Article
Reference
Search for Pair Production of Supersymmetric Top Quarks in Dilepton Events from pp Collisions at s√=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present the results of a search for pair production of the supersymmetric partner of the top quark (the top squark t1) decaying to a b quark and a chargino χ±1 with a subsequent χ±1 decay into a neutralino χ01, lepton ℓ, and neutrino ν. Using a data sample corresponding to 2.7 fb−1 of integrated luminosity of pp collisions at s√=1.96 TeV collected by the CDF II detector, we reconstruct the mass of top squark candidate events and fit the observed mass spectrum to a combination of standard model processes and t1t1 signal. We find no evidence for t1t1 production and set 95% C.L. limits on the masses of the top squark and the neutralino for several values of the chargino mass and the branching ratio B(χ±1→χ01ℓ±ν).
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Pair Production of Supersymmetric Top Quarks in Dilepton Events from pp Collisions at s√=1.96 TeV. Physical Review Letters , 2010, vol. 104, no. 25, p. 251801
DOI : 10.1103/PhysRevLett.104.251801
Available at:
http://archive-ouverte.unige.ch/unige:38654
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for Pair Production of Supersymmetric Top Quarks in Dilepton Events from p p Collisions at ffiffiffi
p s
¼ 1:96 TeV
T. Aaltonen,24J. Adelman,14T. Akimoto,56B. A´ lvarez Gonza´lez,12,tS. Amerio,44b,44aD. Amidei,35A. Anastassov,39 A. Annovi,20J. Antos,15G. Apollinari,18A. Apresyan,49T. Arisawa,58A. Artikov,16W. Ashmanskas,18A. Attal,4
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(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 Bologna, I-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
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
22Glasgow University, Glasgow G12 8QQ, United Kingdom
251801-2
23Harvard University, Cambridge, Massachusetts 02138, USA
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
25University of Illinois, Urbana, Illinois 61801, USA
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
28Center 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
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, 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
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
38University of New Mexico, Albuquerque, New Mexico 87131, USA
39Northwestern University, Evanston, Illinois 60208, USA
40The Ohio State University, Columbus, Ohio 43210, USA
41Okayama University, Okayama 700-8530, Japan
42Osaka City University, Osaka 588, Japan
43University of Oxford, Oxford OX1 3RH, United Kingdom
44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44bUniversity of Padova, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
47bUniversity of Pisa, I-56127 Pisa, Italy
47cUniversity of Siena, I-56127 Pisa, Italy
47dScuola Normale Superiore, I-56127 Pisa, Italy
48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
49Purdue University, West Lafayette, Indiana 47907, USA
50University of Rochester, Rochester, New York 14627, USA
51The Rockefeller University, New York, New York 10021, USA
52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
52bSapienza Universita` di Roma, I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy
55bUniversity of Trieste/Udine, I-33100 Udine, Italy
56University of Tsukuba, Tsukuba, Ibaraki 305, Japan
57Tufts University, Medford, Massachusetts 02155, USA
58Waseda University, Tokyo 169, Japan
59Wayne State University, Detroit, Michigan 48201, USA
60University of Wisconsin, Madison, Wisconsin 53706, USA
61Yale University, New Haven, Connecticut 06520, USA (Received 6 December 2009; published 23 June 2010)
We present the results of a search for pair production of the supersymmetric partner of the top quark (the top squark~t1) decaying to abquark and a chargino ~1 with a subsequent~1 decay into a neutralino~01, lepton ‘, and neutrino. Using a data sample corresponding to 2:7 fb1 of integrated luminosity ofppcollisions at ffiffiffi
ps
¼1:96 TeVcollected by the CDF II detector, we reconstruct the mass of top squark candidate events and fit the observed mass spectrum to a combination of standard model processes and~t1~t1signal. We find no evidence for~t1~t1production and set 95% C.L. limits on the masses of
the top squark and the neutralino for several values of the chargino mass and the branching ratio Bð~1 !~01‘Þ.
