Article
Reference
Search for Pair Production of Scalar Top Quarks in R -Parity Violating Decay Modes in pp Collisions at s√=1.8 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present the results of a search for pair production of scalar top quarks (t˜1) in an R-parity violating supersymmetry scenario in 106 pb−1 of pp¯ collisions at s√=1.8 TeV collected by the Collider Detector at Fermilab. In this mode each t˜1 decays into a τ lepton and a b quark. We search for events with two τ's, one decaying leptonically (e or μ) and one decaying hadronically, and two jets. No candidate events pass our final selection criteria. We set a 95%
confidence level lower limit on the t˜1 mass at 122 GeV/c2 for Br(t˜1→τb)=1.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Pair Production of Scalar Top Quarks in R -Parity Violating Decay Modes in pp Collisions at s√=1.8 TeV. Physical
Review Letters , 2004, vol. 92, no. 05, p. 051803
DOI : 10.1103/PhysRevLett.92.051803
Available at:
http://archive-ouverte.unige.ch/unige:38065
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for Pair Production of Scalar Top Quarks in R-Parity Violating Decay Modes in pp Collisions at
p s
1:8 TeV
D. Acosta,14T. Affolder,25H. Akimoto,51M. G. Albrow,13D. Ambrose,37D. Amidei,28K. Anikeev,27J. Antos,1 G. Apollinari,13T. Arisawa,51A. Artikov,11T. Asakawa,49W. Ashmanskas,2F. Azfar,35P. Azzi-Bacchetta,36 N. Bacchetta,36H. Bachacou,25W. Badgett,13S. Bailey,18P. de Barbaro,41A. Barbaro-Galtieri,25V. E. Barnes,40
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E. Brubaker,25N. Bruner,31J. Budagov,11H. S. Budd,41K. Burkett,18G. Busetto,36K. L. Byrum,2S. Cabrera,12 P. Calafiura,25M. Campbell,28W. Carithers,25J. Carlson,28D. Carlsmith,52W. Caskey,5A. Castro,3D. Cauz,48 A. Cerri,38L. Cerrito,20A.W. Chan,1P. S. Chang,1P. T. Chang,1J. Chapman,28C. Chen,37Y. C. Chen,1M.-T. Cheng,1
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F. DeJongh,13S. Dell’Agnello,15M. Dell’Orso,38S. Demers,41L. Demortier,42M. Deninno,3D. De Pedis,43 P. F. Derwent,13T. Devlin,44C. Dionisi,43J. R. Dittmann,13A. Dominguez,25S. Donati,38M. D’Onofrio,38T. Dorigo,36
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K. Goulianos,42C. Green,40A. Gresele,3G. Grim,5C. Grosso-Pilcher,10M. Guenther,40G. Guillian,28 J. Guimaraes da Costa,18R. M. Haas,14C. Haber,25S. R. Hahn,13E. Halkiadakis,41C. Hall,18T. Handa,19R. Handler,52 F. Happacher,15K. Hara,49A. D. Hardman,40R. M. Harris,13F. Hartmann,22K. Hatakeyama,42J. Hauser,6J. Heinrich,37
A. Heiss,22M. Hennecke,22M. Herndon,21C. Hill,7A. Hocker,41K. D. Hoffman,10R. Hollebeek,37L. Holloway,20 S. Hou,1B. T. Huffman,35R. Hughes,33J. Huston,29J. Huth,18H. Ikeda,49C. Issever,7J. Incandela,7G. Introzzi,38 M. Iori,43A. Ivanov,41J. Iwai,51Y. Iwata,19B. Iyutin,27E. James,28M. Jones,37U. Joshi,13H. Kambara,16T. Kamon,45 T. Kaneko,49J. Kang,28M. Karagoz Unel,32K. Karr,50S. Kartal,13H. Kasha,53Y. Kato,34T. A. Keaffaber,40K. Kelley,27 M. Kelly,28R. D. Kennedy,13R. Kephart,13D. Khazins,12T. Kikuchi,49B. Kilminster,41B. J. Kim,24 D. H. Kim,24
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P. Sphicas,27F. Spinella,38M. Spiropulu,10L. Spiegel,13J. Steele,52A. Stefanini,38J. Strologas,20F. Strumia,16 D. Stuart,7A. Sukhanov,14K. Sumorok,27T. Suzuki,49T. Takano,34R. Takashima,19K. Takikawa,49P. Tamburello,12
M. Tanaka,49B. Tannenbaum,6M. Tecchio,28R. J. Tesarek,13P. K. Teng,1K. Terashi,42S. Tether,27J. Thom,13 A. S. Thompson,17E. Thomson,33R. Thurman-Keup,2P. Tipton,41S. Tkaczyk,13D. Toback,45K. Tollefson,29 D. Tonelli,38M. Tonnesmann,29H. Toyoda,34W. Trischuk,47J. F. de Troconiz,18J. Tseng,27D. Tsybychev,14N. Turini,38
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T. Wilkes,5H. H. Williams,37P. Wilson,13B. L. Winer,33D. Winn,28S. Wolbers,13D. Wolinski,28J. Wolinski,29 S. Wolinski,28M. Wolter,50S. Worm,44X. Wu,16F. Wu¨rthwein,27J. Wyss,38U. K. Yang,10W. Yao,25G. P. Yeh,13P. Yeh,1
