Article
Reference
Search for the supersymmetric partner of the top quark in pp collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a search for the lightest supersymmetric partner of the top quark in proton-antiproton collisions at a center-of-mass energy s√=1.96 TeV. This search was conducted within the framework of the R parity conserving minimal supersymmetric extension of the standard model, assuming the stop decays dominantly to a lepton, a sneutrino, and a bottom quark. We searched for events with two oppositely-charged leptons, at least one jet, and missing transverse energy in a data sample corresponding to an integrated luminosity of 1 fb−1 collected by the Collider Detector at Fermilab experiment. No significant evidence of a stop quark signal was found. Exclusion limits at 95% confidence level in the stop quark versus sneutrino mass plane are set. Stop quark masses up to 180 GeV/c2 are excluded for sneutrino masses around 45 GeV/c2, and sneutrino masses up to 116 GeV/c2 are excluded for stop quark masses around 150 GeV/c2.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for the supersymmetric partner of the top quark in pp collisions at s√=1.96 TeV. Physical Review. D , 2010, vol. 82, no.
09, p. 092001
DOI : 10.1103/PhysRevD.82.092001
Available at:
http://archive-ouverte.unige.ch/unige:38667
Disclaimer: layout of this document may differ from the published version.
Search for the supersymmetric partner of the top quark in p p collisions at ffiffiffi p s
¼ 1:96 TeV
T. Aaltonen,22B. A´ lvarez Gonza´lez,10,wS. Amerio,42aD. Amidei,33A. Anastassov,37A. Annovi,18J. Antos,13 G. Apollinari,16J. A. Appel,16A. Apresyan,47T. Arisawa,56A. Artikov,14J. Asaadi,52W. Ashmanskas,16B. Auerbach,59 A. Aurisano,52F. Azfar,41W. Badgett,16A. Barbaro-Galtieri,27V. E. Barnes,47B. A. Barnett,24P. Barria,45c,45aP. Bartos,13 M. Bauce,42b,42aG. Bauer,31F. Bedeschi,45aD. Beecher,29S. Behari,24G. Bellettini,45b,45aJ. Bellinger,58D. Benjamin,15 A. Beretvas,16A. Bhatti,49M. Binkley,16,aD. Bisello,42b,42aI. Bizjak,29,ccK. R. Bland,5C. Blocker,7B. Blumenfeld,24
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R. Culbertson,16D. Dagenhart,16N. d’Ascenzo,43,uM. Datta,16P. de Barbaro,48S. De Cecco,50aG. De Lorenzo,4 M. Dell’Orso,45b,45aC. Deluca,4L. Demortier,49J. Deng,15,dM. Deninno,6aF. Devoto,22M. d’Errico,42b,42a A. Di Canto,45b,45aB. Di Ruzza,45aJ. R. Dittmann,5M. D’Onofrio,28S. Donati,45b,45aP. Dong,16T. Dorigo,42aK. Ebina,56
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S. Pagan Griso,42b,42aC. Pagliarone,53aE. Palencia,10,gV. Papadimitriou,16A. A. Paramonov,2J. Patrick,16 G. Pauletta,53B,53aM. Paulini,11C. Paus,31D. E. Pellett,8A. Penzo,53aT. J. Phillips,15G. Piacentino,45aE. Pianori,44
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E. Vataga,45d,45aF. Va´zquez,1,lG. Velev,16C. Vellidis,3M. Vidal,30I. Vila,10R. Vilar,10M. Vogel,36G. Volpi,45b,45a P. Wagner,44R. L. Wagner,16T. Wakisaka,40R. Wallny,9S. M. Wang,1A. Warburton,32D. Waters,29M. Weinberger,52
W. C. Wester III,16B. Whitehouse,55D. Whiteson,44,dA. B. Wicklund,2E. Wicklund,16S. Wilbur,12F. Wick,25 H. H. Williams,44J. S. Wilson,38P. Wilson,16B. L. Winer,38P. Wittich,16,hS. Wolbers,16H. Wolfe,38T. Wright,33 X. Wu,19Z. Wu,5K. Yamamoto,40J. Yamaoka,15T. Yang,16U. K. Yang,12,qY. C. Yang,26W.-M. Yao,27G. P. Yeh,16
K. Yi,16,nJ. Yoh,16K. Yorita,56T. Yoshida,40,kG. B. Yu,15I. Yu,26S. S. Yu,16J. C. Yun,16A. Zanetti,53a Y. Zeng,15and S. Zucchelli6b,6a
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798, USA
6aIstituto Nazionale di Fisica Nucleare, 6-40127 Bologna, Italy
6bUniversity of Bologna, I-40127 Bologna, Italy
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
13Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708, USA
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
1University of Florida, Gainesville, Florida 32611, USA
18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
19University of Geneva, CH-1211 Geneva 4, Switzerland
20Glasgow University, Glasgow G12 8QQ, United Kingdom
21Harvard University, Cambridge, Massachusetts 02138, USA
22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
26Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon 305-806, Korea;
Chonnam National University, Gwangju 500-757, Korea;
Chonbuk National University, Jeonju 561-756, Korea
27Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
28University of Liverpool, Liverpool L69 7ZE, United Kingdom
29University College London, London WC1E 6BT, United Kingdom
30Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics: McGill University, Montre´al, Que´bec, Canada H3A 2T8;
Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;
University of Toronto, Toronto, Ontario, Canada M5S 1A7;
and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy;
42bUniversity of Padova, I-35131 Padova, Italy
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
45aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy;
45bUniversity of Pisa, I-56127 Pisa, Italy;
45cUniversity of Siena, I-56127 Pisa, Italy;
45dScuola Normale Superiore, I-56127 Pisa, Italy;
