PL, England, tralinoχ˜0 (seeFig.2),oneofthemostpowerfultests
2
nNagasaki Institute of Applied Science, Nagasaki, Japan, of SUSY at hadron colliders.
oUniversitydeOviedo,E-33007Oviedo,Spain,pQueenMary,
The paper is organized as follows. Section II con-
UniversityofLondon,London,E14NS,England,qTexasTech
University, Lubbock, TX 79409, rIFIC(CSIC-Universitat de tains a brief description of the CDF detector. Sec-
Valencia),46071Valencia,Spain, tion III presents the lepton identification procedure
5
pseudorapidity range η < 2.0. For the leptons in
| |
oursearch,theresolutionontheimpactparameteris
40 µm, including 30 µm for the beam size. Sur-
≈ ≈
rounding the tracking system is a solenoidal magnet
which provides a 1.4 T field aligned parallel to the
beam. Fromthecurvatureofatrackinthe magnetic
field, we determine the transverse momentum pT of
charged particles. The momentum resolution of the
outer tracking is σ(pT)/p2T =0.0017 c/GeV.
Twolayersofsamplingcalorimeters,onefordetect-
ing electromagnetic particles and the other to mea-
sure the remaining hadronic energy, cover the range
η < 3.6. The central electromagnetic calorimeter
FIG.2: Charginoandneutralinodecaymodes. Theℓ˜and (|C|EM) surrounds the solenoid within η < 1.1. It
| |
the ν˜ are the SUSY counterparts of the lepton and the consistsof leadsheets separatedby polystyrenescin-
neutrino. tillator with an energy resolution of 13.5%/√ET
⊕
1.5%, where ET = E sinθ is measured in GeV.
| |
TheCEMissegmentedinto 24wedgesperside; each
wedge spans an angle of approximately15 degrees in
and measurement of misidentification rate. In Sec-
φandisdividedintotentowersof∆η =0.11. Atnor-
tionIV we discuss the backgroundsandin SectionV
mal incidence the total depth corresponds to about
we describe the event selection. In Section VI we
present the estimated systematic uncertainties, fol- 18 radiation lengths (X0). A proportional chamber
(CES)isembeddedineachCEMwedgeattheshower
lowed by the validation of the analysis procedure in
maximum and provides good spatial resolution and
SectionVII. Thesearchespresentedinthispaperare
shower shape information used for electron and pho-
not targeted at a specific model, but rather are de-
ton identification. The central hadronic calorimeter
signed to cover a large range of possible new physics
(CHA), positioned outside the CEM, matches the
scenarios in which events with three leptons and sig-
CEMsegmentationinto 24 wedges but uses steel ab-
nificant missing transverse energy are predicted at
sorbers interspersed with acrylic scintillators. There
rates larger than the SM predictions. Nevertheless,
are 23 layers in the CHA and each layer is com-
based on the results presented in Section VIII, we
posed of one inch of steel and one centimeter of
set limits as a function of the chargino mass in sev-
scintillator. The end wall calorimeter and the end
eral SUSY scenarios (Section IX). The results of the
plug calorimeter complete the coverage in the re-
analysis presented in this paper are combined with
gions 0.8 < η < 1.2 and 1.1 < η < 3.6, re-
results of similar searches carriedout at CDF to fur- | | | |
spectively. The plug calorimeter consists of a lead-
ther improve the sensitivity [10].
scintillator electromagnetic section (PEM) and an
iron-scintillator hadronic section (PHA). The PEM
resolution is 16%/√ET 1%. The PEM also con-
II. EXPERIMENTAL APPARATUS ⊕
tains a shower maximum detector (PES).
