Article
Reference
Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks in pp collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at s√=1.96 TeV, using 295 pb−1 of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle.
The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to 132 GeV/c2 for a neutralino mass of 48 GeV/c2, and a supersymmetric bottom quark mass up to 193 GeV/c2 for a neutralino mass of 40 GeV/c2.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks in pp collisions at s√=1.96 TeV.
Physical Review. D , 2007, vol. 76, no. 07, p. 072010
DOI : 10.1103/PhysRevD.76.072010
Available at:
http://archive-ouverte.unige.ch/unige:38438
Disclaimer: layout of this document may differ from the published version.
Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks in p p collisions at
p s
1:96 TeV
T. Aaltonen,23A. Abulencia,24J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,17S. Amerio,43D. Amidei,35 A. Anastassov,52K. Anikeev,17A. Annovi,19J. Antos,14M. Aoki,55G. Apollinari,17T. Arisawa,57A. Artikov,15 W. Ashmanskas,17A. Attal,3A. Aurisano,53F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43W. Badgett,17
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C. S. Moon,28R. Moore,17M. Morello,46P. Movilla Fernandez,29J. Mu¨lmensta¨dt,29A. Mukherjee,17Th. Muller,26
R. Mumford,25P. Murat,17M. Mussini,5J. Nachtman,17A. Nagano,55J. Naganoma,57K. Nakamura,55I. Nakano,40 A. Napier,56V. Necula,16C. Neu,45M. S. Neubauer,9J. Nielsen,29,nL. Nodulman,2O. Norniella,3E. Nurse,31S. H. Oh,16
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A. Savoy-Navarro,44T. Scheidle,26P. Schlabach,17E. E. Schmidt,17M. P. Schmidt,60M. Schmitt,38T. Schwarz,7 L. Scodellaro,11A. L. Scott,10A. Scribano,46F. Scuri,46A. Sedov,48S. Seidel,37Y. Seiya,41A. Semenov,15 L. Sexton-Kennedy,17A. Sfyrla,20S. Z. Shalhout,58M. D. Shapiro,29T. Shears,30P. F. Shepard,47D. Sherman,22 M. Shimojima,55,kM. Shochet,13Y. Shon,59I. Shreyber,20A. Sidoti,46P. Sinervo,34A. Sisakyan,15A. J. Slaughter,17 J. Slaunwhite,39K. Sliwa,56J. R. Smith,7F. D. Snider,17R. Snihur,34M. Soderberg,35A. Soha,7S. Somalwar,52V. Sorin,36
J. Spalding,17F. Spinella,46T. Spreitzer,34P. Squillacioti,46M. Stanitzki,60A. Staveris-Polykalas,46R. St. Denis,21 B. Stelzer,8O. Stelzer-Chilton,42D. Stentz,38J. Strologas,37D. Stuart,10J. S. Suh,28A. Sukhanov,18H. Sun,56I. Suslov,15
T. Suzuki,55A. Taffard,24,pR. Takashima,40Y. Takeuchi,55R. Tanaka,40M. Tecchio,35P. K. Teng,1K. Terashi,50 J. Thom,17,dA. S. Thompson,21E. Thomson,45P. Tipton,60V. Tiwari,12S. Tkaczyk,17D. Toback,53S. Tokar,14 K. Tollefson,36T. Tomura,55D. Tonelli,46S. Torre,19D. Torretta,17S. Tourneur,44W. Trischuk,34S. Tsuno,40Y. Tu,45 N. Turini,46F. Ukegawa,55S. Uozumi,55S. Vallecorsa,20N. van Remortel,23A. Varganov,35E. Vataga,37F. Vazquez,18,i
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B. Whitehouse,56D. Whiteson,45,pA. B. Wicklund,2E. Wicklund,17G. Williams,34H. H. Williams,45P. Wilson,17 B. L. Winer,39P. Wittich,17,dS. Wolbers,17C. Wolfe,13T. Wright,35X. Wu,20S. M. Wynne,30A. Yagil,9K. Yamamoto,41
J. Yamaoka,52T. Yamashita,40C. Yang,60U. K. Yang,13,jY. C. Yang,28W. M. Yao,29G. P. Yeh,17J. Yoh,17K. Yorita,13 T. Yoshida,41G. B. Yu,49I. Yu,28S. S. Yu,17J. C. Yun,17L. Zanello,51A. Zanetti,54I. Zaw,22X. Zhang,24
J. Zhou,52and S. Zucchelli5
(CDF Collaboration)a
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Comenius University, 842 48 Bratislava, Slovakia;
Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708, USA
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138, USA
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801, USA
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
28Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
SungKyunKwan University, Suwon 440-746, Korea
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
34Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37University of New Mexico, Albuquerque, New Mexico 87131, USA
38Northwestern University, Evanston, Illinois 60208, USA
39The Ohio State University, Columbus, Ohio 43210, USA
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
48Purdue University, West Lafayette, Indiana 47907, USA
49University of Rochester, Rochester, New York 14627, USA
50The Rockefeller University, New York, New York 10021, USA
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155, USA
57Waseda University, Tokyo 169, Japan
qVisiting scientist from IFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain.