DOI:10.1103/PhysRevLett.104.251801 PACS numbers: 12.60.Jv, 14.65.Ha, 14.80.Ly
Supersymmetry (SUSY) [1] is a plausible extension to the standard model (SM) of particle physics that naturally solves the hierarchy problem, predicts the unification of the gauge coupling constants, and provides a possible candi- date for dark matter. In SUSY, a new spin-based symmetry turns a bosonic state into a fermionic state (and vice versa), postulating the existence of a superpartner for each of the known fundamental particles. To be reconciled with ex- perimental data, SUSY must be broken, and thus super- symmetric particles are expected to be much heavier than their SM partners. An exception to this might come from the partner of the top quarkt, the top squark, whose low- mass eigenstate~t1may be lighter than the top quark due to the substantial top-Yukawa coupling [2]. This mass in- equality m~t1 &mt is favored in supersymmetric electro- weak baryogenesis scenarios [3].
In canonical SUSY models R parity is conserved, the lightest supersymmetric particle (LSP) is the neutralino~01, and top squarks~t1are expected to be pair produced via the strong interaction. The~t1~t1cross section depends primarily on the mass of the top squarkm~t1, and at the Tevatron is expected to be an order of magnitude smaller than that for top quarks of the same mass [4,5]. If the chargino~1 is lighter than the~t1, the decay channel~t1!b~1 becomes dominant. Subsequent chargino decays via ~1 !~01‘ result in experimental event signatures with two energetic, oppositely charged leptons, two jets from the bottom quarks, and a large imbalance in energy from the lack of detection of the neutrinos and neutralinos. This event signature is identical to the dilepton final state of top pair decays (tt!WþbWb!‘þb‘00b). Therefore, an admixture of top squark events with the top dilepton events could impact measurements of the properties of the top quark, such as the mass value. This search was in part motivated by apparent inconsistencies in the top mass measurements between different top decay channels ob- served in the early CDF and D0 Run II data [6]. Previous searches for top squark decays~t1!b~1 [7] did not ex- clude any region in the SUSY parameter space.
In this Letter we present the results of a search for pair production of scalar top quarks, each decaying as ~t1 ! b~1 !b~01‘. We analyze a data set corresponding to 2:7 fb1 of integrated luminosity from pp collisions col- lected by the upgraded Collider Detector at Fermilab (CDF II) [8,9], and fit the data with the ~t1~t1 production hypothesis.
We identify and record events containingeorcandi- dates with large transverse momenta (pT 18 GeV=c) using high-speed trigger electronics. The performance of the trigger and lepton identification (ID) algorithms is
described elsewhere [10]. We identify final state quarks as jets of hadrons in the calorimeter. Jet reconstruction employs an iterative cone-based clustering algorithm that associates calorimeter energy deposits within a cone of R ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðÞ2þ ðÞ2
p ¼0:4. The energies of recon- structed jets and the missing transverse energy (E6 T) [11]
are corrected for detector nonuniformity and other effects [12]. Bottom quark candidates are identified (or
‘‘btagged’’) through the presence within the jet cone of a displaced secondary vertex arising from the decay of a long-lived bottom hadron [13].
The first stage of the ~t1~t1 candidate event selection requires two leptons (eor) withpT>20 GeV=c,jj<
2:0ð1:0ÞforeðÞ, at least one of which is isolated [14], and m‘þ‘0>20 GeV=c2. We also require two or more jets withET>12 GeVwithin the regionjj<2:4, andE6 T>
20 GeV. For events with leptons compatible with originat- ing from aZboson in the mass window from76 GeV=c2to 106 GeV=c2, we apply a requirement on the missing trans- verse energy significance E6 sigT >4 ffiffiffiffiffiffiffiffiffiffi
pGeV
[15]. Selected events are divided into two categories based on whether any of the jets is identified as abjet (b-tagged channel) or not (non-b-tagged channel). Further optimized event se- lection criteria are used in the last stage of the analysis.
The dominant SM process that contributes to the dileptonþjetsþE6 T event signature is tt production.
Other SM processes include Z=þjets, diboson, and Wþjets production, where a real lepton comes from the W decay and one of the jets is misidentified as a second lepton. We use thePYTHIAv6.216 Monte Carlo (MC) event generator [16] to simulate~t1~t1,tt, and diboson processes.
The ~t1~t1 signal is normalized according to the next-to- leading order (NLO) theoretical cross section obtained fromPROSPINO2[17] using the CTEQ6M [18] parton den- sity functions (PDF). For tt we use the NLO theoretical cross section value of 7.3 pb [5], corresponding to the world average top mass of 172:5 GeV=c2 [19], which is dominated by the measurements in the leptonþjets chan- nel of ttdecays. Diboson processes (WW, WZ, ZZ) are normalized to their NLO theoretical cross sections [20].