Themuonsystemisinstalledoutsidethecalorime-
The CDF II detector [11] is a general-purpose ters. The innermost four-layer drift chamber system
detector with approximate azimuthal and forward- (CMU) can detect minimum ionizing particles with
backward symmetry. CDF combines precision transverse momenta larger than 1.4 GeV/c. An ad-
charged-particletrackingwith projective calorimeter ditional four-layer drift chamber (CMP) is located
towers and muon detection. In the detector coor- outside the magnet return yoke and detects parti-
dinate system, φ is the azimuthal angle around the cles with pT > 2.0 GeV/c. The CMU-CMP cover-
beam axis and η is the pseudorapidity defined as age (η < 0.6) is extended up to η < 1.0 by the
η = lntan(θ/2), where θ is the polar angle from centra|l|muon extension chambers |(C|MX). Outside
−
the beam axis. The radial distance to the beam axis the CMP and CMX chambers are scintillator detec-
is referred to as r. tors providing additional timing measurements. The
The tracking system is composed of an inner sili- last set of muon detectors (IMU) covers the region
con detector (1.5 < r < 29.0 cm) and an outer drift 1.0 < η < 1.5. The information from the IMU
chamber (COT, 40 < r < 140 cm). These detec- chambe|rs|is not used in this analysis.
tors provide three-dimensional vertex measurement Theluminosityismeasuredfromthetotalinelastic
and track reconstruction efficiency above 90% in the
6
pp¯ cross-sectionusing Cherenkovcounters locatedin theshowerprofileintheCES,andtotestbeamdata.
the 3.7< η <4.7 region. Asimilarprocedurebasedontheχ2 fromcomparing
| |
The CDF trigger has a three-level architecture. the tower energy distribution is applied to electrons
The first level (L1) is a custom-designed hardware reconstructed in the plug calorimeter. In this case
trigger which makes a fast trigger decision based on only tracks within η < 2 and with silicon hits are
| |
preliminary informationfrom the tracking,calorime- accepted. The collimationoftheshowershapeinthe
ter, and muon systems with an average accept rate PES is also restricted, by requiring that the energy
of 25 kHz. The second level (L2) uses both custom in the middle five strips of a PES cluster should be
hardware and a software-based event reconstruction morethan65%ofthe energyinallnine strips. If the
with an accept rate of 750 Hz. The third level (L3) track associated to the candidate electron is consis-
uses the offline reconstruction software and selects tent with coming from a γ e+e− conversion, the
→
events for storage with a rate of up to 85 Hz [12]. candidateelectronisrejected. Thephotonconversion
identification algorithm defines an electron as origi-
nating from a conversionif the azimuthal separation
ofthe electroncandidateandanyoppositely-charged
III. LEPTON IDENTIFICATION
track at the tangency point (D = R ∆φ where
xy
×
R is the conversion radius) is less than 0.2 cm and
A. Lepton identification probability
the difference in polarangle (∆cotθ) is smaller than
0.04. The measurement of the conversion identifica-
We use different constraints on identification vari- tion efficiency is described in Section IVB1.
ables for high-pT (pT > 20 GeV/c) and low-pT (pT Trackswith small energy deposits in the calorime-
< 20 GeV/c) leptons due to different detection char- ters and matched stubs [14] in the CMU and CMP
acteristics and also due to the trigger requirements. (or CMX only) muon chambers are candidates for
These identification criteria are described below and the CMUP (CMX) muon category. The matching
are summarized in Tables I through III. between the extrapolated track and the stub in the
Reconstructedcentraltracksmusthaveatleastfive chamber(∆x, wherexis the locallinearcoordinate
hits out of 12 possible in at least three (two) out of in the tra|nsve|rse plane) has to be within a certain
four axial (stereo) COT super layers, to ensure high range (refer to Table III). If a track has pT less
reconstruction efficiency and purity. We accept only than 20 GeV/c, the effect of multiple scattering is
tracks originating within 60 cm from the center of enhanced and thus we set a less stringent require-
±
the detector, and we apply a cut on the impact pa- ment. For CMX muons we restrict our selection to
rameter(d0,seeTableI)tosuppresscosmicraysand tracks that pass through all eight super layers of the
secondary vertices. The impact parameter is the ra- COT.Theefficiencyoffindingastubinthefirstplace
dial distance of closest approach between the track is measured separately and combined with the other
and the beam line. For each beam-constrained COT identification measurements.