46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
47Purdue University, West Lafayette, Indiana 47907, USA
48University of Rochester, Rochester, New York 14627, USA
49The Rockefeller University, New York, New York 10065, USA
50aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy;
50bSapienza Universita` di Roma, I-00185 Roma, Italy
51Rutgers University, Piscataway, New Jersey 08855, USA
52Texas A&M University, College Station, Texas 77843, USA
53aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy;
53BUniversity of Trieste/Udine, I-33100 Udine, Italy
aDeceased
bWith visitors from University of Massachusetts Amherst, Amherst, MA 01003, USA
cWith visitors from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy
dWith visitors from University of California Irvine, Irvine, CA 92697, USA
eWith visitors from University of California Santa Barbara, Santa Barbara, CA 93106 , USA
fWith visitors from University of California Santa Cruz, Santa Cruz, CA 95064, USA
gWith visitors from CERN, CH-1211 Geneva, Switzerland
hWith visitors from Cornell University, Ithaca, NY 14853, USA
iWith visitors from University of Cyprus, Nicosia CY-1678, Cyprus
jWith visitors from University College Dublin, Dublin 4, Ireland
kWith visitors from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017
l(k) With visitors from Universidad Iberoamericana, Mexico D.F., Mexico
mWith visitors from Iowa State University, Ames, IA 50011, USA
nWith visitors from University of Iowa, Iowa City, IA 52242, USA
oWith visitors from Kinki University, Higashi-Osaka City, Japan 577-8502
ccWith visitors from On leave from J. Stefan Institute, Ljubljana, Slovenia
bbWith visitors from Yarmouk University, Irbid 211-63, Jordan
aaWith visitors from University of Virginia, Charlottesville, VA 22906, USA
zWith visitors from Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile
yWith visitors from IFIC (CSIC-Universitat de Valencia), 56071 Valencia, Spain
xWith visitors from Texas Tech University, Lubbock, TX 79609, USA
wWith visitors from Universidad de Oviedo, E-33007 Oviedo, Spain
vWith visitors from University of Notre Dame, Notre Dame, IN 46556, USA
uWith visitors from National Research Nuclear University, Moscow, Russia
tWith visitors from Nagasaki Institute of Applied Science, Nagasaki, Japan
sWith visitors from Muons, Inc., Batavia, IL 60510, USA
rWith visitors from Queen Mary, University of London, London, E1 4NS, United Kingdom
qWith visitors from University of Manchester, Manchester M13 9PL, United Kingdom
pWith visitors from Kansas State University, Manhattan, KS 66506, USA
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA (Received 3 September 2010; published 2 November 2010)
We present a search for the lightest supersymmetric partner of the top quark in proton-antiproton collisions at a center-of-mass energy ffiffiffi
ps
¼1:96 TeV. This search was conducted within the framework of the R parity conserving minimal supersymmetric extension of the standard model, assuming the stop decays dominantly to a lepton, a sneutrino, and a bottom quark. We searched for events with two oppositely-charged leptons, at least one jet, and missing transverse energy in a data sample corresponding to an integrated luminosity of 1 fb1 collected by the Collider Detector at Fermilab experiment. No significant evidence of a stop quark signal was found. Exclusion limits at 95% confidence level in the stop quark versus sneutrino mass plane are set. Stop quark masses up to 180 GeV=c2 are excluded for sneutrino masses around45 GeV=c2, and sneutrino masses up to116 GeV=c2are excluded for stop quark masses around150 GeV=c2.
DOI:10.1103/PhysRevD.82.092001 PACS numbers: 14.80.Ly, 13.85.Rm, 12.60.Jv
I. INTRODUCTION
The minimal supersymmetric standard model [1]
(MSSM) was introduced to solve several problems that arise in the standard model (SM). These include: the hierarchy problem that requires the fine-tuning of theoreti- cal parameters in order to obtain cancellation of large quantum corrections to the Higgs mass; the lack of con- vergence of the strong, weak, and electromagnetic gauge couplings at the grand-unification energy scale; and the lack of a dark matter candidate.
The MSSM assigns a new bosonic counterpart to each SM fermion and likewise a fermionic superpartner to each SM boson. This results in scalar partnersq~L andq~R to the SM quark helicity states qL and qR. There can be two supersymmetric mass eigenstates for each supersymmetric quark (squark) corresponding to the two fermionic degrees-of-freedom of the SM quark. The supersymmetric scalar top quark (stop) mass eigenstates ~t1 and ~t2 are rotated relative to ~tL and ~tR by a mixing angle ~t. In some models [2], m~t1;2mt. The large mass of the top quark and the corresponding large value of the top-to- Higgs coupling constant may lead to a large splitting between m~t1 and m~t2. Consequently the lower mass stop quark eigenstate is expected to be the lightest of all the squarks, with a mass even below the top quark, making its detection at the Tevatron a realistic possibility.