K. Yi,21J. Yoh,13C. Yosef,29T. Yoshida,34I. Yu,24S. Yu,37Z. Yu,53J. C. Yun,13L. Zanello,43A. Zanetti,48 F. Zetti,25and S. Zucchelli3
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
4Brandeis University, Waltham, Massachusetts 02254, USA
5University of California at Davis, Davis, California 95616, USA
6University of California at Los Angeles, Los Angeles, California 90024, USA
7University of California at Santa Barbara, Santa Barbara, California 93106, USA
8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
11Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
12Duke University, Durham, North Carolina 27708, USA
13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
14University of Florida, Gainesville, Florida 32611, USA
15Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
16University of Geneva, CH-1211 Geneva 4, Switzerland
17Glasgow University, Glasgow G12 8QQ, United Kingdom
18Harvard University, Cambridge, Massachusetts 02138, USA
19Hiroshima University, Higashi-Hiroshima 724, Japan
20University of Illinois, Urbana, Illinois 61801, USA
21The Johns Hopkins University, Baltimore, Maryland 21218, USA
22Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
23High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
24Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea; Seoul National University, Seoul 151-742, Korea; and SungKyunKwan University, Suwon 440-746, Korea
25Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
26University College London, London WC1E 6BT, United Kingdom
27Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
28University of Michigan, Ann Arbor, Michigan 48109, USA
29Michigan State University, East Lansing, Michigan 48824, USA
30Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
31University of New Mexico, Albuquerque, New Mexico 87131, USA
051803-2 051803-2
32Northwestern University, Evanston, Illinois 60208, USA
33The Ohio State University, Columbus, Ohio 43210, USA
34Osaka City University, Osaka 588, Japan
35University of Oxford, Oxford OX1 3RH, United Kingdom
36Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
37University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
38Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
39University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
40Purdue University, West Lafayette, Indiana 47907, USA
41University of Rochester, Rochester, New York 14627, USA
42Rockefeller University, New York, New York 10021, USA
43Istituto Nazionale di Fisica Nucleare, Sezione di Roma, University di Roma I, ‘‘La Sapienza,’’ I-00185 Roma, Italy
44Rutgers University, Piscataway, New Jersey 08855, USA
45Texas A&M University, College Station, Texas 77843, USA
46Texas Tech University, Lubbock, Texas 79409, USA
47Institute of Particle Physics, University of Toronto, Toronto M5S 1A7, Canada
48Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
49University of Tsukuba, Tsukuba, Ibaraki 305, Japan
50Tufts University, Medford, Massachusetts 02155, USA
51Waseda University, Tokyo 169, Japan
52University of Wisconsin, Madison, Wisconsin 53706, USA
53Yale University, New Haven, Connecticut 06520, USA (Received 7 May 2003; published 5 February 2004)
We present the results of a search for pair production of scalar top quarks (~tt1) in anR-parity violating supersymmetry scenario in 106 pb1 of pp collisions at ps
1:8 TeV collected by the Collider Detector at Fermilab. In this mode each~tt1decays into alepton and abquark. We search for events with two’s, one decaying leptonically (e or ) and one decaying hadronically, and two jets. No candidate events pass our final selection criteria. We set a 95% confidence level lower limit on the~tt1 mass at122 GeV=c2 forBr~tt1!b 1.