pVisiting scientist from University of California, Irvine, Irvine, CA 92697, USA.
oVisiting scientist from Texas Tech University, Lubbock, TX 79409, USA.
nVisiting scientist from University of California, Santa Cruz, Santa Cruz, CA 95064, USA.
mVisiting scientist from University of London, Queen Mary College, London, E1 4NS, England.
lVisiting scientist from University de Oviedo, E-33007 Oviedo, Spain.
kVisiting scientist from Nagasaki Institute of Applied Science, Nagasaki, Japan.
jVisiting scientist from University of Manchester, Manchester M13 9PL, England.
iVisiting scientist from Universidad Iberoamericana, Mexico D.F., Mexico.
hVisiting scientist from University of Heidelberg, D-69120 Heidelberg, Germany.
gVisiting scientist from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
fVisiting scientist from University College Dublin, Dublin 4, Ireland.
eVisiting scientist from University of Cyprus, Nicosia CY-1678, Cyprus.
dVisiting scientist from Cornell University, Ithaca, NY 14853, USA.
cVisiting scientist from University Libre de Bruxelles, B-1050 Brussels, Belgium.
bVisiting scientist from University of Bristol, Bristol BS8 1TL, United Kingdom.
aVisiting scientist from University of Athens, 15784 Athens, Greece.
. . .
58Wayne State University, Detroit, Michigan 48201, USA
59University of Wisconsin, Madison, Wisconsin 53706, USA
60Yale University, New Haven, Connecticut 06520, USA (Received 17 July 2007; published 25 October 2007)
We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at ps
1:96 TeV, using 295 pb1 of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle. The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to132 GeV=c2 for a neutralino mass of48 GeV=c2, and a super- symmetric bottom quark mass up to193 GeV=c2 for a neutralino mass of40 GeV=c2.
DOI:10.1103/PhysRevD.76.072010 PACS numbers: 12.60.Jv, 13.85.Rm, 14.80.Ly
I. INTRODUCTION
Supersymmetry (SUSY) is an extension of the standard model (SM) of particle physics that overcomes some of the theoretical problems in the SM by introducing a new degree of freedom [1]. In this model a bosonic supersym- metric partner is assigned to every SM fermion helicity state, and a fermionic superpartner to every SM boson.
Thus, the SM quark helicity statesqLandqRacquire scalar partnersq~L and q~R. The mass eigenstates of each super- symmetric quark (squark) can be a mixture of their weak eigenstates, quantified by a mixing angle. The difference in the mass eigenvalues depends on several factors. In the case of the supersymmetric top quark (stop), due to the large top quark mass and the large value of its Yukawa coupling constant (Higgs-to-top coupling), there can be a significant difference in the mass between the two mass eigenstates ~t1 and ~t2. In the case of the supersymmetric bottom quark (sbottom), a large mass difference between the two mass eigenstatesb~1andb~2can occur if the ratio of the vacuum expectation values of the two Higgs fields expected in SUSY is large [2]. In both cases it is likely that the less massive stop (~t1) and sbottom (b~1) could be lighter than the first two generations of supersymmetric quarks. In fact the~t1 could be lighter than the top quark if the top Yukawa coupling strength or the stop mixing is strong enough. The requirement of a light stop is also a feature of many baryogenesis models [3].
In this paper, we describe two analyses searching for stop and sbottom production at the Tevatron using a data sample of295 pb1 integrated luminosity collected with the CDF II detector. Both analyses are performed within theR-parity [4] conserving minimal supersymmetric stan- dard model framework. A consequence of R-parity con- servation is that all SUSY particles are pair produced, and the lightest SUSY particle (LSP) is stable. If the LSP interacts weakly it is a good candidate for cold dark matter and it escapes our detection.