The MSSM possesses a new conserved quantity calledR parity (Rp), defined asRp¼ ð1Þ3ðBLÞþ2S, whereBis the baryon number,Lis the lepton number, andSis the spin.
As a consequence, the lightest supersymmetric particle (LSP) must be stable and is a dark matter candidate.
Because the initial state of pp collisions has Rp¼ þ1 and supersymmetric particles have Rp¼ 1, supersym- metric particles must be pair produced. At the Tevatron
stop quarks are expected to be produced primarily through gluon-gluon (gg) fusion and quark-antiquark (qq) annihilation, withgg fusion dominant at low stop masses (<100 GeV=c2) andqq annihilation dominant at higher stop masses [3].
The produced stop (~t) quarks can decay via several possible channels, depending on the masses of the particles involved. Two-body decays include ~t!t~01, ~t!b~þ1, and~t!c~01, where~01 is the lightest neutralino and ~þ1 the lightest chargino. These decays may not be kinemati- cally possible for a light stop, or they may be suppressed by higher-order diagrams. They are also constrained by exist- ing limits [4,5]. Possible three-body decays include~t! Wþb~01,~t!b~lþl, and~t!blþ~l, where~lis the super- symmetric lepton and ~l the supersymmetric neutrino.
Limits on the supersymmetric lepton and neutralino masses from experiments at the large electron-positron collider (LEP) [4] restrict the range of stop masses avail- able to the first two decay modes.
The decay ~t!blþ~l, which proceeds via a virtual chargino, is the subject of this analysis. We assume the branching ratio for this decay mode is 100%, and that electrons, muons, and taus are equally likely decay prod- ucts. While electrons and muons are detected directly, taus are only included opportunistically in this analysis through their decays into electrons and muons. We also assume that the supersymmetric neutrino decays neutrally into the LSP (or is the LSP), thus escaping undetected and leading to missing transverse energy (ET) [6] in the detector. Since stop quarks are produced in pairs, we search for events with two opposite-charge leptons (ee,e,),ET, and at least one hadronic jet.
Previous searches at LEP and at the Tevatron [4,5] for the stop quark using the same topology as this analysis have produced negative results. These analyses have set
95% confidence level exclusion limits in the stop-sneutrino mass plane. This paper extends the earlier CDF results based on107 pb1 of data at ffiffiffi
ps
¼1:8 TeV to1 fb1 at ffiffiffis
p ¼1:96 TeV.
The structure of this paper is as follows: Sec.IIdetails the detector and the data set, Sec.IIIdescribes the back- ground estimation, Sec. IV the signal predictions and systematic uncertainties, Sec. V event preselection and control samples, Sec.VIexplains the optimization of event selection cuts, and Sec. VII presents the results and conclusions.
II. THE DETECTOR AND DATA SET
The data sample corresponds to an integrated luminosity of 1 fb1 of pp collisions at a center-of-mass energy of 1.96 TeV collected with the CDF detector [7] at the Fermilab Tevatron. Of particular relevance to this analysis are the tracking system, the calorimetry, and the muon detectors.
The tracking system consists of two silicon micro-strip detectors and a multiwire open-cell drift chamber, the central outer tracker (COT). The silicon vertex detector (SVX) and the intermediate silicon layers cover the pseu- dorapidity [6] regionjj<2, while the COT coversjj<
1. The tracking system is surrounded by a superconducting solenoid with a magnetic field of 1.4 T. The relative track momentum resolution provided by the COT ispT=p2T’ 0:0015ðGeV=cÞ1.
Outside the magnet in the radial direction are electro- magnetic and hadronic calorimeters arranged in a projec- tive tower geometry with a tower granularity of ’0:10:26in the central region. The central electromagnetic calorimeter (CEM) utilizes lead- scintillator sampling and covers jj<1:1 with energy resolution for electrons ET=ET’13:5%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ET ðGeVÞ
p
2%. The central electromagnetic shower maximum detec- tor (CES) is located inside the CEM at a depth of six radiation lengths, close to the position of maximum elec- tromagnetic shower development, and is used for the de- termination of the shower shape and for matching the shower location to the track extrapolation. The central hadron calorimeter (CHA) uses iron-scintillator sampling and coversjj<0:9. Its energy resolution for hadrons is ET=ET’75%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ET ðGeVÞ
p 3%.
Additional calorimetry extends the coverage in the for- ward direction and is used in this analysis for calculating ET and jet energies but not for lepton identification. The plug electromagnetic calorimeter covers 1:1<jj<3:6 and is constructed of lead and scintillator layers with an energy resolution for electrons of ET=ET’ 16%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ET ðGeVÞ
p 1%. The plug hadron calorimeter cov- ers1:3<jj<3:6and is constructed of iron and scintil- lator layers with an energy resolution for hadrons of ET=ET’80%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ET ðGeVÞ
p 5%. The iron and scintilla-
tor wall hadron calorimeter covers the intermediate region 0:7<jj<1:3 between the central and plug hadron calorimeters. Its energy resolution is ET=ET’ 75%= ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ETðGeVÞ
p 4%.