Z= events with associated jets are simulated with the
ALPGENv2.13 matrix element generator [21], interfaced to
PYTHIAv6.325, and normalized to data in theZ-mass-peak region. The detector response in all MC samples is mod- eled by a GEANT-based CDF II detector simulation [22].
The Wþjets background is modeled using data by mea- suring relative rates of jets being misidentified as charged leptons in inclusive jet data samples and applying them to data events with exactly one lepton plus jets. We validate the background modeling of dilepton events by comparing 251801-4
the predictions with observations using control samples that are independent of the signal sample. These include samples of events with lowE6 T, events with zero or one jet, and events with same-sign charged leptons.
To enhance the search sensitivity, we perform a kine- matic reconstruction of events under the ~t1~t1 production and decay hypothesis. We use as inputs the measured four- momenta of the two leptons and of the two largestET jets, and the6~ET. Because of the unknown masses of the super- symmetric~1 and~01, and because the two neutrinos and the two massive neutralinos escape detection, the kinemat- ics of~t1~t1 events is severely underconstrained. Therefore, we employ the following strategy. First, we usem~1 as a fixed parameter, and perform the reconstruction for differ- ent values of m~1. Second, we treat the ~01 pair corre- sponding to each~t1 decay as one ‘‘massive particle.’’ To compensate for nonresonant structure of the invariant mass of the~01pair we assign to this ‘‘massive particle’’ a large width. Based on studies carried out on MC samples for a wide range of neutralino masses (m~01 46–90 GeV=c2) we fix the values of m~01 and ~0
1 to 75 GeV=c2 and 10 GeV=c2, respectively. Third, to avoid the twofold am- biguity in assigning abjet to a lepton, we always choose the combination that yields the smallest sum of invariant masses of a pairedbjet and lepton. This approach identi- fies the correct pairing in 90%of cases in the ~t1 mass regime considered.
The system of kinematic equations consists of con- straints imposed on the particle massesm~1,m~01,m~t1 ¼ m~t1, and the requirement of transverse momentum conser- vation: 6~ET ¼ ~pTð~01Þ1þ ~pTð~01Þ2. If the azimuthal di- rections1and2of the four-momenta of theð~01Þ1and ð~01Þ2 pairs are fixed, the event kinematics (with the exception of the singular points12¼k, wherek is an integer) is constrained. There exist four solutions due to the twofold ambiguities in resolving thezcomponents of theð~01Þ1andð~01Þ2four-momenta. We perform a scan of the entire parameter space of azimuthal angles wherein we repeat the reconstruction for different values of (1,2), avoiding the aforementioned singular points. The~t1 mass is reconstructed by minimizing the event2, which takes the following form:
2 ¼ X
k¼1;2
ðmfitð~0
1Þkm~01Þ2 2~0
1
þðmfitð~0
1‘Þkm~1Þ2 2~
1
þðmfitð~0
1‘jÞkmrec~t1 Þ2 2~t
1
þ X
i¼2‘;2jets
ðpfitT;ipmeasT;i Þ 2pT;i :
(1) Here we assume ~
1 2 GeV=c2 and ~t1 1:5 GeV=c2, the k index represents the decay products
from ~t1 or t~1, respectively, and themfit are the invariant masses of the final decay products from~t1 decays. We let the four-momenta of the leptons and the jets vary in the fit, and use the MINUITpackage [23] to minimize the 2. At each step during the minimization procedure the 6~ET is recalculated according to the values ofpfitT of the leptons and jets. The longitudinal components of ð~01Þ1 and ð~01Þ2 are free parameters in the fit with starting values initialized to the values corresponding to solutions of the system of kinematic equations. All four starting values are tried in the fit, but only the one that gives the lowest2 is kept. The valuemrec~t1 at which2is minimized yields the~t1
reconstructed mass for a given pair of the azimuthal angles (1,2). Finally, we integratemrec~t1 ð1; 2Þweighted by the goodness of fit term e2ð1;2Þ over 1 and 2 to obtain the~t1 reconstructed mass for each event. Running the reconstruction algorithm over simulated ~t1~t1 events yields a distribution with a peak near the generated ~t1
mass, as shown in Fig. 1, and provides discrimination between a~t1~t1 signal and SM backgrounds [24].