track, we place a requirement on the fit quality χ2 Other muons in an event are also included if they
normalized by the number of degrees of freedom in fall in the muon category called “central minimum
the track fitting. The efficiency of reconstructing a ionizing objects” (CMIO’s). This category is com-
track is measuredseparatelyin calorimetertriggered posed of tracks with pT greater than 10 GeV/c for
W eν events as described in [13]. which the track does not extrapolate to the fidu-
→
Acandidateelectroninthecentralregionisatrack cial region of the CMU and CMP or CMX cham-
pointing to an electromagnetic calorimeter cluster. bers [15]. In this case we constrain the selection to
If the ratio of the energy measured in the hadronic muon candidate tracks with a non-zero calorimeter
calorimeter to that measured in the electromagnetic energy deposit to suppress tracks entering uninstru-
calorimeter is small, we define it as a “loose” elec- mented parts. This extends the muon coverage to
tron. Additional requirements on the shower shape η < 1.5, with lower efficiency and lower purity for
andtheenergytomomentumratioareimposedtose- |η|>1.2.
lect high purity, “tight” electrons. One such require- | |Since leptons from χ˜± and χ˜0 decays are expected
1 2
ment, the lateral shower sharing profile (Lshr) com- to be well separated from each other and from other
pares the energy sharing between neighboring CEM objects in the event, [16], we restrict our studies to
towers to the expectation from test beam data. We isolated electrons and muons. To decide whether a
also restrict the matching between the shower and lepton is isolated or not we sum up the calorime-
the track, both the distance in the r-φ plane (Q∆x, ter transverse energy (Econe) in a cone of ∆R =
· T
where Q is the lepton charge), and in the r-z plane p∆η2+∆φ2 < 0.4 around, but not including, the
(∆z ). Inadditionwealsorestricttheχ2 ofthefitto
| |
7
energy deposited by the lepton. We require Econe to
T TABLE III: Muon selection criteria
be less than 2 GeV for the loose CMUP and CMX
muons. TheEcone isrequiredtobesmallerthan10% Variable Cut
T
of the muon pT (electron ET) for other lepton cate- Base muon:
gories, or if the muon pT (electron ET) is above 20 Central track
GeV/c (GeV).
E < 2 GeV
EM
E (p 20 GeV/c) < 6 GeV
Had T
≥
TABLEI: Requirementsfor central tracks EHad (pT < 20 GeV/c) < 3.5 + pT/8 GeV
Tight CMUP/CMX muon:
Variable Cut
Base muon
no. axial COT super layers 3 with 5 hits
≥ ≥ Econe/p < 0.1
no. stereo COT super layers 2 with 5 hits T T
z0 ≥ <≥60 cm |∆xCMU| (CMUP) < 7 cm or χ2 < 9
|d0| (no silicon hits) < 0.2 cm |∆xCMP| (CMUP) < 5 cm or χ2 < 9
|d0|(silicon hits) < 0.02 cm |∆xCMX| (CMX) < 6 cm or χ2 < 9
| |
Loose CMUP/CMX muon:
Muon tracks:
As muon1except:
COT exit radius (CMX) > 140 cm
χ2 (first 350 pb−1) < 2.8 ETcone (pT < 20 GeV/c) < 2 GeV
χ2 (otherwise) < 2.3 CMIO muon:
Base muon
Not fiducial toCMUP, CMX
p 10 GeV/c
T
≥
Econe/p < 0.1
TABLE II:Electron selection criteria T T
E + E > 0.1 GeV
EM Had
Variable Cut
Loose electron:
Central track
Not conversion > 20 GeV/c (GeV) is the single-electron or single-
η < 1 muon triggered sample with a pT (ET) threshold of
| |
E /E < 0.055+0.00045 E GeV 18 GeV/c (GeV) used for the analysis itself, as de-
Had EM EM
·
Econe/E < 0.1 scribed in Section IV. Samples of simulated events
T T
neededinthisstudyarealsopresentedinSectionIV.