Radially outside the calorimetry is the muon detection system. The parts of the muon detector relevant to this analysis are the central muon detector (CMU), the central muon upgrade detector (CMP) and the central muon ex- tension (CMX). The CMU consists of four-layer drift chambers and covers the pseudorapidity range jj<0:6. The CMP is made of four layers of single-wire drift cells located behind an additional 3.3 interaction lengths of steel and covers jj<0:6. Muons with reconstructed track stubs found in both the CMU and CMP are labeled CMUP muons. The CMX extends the coverage to jj<
1and is made up of eight layers of drift tubes.
The data were collected using a three-level trigger sys- tem. The first two levels are mostly hardware based, while the third level is software based and is a fast version of the offline event reconstruction package. The online selection requires at least two lepton candidates falling into the following categories: CEM-CEM, CEM-CMUP, CEM- CMX, CMUP-CMUP, and CMUP-CMX, where the lep- tons are labeled by the detector components used in their identification. The triggers used in this analysis had anET threshold of 4 GeV for electrons and a pT threshold of 4 GeV=cfor muons.
All lepton candidates require the presence of a well- reconstructed track in the COT detector. Offline, electrons are required to pass a 2 comparison of the CES lateral shower profile in the r-z view and the profile extracted from electron test-beam data. The shower position in the CES must also match the extrapolated position of the track.
In addition, the lateral shower profile in the CEM towers must be consistent with that expected from test-beam data.
The energy deposited in the calorimeters must be consis- tent with the track momentum measurement and the energy deposition in the CHA must be small. To reduce the background arising from the decay of hadrons in jets, the electrons are required to be isolated. Isolated electrons are selected by requiring that the remaining transverse energy after subtracting the transverse energy associated with the electron in a cone of 0.4 in-space (EisoT ) must be less than 2 GeV ifET20 GeVorEisoT =ETbe less than 0.1 if ET>20 GeV. Electrons arising from photons that convert into eþe pairs are removed by cuts applied to pairs of opposite-charge tracks with small opening angles. The electron identification efficiency was measured with Drell-Yan (DY) electrons to range from 75 to 83% [8]
increasing with the electronET.
Muons are identified by matching tracks with reconstructed track stubs in the CMUP or CMX.
The energy deposited in the calorimeters must be consis- tent with a minimum-ionizing particle. The isolation re- quirement is similar to that for electrons:EisoT <2 GeVif
pT20 GeV=c or EisoT =ET<0:1 if pT>20 GeV=c.
The muon identification efficiency was measured with J=c and Z data to be in the range 90–96% [8] rising with the muonpT.
For both electrons and muons the impact parameter (d0) of the track with respect to the beam line position must be less than 0.2 cm if the track is based on COT information only or less than 0.02 cm if silicon-based tracking is also available. The longitudinal position of the event vertex is required to be within 60 cm of the center of the detector.
Jets are identified in -space as a group of electro- magnetic and hadronic towers using a clustering algorithm with cone sizeR¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðÞ2þ ðÞ2
p ¼0:7. The jet en- ergy resolution is ’0:1ETðGeVÞ 1 GeV for jets with 35< ET<450 GeV. In order to find genuine had- ronic jets and to avoid counting electrons and photons as jets, the fraction of the total energy deposited in the elec- tromagnetic calorimeters is required to be between 0.1 and 0.9. Corrections to the jet energy are applied to take into account -dependent losses, luminosity-dependent mul- tiple interactions, and nonlinearities in calorimeter re- sponse [9]. In this analysis, we require at least one jet withjj<2andET>15 GeVafter corrections.
ETis calculated as the magnitude of the negative vector sum of the transverse energy deposited in the calorimeter towers with jj<3:6 and energy larger than 0.1 GeV.
Since muons are minimum-ionizing particles and deposit only ’2 GeV energy in the calorimetry, the muon mo- mentum from tracking (minus the expected energy depo- sition) is used to correctET. The energy corrections applied to jets with jj<2:4 and ET>10 GeV are also propa- gated to the calculation ofET.
III. BACKGROUND ESTIMATION
Several background sources result in events with dilep- tons, jets, andET. These backgrounds are tt production, other heavy-flavor quark (bb and cc) production, DY production of lepton pairs where mismeasurement of a jet or lepton results in substantialET, diboson production (WW,WZ,ZZ, andW), and events with a lepton and a misidentified or secondary lepton (lþfake). The jets in most backgrounds result from QCD initial-state or final- state radiation (ISR or FSR).
Thett, DY, and diboson backgrounds are estimated by generating Monte Carlo (MC) events using thePYTHIA[10]
event generator followed by aGEANT-based [11] detector simulation. Backgrounds arising from fake leptons and heavy-flavor ðbb; c cÞ are estimated using data-driven methods.
The MCttsamples are normalized to the next-to-lead- ing-order (NLO) cross sectiontt¼6:71 pb[12] with a top mass of175 GeV=c2. TheZ!ee,Z!, andZ! samples are normalized to a leading-order cross section times branching ratio of 1272 pb for mll>10 GeV=c2 times ak-factor of 1.4 [13] to correct for NLO contribu-
tions. The NLO cross sections times branching ratios for WW !ll, WZ!llX, and ZZ!llX are 1.27, 0.365, and 1.513 pb respectively [14]. ForW!eandW! , we use the leading-order cross section of 21.5 pb times akfactor of 1.34 to account for NLO contributions [15].