At the Tevatron, stop and sbottom are expected to be produced in pairs mainly viaggfusion andqqannihilation, as shown in Fig.1. At leading order, their production cross sections depend essentially only on their masses. For a center-of-mass energy of 1.96 TeV, the next-to-leading- order (NLO) cross section, calculated with PROSPINO[5], ranges from 50.3 pb to 0.25 pb for stop and sbottom masses from 80 GeV=c2 to 200 GeV=c2. In the calculation, the renormalization and factorization scales are set to the mass of stop or sbottom (Qrf m~t
1;b~1), and the parton distribu- tion functions (PDFs) are fromCTEQ6M[6].
The stop and sbottom can decay in many channels, depending on the mass difference between stop/sbottom and other SUSY and SM particles. Here we consider the SUSY parameter space where the stop and sbottom are relatively light. In the case of stop, the flavor changing loop decay~t1!c~01 dominates ifm~t1> mcm~0
1, but m~t1<
mbm~
1 and m~t1< mWmbm~0
1. In the sbottom search, the b~1!b~01 is the only relevant decay ifmb~1>
mbm~0
1, but mb~1< mbm~0
2 and mb~1< mtm~
1. The neutralinos (~01;2;3;4) and the charginos (~1;2) are the SUSY partners of the electroweak bosons and are labeled in order of increasing mass. Therefore, we search for the processes pp !~t1~t1 ! c~01c~01, and pp !b~1b~1! b~01b~01, as shown in Fig. 2. We assume ~01 is the LSP. Thus, the experimental signature for stop and sbottom pair production processes is a pair of acollinear heavy- flavor jets (i.e. bandcjets), and large missing transverse energy (E6 T) coming from the escaping LSPs. The searches assume that both stop and sbottom decay very close to the interaction point. For m~t1 100 GeV=c2 (mb~1 160 GeV=c2) and m~0
1 60 GeV=c2, the expected life- time of stop (sbottom) with only the decay~t1 !c~01(b~1! b~01) is of the order of 1015 (1023) seconds which corresponds to a natural decay length of 0:3 m (3 109 m) [7,8].
Previous searches for stop and sbottom have been per- formed at CERN LEP and at the Tevatron [9–17]. For the search topology studied in the present analysis, LEP ex- cludes stop (sbottom) masses smaller than100 GeV=c2 (100 GeV=c2), independent of the difference between stop (sbottom) and neutralino~01masses [9]. Recent results from the D0 Collaboration in the same topology [16,17], based on Run II data, have extended Tevatron’s Run I reach [10,11] by excluding stop masses up to 133 GeV=c2, and sbottom masses up to 220 GeV=c2. These results are also shown in Figs.4and5of this paper.
II. THE DETECTOR AND DATA SAMPLE CDF II is a general-purpose detector that is described in detail elsewhere [18]. The components relevant to this
analysis are briefly described here. The charged-particle tracking system is closest to the beam pipe, and consists of multiple layers of silicon microstrip detectors, which cover a pseudorapidity regionjj<2, and a large open-cell drift chamber covering the pseudorapidity regionjj<1[19–
21]. The silicon microstrips of the silicon detectors have a pitch of 25 to65m, depending on the layer, thus allow- ing a precise measurement of a track’s impact parameter with respect to the primary vertex. The tracking system is enclosed in a superconducting solenoid, which in turn is surrounded by calorimeters. The calorimeter system [22] is organized into electromagnetic and hadronic sections seg- mented in projective tower geometry, and covers the region jj<3:6. The electromagnetic calorimeters use lead- scintillator sampling, whereas the hadron calorimeters use iron-scintillator sampling construction. The transverse
~
t
1χ
∼+b
W
* 1χ
∼0c
b
~1
χ
0∼
b
FIG. 2. The decay channels of stop and sbottom considered in this paper.
g
g
g
b~ , t
~
b~ ,
~t
g
g
b~ , t
~
b~ ,
~t
g
g
b
~ , t
~
b~ ,
~t
q
q
g
b~ , t
~
b~ ,
~t
FIG. 1. Leading-order Feynman diagrams for pair production of stop and sbottom at the Tevatron.