Tight electron:
Additional data samples are used to measure effi-
As electron1 except
ciencies and misidentification probabilities for lower-
E/p (p < 50 GeV/c) < 2
T energyleptons. Theseinclude samplescollectedwith
Lshr < 0.2 single-lepton trigger thresholds of 8 GeV/c for muon
χ2strips < 10 pT and 8 GeV for electron ET, and inclusive central
Q ∆x > -3.0, < 1.5 cm jet samples collected with jet trigger thresholds at
·
∆z < 3.0 cm ET > 20, 50, 70, and 100 GeV.
| |
Plug electron: For high-pT leptons, we measure the identifica-
Track with Silicon hits tionefficienciesusingsame-flavor,oppositely-charged
dilepton candidate events in the invariant mass win-
Not conversion
dowfrom76to106GeV/c2. Werequirethatatleast
η > 1.2 < 2.0
| | one of those candidate leptons fulfills all the tight
E /E < 0.055 GeV
Had EM electronortightCMUP/CMXcriteria,definedinTa-
Econe/E < 0.1
T T bles II and III respectively,andsatisfying the trigger
E/p < 3 requirements. We then measure the efficiencies of
PES 5/9 (see text) > 0.65 ouridentificationcriteriaonthe othercandidate lep-
χ2 < 10 ton. In the case of Z e+e− candidates we sub-
PEM →
tract background using same-charge dilepton events
in the mass window. The effect of background sub-
The primary data sample used to measure trigger traction is found to be negligible in the Z µ+µ−
→
andidentificationefficienciesforleptonswithpT(ET) sample [17].
8
The efficiency of low-pT leptons is measured in
Drell-Yan candidate events requiring same-flavor, 100
oppositely-chargedleptonswith∆φ(ℓ1ℓ2)>160◦. In
order to reject events in which a cosmic ray is recon-
%] 80
structed as a pair of muons, we require the timing of
the trackhits inthe trackingsystemtobe consistent cy [
n
with particles originating from the center of the de- e
ci 60
tector and moving outwards, and reject events with Effi
significant E/T. At least one lepton candidate must n
pass all the identification criteria (to reduce instru- o
mental and non-prompt background) and must sat- cati 40
isfythe8GeV/ctriggerrequirements. Wethenmea- ntifi CEM (Observed Events)
sure the efficiency of the identification variables on e 20
d
thesecondleptoncandidateintheevent. Ineventsin I CEM (Simulated events)
whichbothleptons passthe triggerrequirements,we
use both to determine the efficiency. The remaining 0
10 20 30 40 50 60 70 80
background to be subtracted is estimated in events
Electron E (GeV)
with lepton candidates of the same electric charge. T
Aspartofthecrosschecksandsystematicuncertain-
ties evaluation, we also verify the results using J/ψ FIG. 3: The identification efficiency of tight electrons as
a function of E using theDrell-Yan selection.
and Υ candidate events and sideband subtraction, T
exceptforthe isolationcut,asonly the Drell-Yanse-
lection gives well-isolated,prompt leptons with good 100
statistics in the full pT range. The resulting total
identification efficiency ranges between 75% for for- %] 80
ward electrons and 80% for central electrons to 90% y [
for most muon categories. c
n
In both observed events and Monte Carlo (MC) e
ci 60
simulated events, we check for possible dependence Effi
of the efficiency of identifying leptons on additional n
factors: thenumberofprimaryvertices,thegeometry atio 40
of the detector, and changes in the detector perfor- c
mance and/or configuration over time. We include ntifi CMUP (Observed events)
deviations as part of the uncertainty on lepton iden- de 20
I CMUP (Simulated events)
tification efficiency measurements. Fig. 3 and Fig. 4
show examples of ET (pT) dependence in observed
and simulated events. The dependence is mainly 0
0 10 20 30 40 50 60
causedbyphotonsradiatedbytheleptons;duetothe
Muon p (GeV)
pT spectrum of Drell-Yan events this effect is most T
visible in the 20-30 GeV range with our selection.