The lepton plus fake background consists of events with a genuine lepton plus a ‘‘fake’’ lepton which is either a light hadron misidentified as a lepton or an uninteresting lepton from pion or kaon decay in flight. For muons the fakes can be particles that penetrate the calorimeters and absorbing material and reach the muon detectors or decay in flight to muons. In the case of electrons, fakes are usually jets that are misidentified as electrons, mainly due to neutral pions that decay to photons, which shower in the electromagnetic calorimeters. This background is estimated by examining samples of single-lepton events taken with theET>8 GeVelectron calibration trigger and thepT>8 GeV=cmuon calibration trigger. Events with at least one ET>4 GeVcentral jet (jj<1:1) or one iso- lated track with pT>4 GeV=c passing the muon track quality cuts, excluding the trigger lepton, form the lþ fake-electron and lþfake-muon candidate samples, re- spectively. To determine the background, it is necessary to estimate a fake rate from samples of jet events that contain a negligible number of directly produced leptons.
Four samples containing jets with ET>20, 50, 70, and 100 GeV, respectively, are used. The electron fake rate is defined as the probability of a jet being misidentified as an electron, and the muon fake rate as the probability of an isolated track being misidentified as a muon. The fake rates are determined as a function of jetETfor electrons and of trackpTfor muons. The fake rate for 20 GeV electrons is about 0.0002 fakes per jet and the fake rate for20 GeV=c muons is about 0.004 fakes per track. These fake rates are then applied to each fake-electron and fake-muon candi- date, one at a time, in the single-lepton sample. Events that pass the analysis cuts are assigned a weight, which is the appropriate fake rate, scaled to the integrated luminosity of the dilepton data sample relative to the integrated luminos- ity of the single-lepton sample, taking into account the single-lepton trigger prescale. The sum of the weights then forms the lþfake background. A 50% systematic uncertainty on the misidentifed lepton background is as- signed based upon the differences between the four jet samples used to determine the fake rates.
The background arising from heavy-flavor ðbb; c cÞ is estimated utilizing dilepton events enriched in heavy- flavor events by inverting the normal impact parameter requirements, i.e. jd0j>0:2 cmfor COT-only tracks and jd0j>0:02 cmif the track included information from the SVX, and requiring that at least one lepton pass the inverse d0 cuts. A ‘‘scaling region’’ is defined as the dilepton invariant mass range15< Mll<35 GeV=c2. The scaling region has no requirements imposed onET, the number of
jets, or other kinematic variables. In this region, the only significant contributions to the data sample with normald0 cuts are due to DY, heavy-flavor, and light hadrons mis- identified as leptons. The DY contribution is derived from MC samples, and the misidentified lepton component is estimated using the technique described above. The re- maining events are attributed to heavy flavor (HF) and are used to calculate scaling factors, defined as the ratio of these inferred HF events passing the normald0 cuts to the total number of events passing the above inverse d0 cuts. It is assumed that this scaling ratio is independent of dilepton mass in regions not dominated byZ-boson decays.
To estimate the heavy-flavor background, the resulting scaling factors, typically of order 2, are then applied to inverse d0 events passing cuts appropriate to the various control regions as well as the signal region. No heavy- flavor contribution to the signal region survives our final cuts.
IV. SIGNAL SAMPLES AND SYSTEMATIC UNCERTAINTIES
A total of 74 MC signal samples corresponding to values of m~t¼ ½55;190 GeV=c2 and m~¼ ½45;110GeV=c2 were generated usingPYTHIAand theGEANT-based detec- tor simulation. The events are scaled using the NLO
PROSPINO cross section [16,17] calculated with the CTEQ6M [18] parton distribution functions (PDFs). The cross sections depend on the stop quark mass and only weakly, through higher-order corrections, on other SUSY parameters. Limits on the production cross section can therefore be translated into lower limits on the lightest stop quark mass without reference to other SUSY parame- ters. The branching ratio for the decay~t!blþ~l, where l¼e, , or with equal probability, is assumed to be 100%.
There are several sources of systematic uncertainty on the background estimation. The systematic uncertainty due to the jet energy scale is determined by varying the jet energy corrections by1[9]. The resulting uncertainty varies from less than 1% to 35% and is largest for DY events, which typically contain low-ET jets. The uncer- tainty arising from ISR and FSR is determined by varying the parameters in PYTHIA that control the generation of ISR/FSR. The resulting uncertainty is ’3%. The uncer- tainty on the acceptance arising from the PDFs used in the MC is estimated using the uncertainties on the CTEQ eigenvectors and determined to be 2%. Other systematic uncertainties are 6% on the measurement of the integrated luminosity, 2% on the dilepton trigger efficiency, 2% on lepton identification efficiency, and 50% on the number of misidentified electrons and muons. The uncertainties on the cross sections used in the MC generation of the back- ground are 8% fortt, 2% for DY, 6% forWW, 8% forWZ, 10% forZZ, and 7% forW.
The systematic uncertainties on the MC stop signal estimation are nearly identical to those on the background estimation. The uncertainty arising from the jet energy scale varies from 1% to 11% depending on the stop- sneutrino mass difference. The uncertainties due to ISR/
FSR, PDFs, luminosity, trigger efficiency, and lepton iden- tification are the same as for the background estimation.