DOI: 10.1103/PhysRevLett.92.051803 PACS numbers: 14.80.Ly, 11.30.Er, 12.60.Jv, 13.85.Rm
Many supersymmetry (SUSY) models [1] predict that the first two generations of SUSY partners of the quarks and the leptons (squarks and sleptons) are approximately mass degenerate and heavy. However, the mass of the lightest top squark (~tt1 or ‘‘stop’’) can be relatively light due to a large mixing between the interaction eigenstates,
~ttL and ~ttR. This mixing depends on the top Yukawa cou- pling. Because of the heavy top (t) quark mass,Mt, it is possible thatM~tt1< Mt[2].
R parity (Rp) is a multiplicative quantum number defined asRp 13BL2S, whereS,B, andLare the spin, baryon, and lepton numbers of a particle [3]. Rp distinguishes standard model (SM) particles (Rp 1) from SUSY particles (Rp 1). Conservation of Rp requires SUSY particles to be produced in pairs and to decay ultimately to SM particles plus the stable lightest SUSY particle.Rpconservation is not required by SUSY.
It is motivated phenomenologically by limits on the pro- ton lifetime, the absence of flavor-changing neutral cur- rents, etc. ViableRpviolating (6Rp) models can be built by adding explicit 6Rp terms with trilinear couplings (ijk, 0ijk,00ijk) and spontaneous 6Rp terms with bilinear cou- plings (i) to the SUSY Lagrangian [4,5], wherei,j, andk are generation indices. These couplings allow B or L violating interactions and, if033k or3 is nonzero, a ~tt1
may decay directly to SM final states which are experi- mentally observable.
In pp collisions, stop pairs can be produced via Rp-conserving processes. In6Rp scenarios each stop could decay into a tau () lepton and a bottom (b) quark with a branching ratio, Br, which depends on the coupling con- stants of the particular model. A good final state topology identifies either an electron or a muon (‘eor) from the!‘‘ decay, as well as a hadronically decaying tau (h) lepton, and two or more jets.
We present the results of a search for ~tt1~tt1 !‘hjj events, in the framework of6Rp– minimal supersymmetric standard model (MSSM), using106 pb1ofppcollisions
at
ps
1:8 TeV collected by the Collider Detector at Fermilab (CDF) [6,7] during the 1992 –1995 run of the Tevatron (Run 1). In CDF theppcollision vertex (zvtx) [8]
is measured with a time projection chamber. The trans- verse momentum (pT) of charged particles havingjj<
1:0 is measured by a central tracking chamber (CTC) immersed in a uniform 1.4 T solenoidal magnetic field [8]. Electromagnetic (EM) and hadronic (HAD) calorim- eters, segmented in a projective tower geometry, surround the solenoid and cover the regionjj<4:2. They identify electrons, taus, and jets and measure the missing trans- verse energy (6ET). The central strip chamber (CES),
embedded in the central EM calorimeter near shower maximum, aids in electron identification and 0! identification from h decays. A muon subsystem is lo- cated outside the HAD calorimeter and has trigger cover- age for the regionjj<0:6.
Events must pass a three-level trigger system [6] which requires a single lepton (e or ) with pT >8 GeV=c (jj<1:0 for electrons and jj<0:6 for muons) [9].
Offline, the lepton must havepT >10 GeV=c, originate from the event vertex, and pass more restrictive identi- fication and isolation requirements [7,10]. An event is removed as aZboson candidate if it contains a second, loosely identified same-flavor opposite-sign lepton with 76< M‘‘<106 GeV=c2. All events are required to have jzvtxj 60 cm.
An inclusive‘hsubsample is made by requiring each event to further contain a highpT, isolated, hadronically decaying lepton candidate with pTh >15 GeV=c [11]
andjj<1:0. Ahcandidate is identified as a calorime- ter cluster satisfying the following requirements [12]: (i) not identified as aneor a; (ii) one or three tracks with pT >1 GeV=c in a 10 cone around the calorimeter cluster center; (iii) the scalar sum of thepT of all tracks inR0:4around the cluster center, excluding those in the 10 cone, less than1 GeV=c; (iv) fewer than three 0 !candidates identified in the CES; (v) more than 4 GeV of ET measured in the calorimeter; (vi) 0:5<
ET=pTh <2:0(1.5) for one track (three tracks); (vii) the width of the calorimeter cluster in-! space less than 0.11 (0.13) –0:0250:034 ET GeV=100 for one track (three tracks); and (viii) the invariant mass reconstructed from tracks and0’s less than1:8 GeV=c2. The charge of theh is defined as the sum of the track charges, and is required to have unit magnitude and have the opposite- sign (OS) of the‘. A total of 642 events pass the above requirements; 16 of these have two or more jets (recon- structed by a fixed cone algorithm withR0:4[13]) with ET >15 GeV and jj<2:4. The four ‘hjets candidates found in the search for tt! WbWb [12] pass the kinematic requirements for this search.