FIG.4: Theidentificationefficiency(includingstubfind-
Thepresenceofextraphotonsmeanstheisolationre-
ingefficiency) of CMUPmuonsas afunction of p using
quirement(andforthe muons,alsotheEEM require- theDrell-Yan selection. T
ment) is notfully efficientin thatpT-rangefor Drell-
Yan events. This effect is adequately described by
the Drell-Yan simulation. For very high pT electrons
the efficiency measured in observed events is lower
thantheonemeasuredinDrell-Yansimulatedevents low-pT end are caused by non-isolated, non-prompt
due to the E/p cut becoming inefficient. For CMUP background. Because the MC does not completely
muonstheefficiencymeasuredinobservedeventsand reproduce the identification efficiency found in the
insimulatedeventsshowsthesamedependency with observedevents,wedefineascalefactor(SID)asthe
respect to the muon transverse momentum. The ef- ratio of the identification efficiency measured in the
ficiency measured in observed events is lower than observed events to the identification efficiency found
the one measured in simulated ones because of mis- in the simulated samples. Typical scale factors ap-
modellingofmultiplescattering. Discrepanciesatthe plied to the MC predictions lie between 0.9 and 1.0
and are pT and ET dependent.
9
B. Probability of hadrons to be misidentified as
leptons
Jet: E>20 GeV
0.16 T
Ajetofhadronsisdefinedasaclusterofenergyin
Jet: E>50 GeV
the calorimeter and reconstructed using a fixed cone %]0.14 T
algorithm (∆R = 0.4). A jet can be misidentified as ate [0.12 Jet: ET>70 GeV
an electron if it consists of an energetic track point- e r
ing to a large energy deposit in the electromagnetic Fak 0.1 Jet: ET>100 GeV
cshaloowreimreintetrh.eChahdarrognedickcaaloonrsimaentder,poiorntshowsieththaatldatee- ctron 0.08
e
cay in flight, can also mimic muons. El0.06
e
We use reconstructed jets to estimate the proba- os
o0.04
bility to misidentify them as electrons. In the study L
of misidentified muons, we use tracks with ETcone < 0.02
4 GeV (called “isolatedtracks”). The isolation is re-
quiredto reduce the dependence onthe sample com-
0 5 10 15 20 25 30
position. In the following we will refer to these jets Ejet (GeV)
andtracksas“fakeableobjects”. Sincesuchfakeable T
objects originate from hadrons, we use the four data
FIG.5: Fakerateofloosecentralelectrons. Thesolidline
samples collected with jet-based triggers to measure
representsthelinearfittotheaverageandthedottedlines
their misidentification probability. We expect only a
are 50%.
negligible contribution from inclusive W and Z pro- ±
duction with the gauge bosons decaying into leptons
anddonotapplyanycorrections. Toavoidatrigger-
induced bias, we remove the highest ET jet from the
collection of fakeable objects.
1.2
The misidentification probability, or fake rate, is
Jet: E>20 GeV
T
calculated as the ratio of the number of identified
lepton candidates over the number of fakeable ob- 1 Jet: ET>50 GeV
jects. It is parametrized as a function of the trans- %]
verse energy (transverse momentum) of the jet (iso- ate [ 0.8 Jet: ET>70 GeV
latedtrack)andaveragedoverthe fourjetdatasam- e r Jet: E>100 GeV
k T
ples. The results for one of the electron and muon Fa
n 0.6
categoriesareshowninFig.5andFig.6respectively. o
u
The probability for misidentifying hadrons as muons M
P 0.4
is higher than that for electrons since the muon-type U
M
fakeableobjectisbasedonanisolatedtrackandthus C
more likely to pass our identification cuts. The ap- 0.2
plicationof these rates inthe analysisis describedin
the next section. An uncertainty of 50% is assessed 0
from the variation in the fake rates measured in the 0 5 10 15 20 25
different jet data samples. Ptrack (GeV/c)
T
FIG. 6: Fake rate of CMUP muons. The solid line rep-
IV. SAMPLES OF OBSERVED AND resents the linear fit to the average and the dotted lines
SIMULATED EVENTS are 50%.
±
A. Sample of observed events
The data used in this analysis were collected be- tral muon with a stub in both the CMU and CMP
tween March 2002 and February 2006 via electron- or in the CMX chambers. The data correspond to
based and muon-based triggers. The former requires an integratedluminosity of 1.0 fb−1 and 0.7 fb−1 for
one central (η < 1) electron with ET > 18 GeV, thesamplesbasedontheelectronandmuontriggers,
| |
whereas the latter requires one pT > 18 GeV/c cen- respectively.