We perform an extended likelihood fit of the observed mass spectrum simultaneously in the b-tagged and the non-b-tagged channels. To quantify the level of agreement we employ a modified frequentist method,CLs[25], based on a log-likelihood ratio test statistic, which involves com- puting p values under the hypothesis of the SM back- ground only and the hypothesis of signal plus background. The systematic uncertainties for both signal and background, described below, enter the fit as Gaussian- constrained nuisance parameters. The uncertainties due to kinematic mismodeling are taken into account by allowing the reconstructed mass distributions to change according to the values of the nuisance parameters [26].
Imperfect knowledge of various experimental and theo- retical parameters leads to systematic uncertainties that degrade our sensitivity to a ~t1~t1 signal. The dominant
100 120 140 160 180 200 220 240 260 280 300 320 2Events/20 GeV/c
5 10 15 20 25 30
320
=132.5 GeV/c2 t1
M~
=105.8 GeV/c2 1
M ±
=47.6 GeV/c2 1
M 0
6.0 excluded Signal
t1
~ t1
~
B-Tagged Channel
100 120 140 160 180 200 220 240 260 280 300 320
100 320
-1) CDF Run II data (2.7 fb
2)
=172.5 GeV/c Top (M t
* + Jets Z/
Other SM Non-B-Tagged Channel
2) Mass (GeV/c t1
Reconstructed~
FIG. 1. The reconstructed~t1mass distribution. The dashed line represents an example of the~t1~t1signal distribution.
systematic effect is due to the uncertainties in the NLO theoretical cross sections for~t1~t1 andttproduction. These uncertainties come from two sources: the renormalization and factorization scale (11% and 7% for ~t1~t1 and tt, re- spectively) and PDFs (14% and 7%) [4,5]. We assume that the scale uncertainty is uncorrelated for ~t1~t1 and ttpro- cesses, while the PDF uncertainty is fully correlated. The theoretical uncertainty of the diboson cross sections is 10%
[20], and assumed to be uncorrelated with other systematic uncertainties. The experimental uncertainties applied to MC-based background estimates include those due to jet energy scale (3%),b-tagging probability (5%), lepton ID and trigger efficiencies (1% per lepton), initial and final state radiation (2%), and the integrated luminosity (6%).
The uncertainty on Wþjets is dominated by the uncer- tainties in the rate to misidentify jets as leptons (30%), while the uncertainty on Z=þjets comes from MC mismodeling of the high-E6 T tail, jet multiplicity distribu- tion, andZ=þheavy-flavor contribution (16%).
Prior to looking at data in the signal sample we study the sensitivity of the search, taking into account all systematic effects, for various event selection criteria imposed sepa- rately for theb-tagged and the non-b-tagged channels. An algorithm based on biological evolution (a so-called ge- netic algorithm) [27] is employed to determine the most sensitive selection criteria. Requirements yielding poorer expected 95% C.L. limit are culled, while those improving the limit are bred together until reaching a plateau. This procedure optimizes the event selection criteria directly to produce the best expected 95% C.L. limit in the no-signal hypothesis.
In theb-tagged (non-b-tagged) channel the optimization procedure yields the following event selection criteria [28]:
the leading jet ET is required to be greater than 15 (20) GeV, and the subleading jetET must be greater than 12 (20) GeV. In both channels we requireE6 T >20 GeV, while this requirement is tightened to 50 GeV in the non-b-tagged channel if there is a lepton or jet within an azimuthal angle of 20from the 6~ET direction. Because of the fact that the~t1is a scalar particle, and the top quark is a fermion, the angular distributions of their final decay prod- ucts are very distinct. Therefore we implement an addi- tional topological cut in both the b-tagged and non-b-tagged channels to suppressttevents:
XpT<jj ‘‘
2 325þ215
GeV=c; (2)
whereP
pTis the scalar sum of transverse momenta of the leptons, jets and the E6 T, the jj and ‘‘ are the azimuthal angles between the jets and leptons, respec- tively, and the numerical values are the result of the opti- mization procedure. This requirement rejects about 50% of ttevents and only about 10% of~t1~t1 events.