The dominant backgrounds come from Z=! jets,tt, diboson (WW,WZ,ZZ) production, and fake ‘h combinations from Wjets and QCD events. Monte Carlo (MC) programs with CTEQ4L parton distribution functions (PDFs) [14] and a detector simulation are used to estimate the background rates from Z=,W,tt, and diboson events. All SM processes except W=Zjets events are generated using ISAJET [15];
VECBOS[16] is used for vector boson plus jets production and decay, followed by HERWIG[17] for the fragmenta- tion and hadronization of the quarks and gluons. The cross sections for Z=, tt, and WW production are normalized to CDF measurements [18– 21] and next-to- leading order (NLO) calculations forWZandZZproduc- tion are used [22]. The number of QCD fake events is estimated from the data assuming that the number of OS
events, after subtracting off the nonfake contribution, is identical to the number of like-sign (LS) events observed in the data as expected from QCD sources, i.e.,NQCDOS NLSdataNMCLS.
The final data selection is optimized to maximize the sensitivity for ~tt1~tt1 production over simulated SM back- grounds and LS data. To reduce the Wjets events we require MT‘;6ET<35 GeV=c2 where MT‘;6Et is the transverse mass of the‘and 6ET, defined asMT‘;6ET
2p‘T6ET1cos!‘6ET q
, and!‘6ET is the azimuthal angle difference between the ‘ and 6ET. To reduce the QCD backgrounds we require P
pT‘; h;6ET p‘T pTh 6ET >75 GeV=c. The MT‘;6ET cut precedes the PpT‘; h;6ET cut because of possible charge correla- tions between the lepton fromWdecay and a fakehfrom a jet. Figure 1 shows the MT‘;6ET and P
pT‘; h;6ET distributions for the OS ‘h 2jet sample. A control sample of ‘h0 jet events with similar kinematic requirements [MT‘;6ET<25 GeV=c2, jpp~T‘6E6E~Tj>
25 GeV=c] is selected to show that the backgrounds are well modeled, dominated by realZ!production, and for later use in the acceptance calculations. Figure 2
0 2 4 6 8 10 12
0 20 40 60 80 100 120 140
CDF Run 1, 106 pb-1 Data (OS τh + ≥ 2 jets)
∼t
1t∼1 MC (M t = 100 GeV/c2) _
∼
W+jets + tt- + diboson Z/γ*(→τ+τ-)+jets QCD
MT( ,E/T) (GeV/c2)
Events / 7 GeV/c2
MT( ,E/T) < 35 GeV/c2 cut
0 1 2 3 4 5 6 7 8 9
0 25 50 75 100 125 150 175 200 After MT( ,E/T) cut
ΣpT( ,τh,E/T) (GeV/c)
Events / 15 GeV/c
ΣpT( ,τh,E/T) > 75 GeV/c cut
FIG. 1 (color online). The final data selection criteria for the OS‘h 2 jet sample. The arrows show the final event selection requirements.
051803-4 051803-4
shows the charged track multiplicity of theh’s (remov- ing the1and3-prong requirements) for this sample.
A breakdown of the backgrounds and data is given in Table I. The backgrounds appear well modeled. A total of 3:21:40:3 events are predicted from all SM sources, domi- nated by Z! jets production. No candidate events pass the final~tt1~tt1 selection criteria, which is ex- pected in3%of experiments when taking into account the statistical and systematic uncertainties.
In order to set limits on~tt1~tt1 production and decay, the acceptances and efficiencies are normalized to the rate of Z! 0jet decays using the following relation:
*~tt1~tt1!bb Nobs
~tt1~tt1NBG
~tt1~tt1
NZobsNZBG
RaccRtrig*Z
BrZ!; (1) where Nobs
~tt1~tt1
and NBG
~tt1~tt1
(NZobs and NBGZ ) are the number of candidates observed in the data and expected back- grounds in the 2jet=~tt1~tt1 (0jet=Z) selections, Racc is
the ratio of theZto~tt1~tt1 acceptances andRtrig is the ratio of the trigger efficiencies. The primary advantage of this approach is that potential systematic uncertainties in the estimate of identification and isolation efficiencies are reduced in the ratio of~tt1~tt1 toZproduction.