. . .
energy resolution of the electromagnetic calorimeters is
ET=ET 13:5%
ET GeV
p 2%for the central region (jj<
1), and E=EpE16%GeV1% for the forward region (jj>1). The transverse energy resolution of the hadronic calorimeters isET=ET 75%
ET GeV
p 3%for the central region, and E=EpE80%GeV5% for the forward re- gion. The present analysis exploits the information of the central muon system, which is located outside of the calorimeter and covers the rangejj<1.
The data sample for this analysis was collected using a E6 Tjets trigger, which is implemented in three levels of online event selection. The E6 T is defined as the energy imbalance in the plane transverse to the beam direction [21], and a jet is defined as a localized energy deposition in the calorimeter. In the first and second levels of the trigger, E6 T is required to be greater than 25 GeV and is calculated by summing over calorimeter trigger towers with trans- verse energies above 1 GeV. In the second level there must be at least two jets withET>10 GeV. In the third level, E6 T is recalculated using the full calorimeter segmentation with a tower energy threshold of 100 MeV and is required to be greater than 35 GeV.
In the offline processing, jets are reconstructed from the calorimeter towers using a cone algorithm with fixed radius
R
22
p 0:4in space [23]. The jet ET measurements andE6 T are corrected for detector effects [24].
A fraction of events passing the trigger is not frompp collisions, but from beam halo and cosmic ray sources. To remove these events we examine the event electromagnetic fractionFemand charged fractionFch. TheFemis the ratio of the energy measured by the electromagnetic calorimeter to the total energy contained in jets of cone radiusR 0:4withET>10 GeVandjj<3:6. TheFchis the frac- tion of the jet energy carried by measured charged-particle tracks (pT>0:5 GeV=c) averaged over jets with jj<
0:9. The beam halo travels parallel to the beam axis and deposits most of its energy in the electromagnetic section of the calorimeter or in the hadronic section. By requiring Fem>0:1we reject events which contain little energy in the electromagnetic section of the calorimeter. A cosmic ray traversing the CDF detector can deposit energy in the calorimeter without registering a track in the tracking detectors. If the beam halo background events and cosmic ray background events, as described above, do not overlap with beam crossing events that produce hard pp collisions, then there will be little activity in the tracking detectors. Therefore, by requiring Fch>0:1 we reject backgrounds that have little tracking activity. More de- tailed explanations of these two variables and how they reduce the non-pp collision events are described in [25].
We also reject events if the reconstruction cone of any jet in the event enters an uninstrumented region of the calorimeter.
III. BACKGROUND SOURCES AND EVENT SELECTION
The dominant backgrounds to the stop and sbottom searches in the jets and E6 T signature are production of multijet, W or Z boson with jets, single top, tt, and di- boson (WW=WZ=ZZ) final states. We use the ALPGEN
generator to simulate the W and Z boson plus parton production, with HERWIG used to model parton showers [26,27]. Multijet, top quark, and di-boson production are simulated withPYTHIA[28]. All the Monte Carlo (MC) SM background samples were generated using the CTEQ5L
PDFs [29]. In generating the multijet sample, we select events where there are borcoutgoing partons/hadrons in the final state [heavy-flavor (HF) multijet]. To normalize the SM top background samples, we use the NLO cross section values for single top andttproduction [30–32]. We use theMCFMprogram to obtain the NLO cross sections for W=Zjets and di-boson production [33,34]. The HF multijet sample is normalized using kinematic regions in the data that are dominated by multijet production. In multijet production events, the E6 T is usually due to jet energy mismeasurement. In this case theE6~T tends to point in the same direction as the jet whose energy is mismeas- ured. In theE6 T jets data sample used in this analysis, the events at higher E6 T are dominated by non-multijet SM contributions. Therefore, the multijet dominant kinematic regions are lowE6 T regions (50< E6 T<70 GeV), and re- gions where a jet is aligned in the direction of E6 T. We obtain an average normalization factorkHFmultijet 1:46 0:37where the uncertainty is mainly due to the uncertainty from jet energy calibration and resolution [24]. The kHFmultijet factor is used to normalize the HF multijet sam- ple to estimate the HF multijet contribution in the signal region.
Data selection is optimized by maximizing the statistical significance of a simulated stop/sbottom signal over the expected background events in the data. The optimization is performed prior to examining the signal regions of the data. As the signal production cross section and event kinematics (for example jet ET, E6 T) could vary signifi- cantly across stop and sbottom masses, we determine separate sets of optimized cuts for three mass ranges.