V. INITIAL EVENT SELECTION AND CONTROL SAMPLES
We first define a presignal region by applying several event selection cuts to significantly reduce background and provide a data sample loosely consistent with the stop quark signature. We subsequently optimize additional cuts to improve the sensitivity of the search. This is done prior to revealing the contents of the data in the presignal region. At the presignal stage, the following cut require- ments are applied: two opposite-charge leptons, one with pT>10 GeV=cand the other withpT>5 GeV=c;mll>
15 GeV=c2, in order to remove sequential B-hadron de- cays and low mass resonances;mll<76 GeV=c2 ormll>
106 GeV=c2 for same-flavor dilepton events, in order to eliminate Z boson-events; at least one jet with corrected ET>15 GeV and jj<2; ET>15 GeV; Rðe;highest-ETjetÞ>0:4; Rðl; lÞ>0:4; and >
20 betweenET and each of the leptons and the highest ETjet.
Figure 1 shows theET distributions separately for ee, e, andevents, as well as the summed distribution in the presignal region. The expectedETdistribution for stop quark events with stop mass 150 GeV=c2 and sneutrino mass75 GeV=c2 is also shown scaled up by a factor of 5.
Figures.2–4show the corresponding plots forHT, thepT of the highestpTlepton (pT1), and thepTof the next-to- highest pT lepton (pT2). HT is defined as HT¼ ETþpT1þpT2þETj, whereETjis the transverse energy of the highestpT jet. TableIlists the sources of expected background for the presignal region and the number of observed events. Good agreement of data with the back- ground estimations is observed.
To check the accuracy of our estimation of SM backgrounds, a number of ‘‘control regions’’ are defined.
One control region consists of same-flavor, opposite- charge lepton events with invariant mass 76< mll<
106 GeV=c2 (the Z region). No jet orET requirements are imposed. The background is expected to arise almost entirely from DY processes. Table IIgives the number of observed events in the Z region and the expected back- ground. Good agreement is seen, demonstrating accurate modeling of the predominantly DY background.
Another control region consists of events where we requireET>15 GeVand no jets with ET>15 GeV. All the other presignal cuts are applied except for the >
20 cut betweenETand each of the two highestpTleptons and the highestET jet. This control region predominantly
FIG. 1 (color online). ET distributions in the presignal region for (a) theeechannel, (b) theechannel, (c) thechannel, and (d) the three channels combined. Data are shown as the points with error bars (statistical only). Shown as stacked histograms are the backgrounds arising from misidentified hadrons and decays in flight (lþfake),bbandcc(HF), DY, dibosons, andtt. For reference, the expected signal forðm~t; m~Þ ¼ ð150;75ÞGeV=c2, multiplied by five, is shown as the dashed line.
FIG. 2 (color online). HTdistributions in the presignal region shown as in Fig.1.
FIG. 3 (color online). pT distributions of the highestpT lepton in the presignal region shown as in Fig.1.
FIG. 4 (color online). pTdistributions of the next-to-highestpTlepton in the presignal region shown as in Fig.1.
TABLE I. Expected backgrounds [DY,tt, diboson, misidentified hadrons and decays in flight (lþfake), andbbandcc(HF)] and observed events in the presignal region. The first uncertainty listed is statistical and the second systematic. The systematic uncertainty includes a 6%
uncertainty on the luminosity common to all entries.
ee e
DY 72:84:826:3 26:62:75:4 62:44:128:4
tt 6:10:10:7 13:10:11:4 4:20:10:5 diboson 3:50:10:6 6:20:11:1 2:10:00:4
lþfake 21:60:210:8 24:90:412:4 5:40:22:7
HF 9:14:17:4 30:67:910:5 8:54:36:7
Total 113630 101818 83630
Data 110 76 89
TABLE III. Expected backgrounds [DY,tt, diboson, misidentified hadrons and decays in flight (eþfake,þfake), andbbandcc(HF)] and observed events for the high-ET/no jet control region. The first uncertainty listed is statistical and the second systematic. The systematic uncertainty includes a 6% uncertainty on the luminosity common to all entries.
ee e
DY 167726 9457 108519
tt 0:10:00:0 0:20:00:0 -
diboson 9:30:00:9 17:90:11:7 5:90:00:6
eþfake 23:10:111:5 11:80:35:9 -
þfake - 14:80:17:4 4:70:22:4
HF 15:65:39:6 62:611:93:8 26:28:16:7
Total 215930 2021313 1451020
Data 186 167 114
TABLE II. Expected DY background and observed events in theZcontrol region. The first uncertainty listed is statistical and the second systematic. The systematic uncertainty includes a 6% uncertainty on the luminosity common to both channels.
ee
DY 12 31463956 690443568
Data 12 461 7111
TABLE IV. Expected backgrounds [DY,tt, diboson, misidentified hadrons and decays in flight (eþfake,þfake), andbbandcc(HF)] and observed events for the low-ET/no jet control region. The first uncertainty listed is statistical and the second systematic. The systematic uncertainty includes a 6% uncertainty on the luminosity common to all entries.
ee e
DY 625443644 201821 468135526
tt - - -
diboson 1:40:00:1 2:50:00:2 0:80:00:1
eþfake 2630132 95148 -
þfake - 105052 40020
HF 82686138 110272110 5547166
Total 734596671 150572132 527680530
Data 7448 1687 5344
tests the modeling of the highETtail of DY and HF events.