10
B. Background Samples convertintooppositely-chargedelectrons[23],wecan
predictthepossibleφlocationoftheclusterbasedon
the charge of the seed electron. In Fig. 7 the “cor-
In the search based on three leptons and miss-
rect” and “incorrect” sides with respect to the seed
ing transverse energy the SM backgrounds are Wγ,
WZ/γ∗, ZZ/γ∗, tt¯and Drell-Yan production, along electron are defined. Furthermore the electrons from
with hadrons misidentified as leptons. The b¯b con- γ conversions are expected to have the same z co-
ordinate at the CES, since the magnetic field B is
tamination is suppressed because the soft and typ-
along the z direction. Based on this, a candidate
ically non-isolated leptons from B decays are re-
photon conversionis a seed electron accompaniedby
jected by our lepton selection. The first set of back-
a CES cluster locatedon the “correct”side and hav-
grounds are estimated using a Monte Carlo tech-
nique, whereas the contribution from misidentified ing ∆zseed,cluster < 20 cm. In order to improve the
| |
hadrons is measured using observed events (Sec-
tion III). The simulated samples are generated Incorrect Side Correct Side
using pythia [18] version 6.216 with the underly-
seed partner
ing event model tuned to the CDF observed events
cluster cluster
[19]. In the case of the WZ sample, pythia is used e− e+
only for the parton showering and the hadronization
of events that are generated with the leading-order
matrix element programmadevent [20]. photon
All simulated background samples were run through
B
the full CDF detector simulation, which is based
on the geant [21] framework, and the same recon-
structionalgorithm[22] thatis used forthe observed
events. All simulation-driven background estimates
are corrected for the different trigger efficiency (see
for instance [13]) and identification efficiency mea- FIG. 7: Sketch of the r φ view of a photon conversion
−
sured in observed events with respect to the one in signaturewithCESclusterslocations. Themagneticfield
B is along thez direction.
simulated events (Section III). An additional correc-
tion factor (Sconv) is needed for the Drell-Yan pro-
duction, as explained in the next section.
purity of the sample of candidate conversions,we re-
To avoid overestimation of the background due to
ject events in which the seed electron comes from a
hadrons misidentified as leptons, we require each
W and is accompanied by a bremsstrahlung photon
identifiedleptoninsimulatedeventstooriginatefrom
the hard interaction (this does not apply to the tt¯ byrequiringE/Ttobelessthan10GeV.Furthermore,
if the invariant mass of the seed electron and a sec-
background where we only ask for three electrons or
ondsame-flavorlepton in the eventfalls in the range
muons).
from 50 to 106 GeV/c2, the event is considered non-
conversionbackground(Z +bremsstrahlungphoton)
and rejected. Events in which the bremsstrahlung
1. Drell-Yan photonconvertsaresuppressedbyrejectingelectrons
having the sum of the measured energy deposit in
Events from Z/γ ℓℓ constitute a background the electromagnetic calorimeter larger than the cor-
∗ →
to our search if an additional lepton is present in responding track momentum. Several other back-
the event. In this section we present the estimate of grounds mimic the conversion candidate signature,
this background contribution when the third lepton suchaselectronsaccompaniedbyaπ0 (decayinginto
comes from a photon radiated from one of the pri- γγ)oraK± (decayinginthedetectorandproducing
mary leptons, and has converted into an e+e− pair. a shower in the electromagnetic calorimeter as well
In order to measure the efficiency of the conver- as in the hadronic calorimeter), or photons from ex-
sion identification algorithm described in Section III tra interactions and jets. These components of the
wecollectapuresampleofcandidateconversionsus- backgroundareexpectedtocontributeequallytothe
ingacalorimeterbasedapproachwhichdoesnotrely “correct” and “incorrect” sides. Consequently, they
ontrackinginformation. Thesampleconsistsofiden- can be estimated by the number of events with clus-
tified electrons with pT larger than 8 GeV/c (called tersonthe“incorrect”side. Wemeasuretheremain-
“seedelectrons”)accompaniedbyanadditionalclus- ingbackgroundintheincorrectsidethroughafitand
ter found in the shower max detector. Since photons subtract it from the signal.