The ‘‘low,’’ ‘‘medium,’’ and ‘‘high’’ mass ranges used for the stop (sbottom) search are m~t1<100 GeV=c2, 100m~t1<120 GeV=c2, and m~t1 120 GeV=c2 (mb~1<140 GeV=c2, 140mb~1<180 GeV=c2, and mb~1180 GeV=c2). The selection cuts for the stop and sbottom are summarized in TablesIandIIrespectively. At the initial stage of event selection, the data sample is dominated by multijet background events. We employ several selection cuts to minimize their contribution. To reduce multijet background and avoid regions where the E6 Ttrigger is inefficient, we requireE6 T>50 GeV. TheE6 T trigger is 75%efficient in this region. Next we require
that there be only two or three reconstructed jets injj<2.
Events with any additional jets withET>8 GeVand2<
jj<3:6are rejected. Most of the time the two highest-ET jets in multijet events are antiparallel. However, this is not the case in the stop (sbottom) pair production since thec (b) jet recoils against the ~01 in the stop (sbottom) decay.
Therefore, we require that the opening angle between the two highestET jets be less than 160 degrees in the plane perpendicular to the beam. The largeE6 Tin multijet events that survive the earlier cuts is usually due to jet energy mismeasurement. Thus, to further reduce this background we require a minimum azimuthal separation between the direction of the jets and E6~T of Jet; E6 T>45. For
multijet events that pass the minimum azimuthal separa- tion requirement, but have large E6 T due to jet energy mismeasurement, the magnitudes of theE6 Tand the second leading jet’s transverse energy are often anticorrelated.
Therefore, we require the sumETJet2 E6 T to be above the values listed in TablesIandII.
In stop (sbottom) pair production, where each stop (sbottom) decays into a charm (bottom) quark and a neu- tralino, the vector sum of the transverse energy of the twoc jets (bjets) should balance against the missing transverse energy from the escaping neutralinos. We explore the correlation between the missing transverse energy and the transverse energy of the first and second leading jets
TABLE II. The event selection cuts for pair production of sbottom in the low, medium, and high sbottom mass regions. Jet1, Jet2, and Jet3 are, respectively, the first, second, and third leading jets. The cuts that are listed only under the medium column are common for all three mass ranges.
Mass range (GeV=c2) Low Medium High
<140 140 –180 >180
E6 T(GeV) >50 >55 >65
ETJet1 (GeV) >35 >55 >75
ETJet2 (GeV) >15 >15 >35
ETJet3 (GeV) >15 >15 >15
jj: Jet1, Jet2, Jet3 j1j<1:2,j2j<1:5,j3j<2:0 Veto additional jets ET>8 GeV,2<jj<3:6
Jet1;Jet2accepted range (deg) 60 –160 50 –160 40 –160 Jet; E6 Taccepted range (deg) 45–180 45–180 45–180 ETJet2 E6 T(GeV) >80 >120 >160
EvTJ12MET (GeV) <15 <15 <15
Lepton Veto YES YES YES
minimum # tracks in jet (jj<1) 4 4 4
JET-PROBABILITYTagging >1tag (JP<1%)
TABLE I. The event selection cuts for pair production of stop in the low, medium, and high stop mass regions. Jet1, Jet2, and Jet3 are, respectively, the first, second, and third leading jets.
The cuts that are listed only under the medium column are common for all three mass ranges.
Mass range (GeV=c2) Low Medium High
<100 100 –120 >120
E6 T(GeV) >50 >50 >50
ETJet1 (GeV) >35 >45 >55
ETJet2 (GeV) >15 >15 >25
ETJet5 (GeV) >15 >15 >15
jj: Jet1, Jet2, Jet3 j1j<1:2,j2j<1:5,j3j<2:0 Veto additional jets ET>8 GeV,2<jj<3:6
Jet1;Jet2accepted range (deg) 70 –160 70 –160 70 –160 Jet; E6 Taccepted range (deg) 45–180 45–180 45–180 ETJet2 E6 T(GeV) >65 >85 >105
EvTJ12MET (GeV) <15 <15 <15
Lepton Veto YES YES YES
minimum # tracks in jet (jj<1) 4 4 4
JET-PROBABILITYTagging >1tag (JP<5%) . . .
through the variableEvT
J12MET, which is defined as
EvTJ12MET E~TJ1E~TJ2E6~T
E~TJ1E~TJ2 jE~TJ1E~TJ2j
: (1)
E~TJ1andE~TJ2are, respectively, the vectors of the transverse energy of the first and second leading jets. For pair pro- duction of stop (sbottom),EvT
J12MET is expected to be insig- nificant, i.e. within the calorimeter jet energy resolution.