TableIIIgives a breakdown of the expected backgrounds and the number of observed events for this control region.
Reasonable agreement is observed between the expected background and data. The significant number of e DY events (comparable to the number ofeeandevents) is due toZ! decays, where one subsequently decays to an electron and the other to a muon.
Two additional control regions are defined as having ET<15 GeV and either the presence or absence of jets withET>15 GeV. These control regions are sensitive to the modeling of heavy flavor and the photon component of DY. All other cuts are the same as for the previous control region. Tables IV and V list the expected backgrounds and the number of observed events for these control re- gions. In both cases, the data agree well with the expected backgrounds.
VI. OPTIMIZATION OF CUTS
The signal regions are determined by optimizing cuts on variables that minimize the limit on the~t~tproduction cross section determined with Bayesian methods [19]. The ee, e, and final states are tuned simultaneously and flavor-independent cut values are obtained. The most use- ful variables for discriminating stop quark signal from background areET;HT; the absolute value of the difference in azimuthal angle between the dilepton system andET; pT2; the transverse momentum of the dilepton system (pllT);
and the transverse mass between each lepton (l) andET, defined as mTi¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2pTiET½1cosððli;ETÞÞ
p , where
the index i labels the lepton.
The optimum values for the cut variables depend on the mass difference (m¼m~tm~), which we group into four sets labeledathroughdinmbands parallel to and below them~t¼m~þmbkinematic limit. Four cut groups were chosen as a compromise between having the cuts near their optimal values for each stop-sneutrino point while keeping the number of cut groups to a minimum. The definitions of the four cut groups and the values of the cuts used are given in TableVI.
The ET distributions for the signal region are shown in Fig. 5 for cut group b, where the other cuts listed in Table VI are all applied. The individual back- grounds are shown as well as the data. For reference, the expected signal from the stop-sneutrino mass point ð140;90Þ GeV=c2is also shown added to the stacked back- grounds. The vertical line represents the lower bound placed onET for this cut group. TheET cut is the most effective at reducing the DY, bb and cc, and lþfake backgrounds. In particular, the DY background is expected to show a small energy imbalance in the detector, while the reference-point signal events are characterized by large energy imbalance. The cut onET reduces the DY back- ground by more than a factor of 10 even when all other final cuts have been applied, while reducing the expected signal by only 20%.
As shown in Fig.6, the cut on the absolute value of the azimuthal angle between the dilepton system andET is useful in further surpressing the dominant DY background.
This cut discriminates against DY events where theET arises from mismeasurement of the leptons or from events where neutrinos from the decay result in ET aligned with the leptons coming from the decay.
Leptons arising from stop quark decay are typically more energetic than those coming fromborcquark decay because of the higher stop quark mass. A cut onpT2, whose distribution is shown in Fig. 7, removes all thebbandcc TABLE V. Expected backgrounds [DY,tt, diboson, misidentified hadrons and decays in flight
(eþfake,þfake), andbbandcc(HF)] and observed events for the low-ET/one or more jet control region. The first uncertainty listed is statistical and the second systematic. The systematic uncertainty includes a 6% uncertainty on the luminosity common to all entries.
ee e
DY 116119223 4236 100416204
tt 0:30:00:0 0:60:00:1 0:20:00:0 diboson 1:40:00:1 0:60:00:1 1:00:00:1
eþfake 2150107 61131 -
þfake - 94047 28014
HF 1442132 2472670 1702746
Total 152128250 4452690 120432209
Data 1443 351 1246
TABLE VI. Table of cuts for the cut groups defining the four signal regions.
Variable Cut group
a b c d
m(GeV=c2) 5–47.5 47.5–72.5 72.5–87.5 >87:5
ET(GeV) >25 >32 >32 >32
ðpllT; ETÞ(deg) >60 >60 >60 >60
pT2 (GeV=c) >7 >7 >7 >7
HT (GeV) <170 120–225 130–290 >165
mT (GeV=c2) >15 >11 - -
pllT (GeV=c) <cm1 - - -
FIG. 5 (color online). The cut groupb ETdistributions for (a) theeechannel, (b) theechannel, (c) thechannel, and (d) the three channels combined with all other cuts applied. Data are shown as the points with error bars (statistical only). Shown as stacked histograms are the backgrounds arising from misidentified hadrons and decays in flight (lþfake),bbandcc(HF), DY, dibosons, and tt. MC predictions forðm~t; m~Þ ¼ ð140;90ÞGeV=c2are stacked on top of the sums of backgrounds and are shown as the dashed line.
The cut selects events to the right of the vertical line.
FIG. 6 (color online). The cut groupbdistributions of the absolute value of the azimuthal angle between the dilepton system andET shown as in Fig.5.
FIG. 7 (color online). The cut groupb pT2 distributions shown as in Fig.5.
FIG. 8 (color online). The cut groupb HTdistributions shown as in Fig.5. The cut selects events between the vertical lines.
background remaining after the other cuts have been ap- plied. Only 5% of the reference-point signal events are removed by this cut.