After applying these event selection requirements we obtain the numbers of predicted and observed events listed in Table I. The data distributions of the reconstructed~t1
mass in both channels are shown in Fig.1, together with the expectations from SM processes and an example of ~t1~t1
signal. The data are consistent with the SM alone and there is no evidence of~t1~t1 production. We use these results to calculate the 95% C.L. exclusion limits in them~01 vsm~t1
plane for several values of the branching ratio Bð~1 ! ~01‘Þ and m~1, assuming equal branching ratios into different lepton flavors and Bð~t1 !~1bÞ ¼100%. The limits for two different values of chargino mass are pre- sented in Fig.2. For a given branching ratio of the pair of top squarks decaying into leptons, equal to B2ð~1 ! ~01‘Þ, we exclude the top squark and neutralino masses below the respective curve shown in the plot. The values B2ð~1 !~01‘Þ are expected to range from almost 100%, corresponding to the scenario with light sleptons and sneutrinos (m‘~,m~*m~t1), where the leptonic decay of the chargino goes mostly through virtual sleptons and sneutrinos, down to 11%, where sleptons and sneutrinos are heavy (m‘~,m~ mW) and the chargino decay through a virtualWis dominant. For the scenario corresponding to the case in which the masses of the chargino and neutralino are near the current lower LEP exclusion limits, m~1 ¼ 105:8 GeV=c2, m~01 ¼47:6 GeV=c2 [29], we exclude a top squark with masses between 128 and 135 GeV=c2 at 95% C.L. independent of the value of B2ð~1 !~01‘Þ. The limits obtained are applicable to anyRparity conserv- ing SUSY scenario where the neutralino is the LSP and the
~t1 decays exclusively into ~1b, and are the first lower limits on~t1 mass in this mode.
In conclusion, we have presented the results of a search for pair production of supersymmetric top quarks decaying
TABLE I. The expected event yields from SM processes with the total uncertainties and the observed numbers of events in the signal region.
Events per2:7 fb1in the signal region.
Top Z=þjets Diboson Wþjets Total Data
btag 49:06:9 4:00:4 0:50:1 2:80:9 56:47:2 57
no tag 25:23:3 25:05:6 6:01:3 9:82:9 65:99:8 65
251801-6
via~t1 !b~1 !b~01‘using a data sample correspond- ing to 2:7 fb1 of integrated luminosity in 1.96 TeV pp collisions. Our fit to the observedmrec~t1 distribution reveals no evidence for~t1~t1 production, and we place the world’s first limits on the masses of~t1 and~01for several values of m~1 and branching ratio ofBð~1 !~01‘Þin this mode.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, U.K.;
the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic
Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.
aDeceased.
bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.
cVisitor from Universiteit Antwerpen, B-2610 Antwerp, Belgium.
dVisitor from University of Bristol, Bristol BS8 1TL, U.K.
eVisitor from Chinese Academy of Sciences, Beijing 100864, China.
fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
gVisitor from University of California Irvine, Irvine, CA 92697, USA.
hVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.
iVisitor from Cornell University, Ithaca, NY 14853, USA.
jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.
kVisitor from University College Dublin, Dublin 4, Ireland.
lVisitor from University of Edinburgh, Edinburgh EH9 3JZ, U.K.
mVisitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.
nVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.
oVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.
pVisitor from Queen Mary, University of London, London, E1 4NS, U.K.
qVisitor from University of Manchester, Manchester M13 9PL, U.K.
rVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.
sVisitor from University of Notre Dame, Notre Dame, IN 46556, USA.
tVisitor from University de Oviedo, E-33007 Oviedo, Spain.
uVisitor from Texas Tech University, Lubbock, TX 79609, USA.
vVisitor from IFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain.
wVisitor from University of Virginia, Charlottesville, VA 22904, USA.
xVisitor from Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany.
yOn leave from J. Stefan Institute, Ljubljana, Slovenia.
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120 130 140 150 160 170 180
2) GeV/c0 1m(
50 60 70 80
90 ±)=105.8 GeV/c2
m( 1
1b)=1
±
t1
BR(~
Observed 95% CL
-1) CDF Run II (2.7 fb
l)=0.11
1 0 1
(±
BR2
l)=0.25
1 0 1
( ±
BR2
l)=0.50
1 0 1
( ±
BR2
l)=1.0
1 0 1
(±
BR2
a)
) GeV/c2
t1
m(~
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2) GeV/c0 1m(
50 60 70 80
90 ±)=125.8 GeV/c2
m( 1
l)=0.25
1 0 1
( ±
BR2
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1 0 1
(±
BR2
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1 0 1
( ±
BR2
Excluded by LEP
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