For multijet production, due to its high jet multiplicity, the missing transverse energy caused by the mismeasurement of a single jet’s energy, may not balance against the trans- verse energy of the first and second leading jets. In top quark pair production, the missing transverse energy caused by the escaping neutrino in the W boson decay also does not necessarily balance against the first and second leading jets in the event. Therefore,EvT
J12METis large in these background events. To reduce the contribution from top quark production, and to further reduce multijet background, we require EvTJ12MET<15 GeV for both stop and sbottom analyses.
To reduce the background contribution fromW=Zjets and top quark production, we reject events with one or more identified leptons. Candidate electrons must have a track associated with a cluster in the electromagnetic calo- rimeter with ET>10 GeV. Its electromagnetic to had- ronic energy ratio and shower profile must also be consistent with that expected for electrons. Candidate muons are identified as tracks withpT>10 GeV=c that extrapolate to hits in the muon chambers and to energy deposited in the calorimeters consistent with a minimum ionizing particle. To increase lepton detection efficiency, candidate leptons (electrons, muons, taus) are also identi- fied by isolated tracks withpT>10 GeV=c. The number of tracks associated with the first and second leading jets should be four or more. This selection reduces contribu- tions from Wjets and Zjets in which the gauge bosons decay into tau leptons.
For the stop and sbottom search, the signal contains a large fraction of heavy-flavor jets compared to the SM background, which is dominated by light-flavor jets. To enhance the signal over SM background, we identify the heavy-flavor jets using theJET-PROBABILITY(JP) algorithm [35]. Jets from heavy-flavor partons are characterized by secondary decays that are displaced from the primary vertex; thus their tracks have a large impact parameter.
Light-flavor jets appear to come from the primary interac- tion and their tracks’ impact parameters are consistent with the primary vertex (within the resolution of the tracking detector). The JP algorithm examines the impact parameter of each track from a candidate jet and computes a proba- bility that the jet is a light/heavy jet. Jets from the primary (secondary) vertex are assigned a large (small) JP value.
For the stop (sbottom) search at least one jet was required to have aJP<5%(JP<1%). A looser cut is used to tagc jets as their lifetime is shorter compared to b jets. The efficiency to tag a fiducial c or b jet increases with the transverse energy of the jet, and plateaus atET 80 GeV.
The average efficiency to tag a fiducialcjet (bjet) with JP value ofJP<5%(JP<1%) is 17%( 40%) [35].
Light-Flavor Background
A significant source of background is that due to mis- identification of light-flavor jets as heavy flavor. In this case one or more light-flavor jets are tagged as acjet orb jet by the JP algorithm (mistag). A detailed description of the mistag rate is given in [35]. Here we provide a brief summary of its measurement and how it is applied in the stop and sbottom searches. The mistag rate is measured in inclusive jet samples. The rate of misidentifying a light- flavor jet as a heavy-flavor jet is about 1%( 5%) for tagging atJP<1%(JP<5%). Uncertainties on the mistag rates are due to uncertainties in the contribution of long- lived’s and K’s in the light-flavor jets and uncertainties in the effect of particle interactions in the detector materi- als; these relative uncertainties are 9% for JP<1%, and 13% forJP<5%. An additional relative uncertainty on the
TABLE III. The number of observed data events, and the number of expected events from standard model sources in the stop signal region. The first uncertainty is from limited simulation statistics and the second is from systematic uncertainties.