Thettbackground is especially difficult to reduce with- out severely impacting the efficiency for stop quark detec-
tion. However, because the sneutrino carries a significant fraction of the available energy, the remaining stop quark decay products typically have less energy than their coun- terparts from top quark decay. As a result theHTdistribu- tion, as shown in Fig. 8, peaks at lower values for stop FIG. 9 (color online). The cut groupb mTdistributions shown as in Fig.5.
TABLE VII. Expected backgrounds and observed events for cut groups a–d. The uncertainties represent the statistical and systematic uncertainties added in quadrature. The systematic uncertainty includes a 6% uncertainty on the luminosity common to all entries. Also shown are the number of expected ~t~t events for (stop, sneutrino) masses (130, 95), (140, 90), (150, 75), and ð180;50ÞGeV=c2 in cut groupsa,b,c, andd, respectively.
cut group flavor DY tt diboson lþfake Total Background ~t~t Data
a ee 0:90:4 0:10:0 0:50:1 1:70:9 3:30:9 3:20:5 1
e 0:30:1 0:30:0 1:00:2 2:01:0 3:61:0 6:60:8 2
0:60:4 0:10:0 0:30:1 0:20:1 1:10:4 2:90:4 1
eeþeþ 1:80:7 0:50:1 1:80:3 3:91:7 7:91:9 12:81:4 4
b ee 0:50:3 1:60:2 1:30:2 1:60:8 4:90:9 3:70:4 3
e 0:30:1 3:20:3 2:80:5 2:51:3 8:71:4 8:80:8 11
0:30:1 1:10:1 1:00:2 0:20:1 2:40:3 3:00:4 8
eeþeþ 1:00:4 5:80:6 5:00:9 4:31:8 16:12:3 15:61:4 22
c ee 0:80:4 3:30:4 1:40:2 1:30:6 6:70:8 6:10:5 7
e 0:30:1 7:10:8 2:90:5 2:11:1 12:51:4 12:60:9 13
0:40:2 2:30:3 1:00:2 0:20:1 3:90:4 4:20:4 9
eeþeþ 1:50:5 12:71:4 5:30:9 3:61:5 23:12:6 22:91:6 29
d ee 0:40:1 4:40:5 0:90:2 0:60:3 6:20:6 3:40:3 5
e 0:10:0 9:41:0 1:90:3 1:40:7 12:71:3 7:30:5 11
0:50:2 3:00:4 0:70:1 0:20:1 4:40:5 2:30:2 8
eeþeþ 1:00:4 16:81:8 3:50:6 2:10:9 23:32:4 13:00:9 24
quarks than for top quarks. An upper limit on HT is effective at reducing the amount oftt background, while a lower limit helps reduce the other backgrounds.
A transverse mass cut is used in the lowmcut groups (aandb) to remove most of the DY background remaining after the other cuts have been applied. The transverse mass distribution for cut groupbis shown in Fig.9.
The upper bound on the transverse momentum of the dilepton system is applied to the lowestmcut group (a) to reduce thettand diboson backgrounds. Its effectiveness decreases asmincreases. The sliding cut is determined by a linear fit to the optimal dilepton pT values as a function ofmin cut groupa:pllT<ðcm1Þ GeV=c.
VII. RESULTS AND CONCLUSIONS
The numbers of expected background and observed data events for cut groups a–d are given in Table VII. In general, the agreement between data and SM background estimations is good, although the statistical uncertainties are large. The largest deviation is an excess of data over background expectations for events with large ET. Examination of individual event properties found no evi- dence for cosmic rays or pion or kaon decays in flight.
These results are not independent observations, since there is a large overlap in events between cut groupsb,c, andd.
The largest muon excess, found in cut groupb, has a 0.4%
probability for the modeled background to fluctuate up to
the number of observed events or more (p value) when considering this cut group alone. Thepvalue includes the effects of the estimated background uncertainty.
Combining all channels in cut groupbraises the probabil- ity to 12.4%. The corresponding one-sided Gaussian sig- nificances are2:6and1:2, respectively.
A joint likelihood is formed from the product of the individual channel likelihoods. Using this likelihood, we apply a Bayesian method [19] with a flat prior for the signal to set 95% confidence level upper limits on the production cross section at each considered point in the stop-sneutrino mass plane. Systematic uncertainties are incorporated by convolving the Poisson probabilities for the signal with Gaussian distributions representing each background un- certainty. The three dilepton flavor channels are incorpo- rated into the statistical analysis simultaneously (but individually), with full treatment of correlated and uncor- related uncertainties. The correlations between the various backgrounds and the MC-generated signal are also ac- counted for.
One-dimensional curves of the upper cross section limits and the theoretical cross section are shown in Fig. 10for groups of points with fixed m. The cross section upper limits for a givenmtend to be almost independent of the stop mass.
To determine the observed (expected) exclusion contour in the stop-sneutrino mass plane, we set the number of events equal to the number of events in the data (total expected background). We then calculate the 95% confi- dence level upper limits on the stop pair cross section for
FIG. 11 (color online). Observed and expected limits in the stop-sneutrino mass plane. The LEP limits are for a mixing angle of zero which provides the greatest reach.
FIG. 10 (color online). Stop cross section upper limits for fixed stop-sneutrino mass differences. The NLOPROSPINO cross sec- tion with CTEQ6M PDFs is shown as the solid curve. The band represents the theoretical uncertainty in the cross section due to uncertainties on the renormalization and factorization scales and the PDFs.