The 95% confidence level (C.L.) limits on *~tt1~tt1! bb in thee,, and combined channels are found using Eq. (1) and come from a Bayesian integration of the likelihood as a function of the cross section, integrating over the correlated and uncorrelated systematic uncer- tainties on the expected signal with a flat prior. TheRacc is a function of the M~tt1 and varies in the range 0:34<
Reacc<2:15 0:35< Racc<1:87 for the e () channel over the range 70< M~tt1<130 GeV=c2. The Rtrig varies between 0:95< Retrig<0:97(0:99< Rtrig<1:00) for the e() channel with an uncertainty of1%. [The accep- tance and trigger efficiencies for theZcontrol sample are 1.19% (0.69%) and 74.5% (83.0%) for thee() channel.]
Assuming lepton universality gives *ZBrZ! *ZBrZ!‘‘ 23112statsyspb [23]. The dominant uncertainty is due to the statistical uncertainty in NZobsNZBG and is 17.0% (24.9%) [24].
Additional uncertainty comes from our estimation of Racc which is dominated by the variation in the ~tt1~tt1 acceptance from choices of the QCD renormalization scaleQ2, PDFs, amount of gluon radiation, the jet energy scale, and the statistical uncertainty in the MC samples [25]. The total uncorrelated uncertainties vary between 17.1 and 17.7% (25.1 and 25.4%), and the total correlated uncertainties vary between 9.3 and 14.1%.
Figure 3 shows the final 95% C.L. upper limits on the cross section times Br for thee,, and combined chan- nels, along with the NLO prediction of the production cross section [26]. The lower limits on M~tt1 are 110 and 75 GeV=c2 for the e and channels, where we have assumedBr1. Combining the two results yields a limit of 122 GeV=c2. Since our analysis does not distinguish the quark flavors in jet reconstruction, these results are equally valid for any 033k coupling. These results sub- stantially improve on the currently most stringent mass limit [27] which excludesM~tt1below93 GeV=c2.
In conclusion, we searched for ~tt1~tt1 production using 106 pb1 data in pp collisions at
ps
1:8 TeV. We examined the ‘h 2 jet final state within an 6Rp SUSY scenario in which each ~tt1 decays to a lepton
TABLE I. Summary of the number of OS events in the data and expectations for the background sources as each selection requirement is applied.
Sample tt Diboson Wjets Z=! QCD Tot Nobs
OS‘h 1:20:3 2:30:8 1016 2259 30118 63121 642
‘h 2jets 1:00:2 0:40:1 3:40:4 7:70:5 83 213 16
MT‘;6ET<35 GeV=c2 0:150:07 0:140:06 0:50:2 6:00:4 83 153 10 PpT‘; h;6ET>75 GeV=c 0:150:07 0:080:03 0:20:1 2:80:3 01:40 3:21:40:3 0
0 10 20 30 40 50 60
0 1 2 3 4 5 6 7
Data (OS τh+0 jets) Z→τ+τ-
QCD
Ntrack(τ)
Events
CDF Run 1, 106 pb-1
FIG. 2 (color online). The number of charged tracks in each h candidate for the opposite-sign (OS) ‘h0 jet control sample. The data are compared to the MC expectation (all backgrounds are summed) which is dominated by real h’s fromZ!production.
and abquark via nonzero0333or3couplings. No events pass our selection criteria and we set a 95% C.L. lower limit on the~tt1 mass at122 GeV=c2 forBr1.
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, Science, Sports and Culture 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 fuer Bildung und Forschung, Germany; the Korea Science and Engineering Foundation (KoSEF), the Korea Research Foundation; and the Comision Interministerial de Ciencia y Tecnologia, Spain.
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(PROSPINO, µ=M∼)t1 95% C.L. upper limit
Combined results eτ + ≥2 jets µτ+ ≥2 jets
CDF Run 1, 106 pb-1 Br(t∼1→τ+b) = 100%
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M∼ (GeV/c2) σ(t1t_ 1→τ+ τ- bb_ ) (pb)∼∼
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FIG. 3 (color online). The 95% C.L. upper limit on cross section times Br for ~tt1~tt1 production compared to the NLO calculations.
051803-6 051803-6
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