Mass range (GeV=c2) Low Medium High
<100 100 –120 >120
Process Events expected
Wjets 11:52:42:6 9:32:32:1 4:01:50:9
Zjets 9:90:52:0 7:30:41:5 4:10:30:8
Di-boson 2:50:10:5 2:00:10:4 0:90:10:2
Top 5:20:20:8 4:90:20:8 3:90:20:6
HF Multijet 32:55:28:1 17:94:04:5 2:61:50:6
Mistag 75:42:210:7 53:52:07:6 27:21:53:9
Total Expected 1376:214:6 94:95:09:9 42:72:64:6
Data 151 108 43
mistag rate of 6.7% (4.7%) for a JP cut ofJP<1%(JP<
5%) is estimated by comparing the observed and predicted tag rates in different data samples [inclusive jet samples taken with different jetET thresholds, and high jet multi- plicity (4 jets) sample]. To estimate the contribution from light-flavor background in the final data sample, we apply the measured mistag rate to the data sample, i.e. we multiply each jet by the mistag probability, after all selec- tion cuts are applied except the heavy-flavor jet tagging. In the estimate of the heavy-flavor background contributions with MC samples, we check for the existence of abor ac parton or hadron within a cone ofR0:4around the jet before we tag it, to avoid double counting of background events due to light-flavor jets. Since a looser tagging requirement is used to tag the c jets in the stop search than to tag thebjets in the sbottom search, we expect the stop analysis to have a larger fraction of mistag back- ground compared to the sbottom analysis. This can be seen in TablesIIIandIV.
IV. DETECTION EFFICIENCY AND SYSTEMATIC UNCERTAINTIES
The total detection efficiencies for the stop and sbottom signals are estimated using thePYTHIAevent generator and the CDF detector simulation program. The samples were generated using the CTEQ5LPDFs, with the renormaliza- tion and factorization scale set to the mass of the squark in the search [29]. The total stop (sbottom) signal selection efficiency in the accessible mass region varies from 0.1% to 3.4% (0.17% to 8.5%). The efficiency increases for higher stop (sbottom) mass and larger mass difference between~t1 (b~1) and~01.
We have estimated the main contributions to the system- atic uncertainty on the signal acceptance and the SM background estimation. The uncertainty due to the jet energy scale for the SM background estimation is of the order of 10%, whereas it varies from 4% to 20% for the signal efficiency, depending on the stop and sbottom masses (larger uncertainty for smaller squark mass). The
systematic uncertainty from NLO cross sections in the SM background varies between 8% to 13%. The systematic uncertainty on the efficiency for tagging a cjet (bjet) is 12% (8.6%) [35]. The uncertainty on the signal acceptance due to modeling of gluon radiation from the initial-state or final-state partons is 5%. This is evaluated from signal MC samples generated with different levels of initial- and final- state radiation. The uncertainty on the signal efficiency due to the PDF choice is determined to be 2%, using the
CTEQ6M uncertainty PDF set. The uncertainty from MC statistics reaches in the most selective search region 50%
for the SM background, and 10% for signal. The uncer- tainty on the trigger efficiency is 5%, and the uncertainty on the luminosity of the data sample is 6% [36]. The quoted uncertainties are relative to the estimated signal and backgrounds.
V. RESULTS AND CONCLUSIONS
The SM contributions and the total number of observed data events are shown in TablesIIIandIVfor the stop and sbottom searches, respectively. We find that after applying all the selection cuts, the number of observed events are consistent with the number of expected SM background events for both stop and sbottom searches. In these two searches, and for all the mass ranges, the largest source of background events is due to misidentification of light- flavor jets as cor bjets. The HF multijet background is the second largest source of SM background in the low mass range. However, its contribution is largely suppressed by the tighter cuts employed in the medium and high mass ranges.
We studied several kinematic distributions. As an ex- ample Fig.3shows the observedE6 T distributions from the data and the predictedE6 T distributions from the SM back- ground for the high mass stop and medium mass sbottom searches after all selection criteria are applied. They are in good agreement with the distributions observed in the data.
No evidence for stop and sbottom production is observed in any of the kinematic regions that we have studied.
TABLE IV. The number of observed data events, and the number of expected events from standard model sources in the sbottom signal region. The first uncertainty is from limited simulation statistics and the second is from systematic uncertainties. The contribution from di- boson background is found to be negligible in the sbottom search.
Mass range (GeV=c2) Low Medium High
<140 140 –180 >180
Process Events expected
Wjets 5:51:21:2 1:50:60:3 0:50:40:1
Zjets 6:00:41:1 2:70:30:5 1:00:20:2
Top 4:20:20:6 2:90:10:4 1:20:10:2
HF Multijet 18:84:04:7 2:61:50:7 02:10
Mistag 20:50:62:3 8:10:50:9 2:00:30:2
Total Expected 55:04:25:9 17:81:71:6 4:72:10:50:5
Data 60 18 3
. . .
