Search for Lepton Flavor Violating Decays of a Heavy Neutral Particle
in
pp
Collisions at
p
s
1
:
8 TeV
D. Acosta,14T. Affolder,7H. 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
B. A. Barnett,21S. Baroiant,5M. Barone,15G. Bauer,27F. Bedeschi,38S. Behari,21S. Belforte,48W. H. Bell,17 G. Bellettini,38J. Bellinger,52D. Benjamin,12J. Bensinger,4A. Beretvas,13J. Berryhill,10A. Bhatti,42M. Binkley,13 D. Bisello,36M. Bishai,13R. E. Blair,2C. Blocker,4K. Bloom,28B. Blumenfeld,21S. R. Blusk,41A. Bocci,42A. Bodek,41 G. Bolla,40A. Bolshov,27Y. Bonushkin,6D. Bortoletto,40J. Boudreau,39A. Brandl,31C. Bromberg,29M. Brozovic,12
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,25L. Cerrito,20A.W. Chan,1P. S. Chang,1P. T. Chang,1J. Chapman,28C. Chen,37Y. C. Chen,1M.-T. Cheng,1
M. Chertok,5G. Chiarelli,38I. Chirikov-Zorin,11G. Chlachidze,11F. Chlebana,13L. Christofek,20M. L. Chu,1 J. Y. Chung,33W.-H. Chung,52Y. S. Chung,41C. I. Ciobanu,33A. G. Clark,16M. Coca,41A. Connolly,25M. Convery,42 J. Conway,44M. Cordelli,15J. Cranshaw,46R. Culbertson,13D. Dagenhart,4S. D’Auria,17S. De Cecco,43F. DeJongh,13
S. Dell’Agnello,15M. Dell’Orso,38S. Demers,41L. Demortier,42M. Deninno,3D. De Pedis,43P. F. Derwent,13 T. Devlin,44C. Dionisi,43J. R. Dittmann,13A. Dominguez,25S. Donati,38M. D’Onofrio,38T. Dorigo,36N. Eddy,20
K. Einsweiler,25E. Engels, Jr.,39R. Erbacher,13D. Errede,20S. Errede,20R. Eusebi,41Q. Fan,41S. Farrington,17 R. G. Feild,53J. P. Fernandez,40C. Ferretti,28R. D. Field,14I. Fiori,3B. Flaugher,13L. R. Flores-Castillo,39G.W. Foster,13 M. Franklin,18J. Freeman,13J. Friedman,27Y. Fukui,23I. Furic,27S. Galeotti,38A. Gallas,32M. Gallinaro,42T. Gao,37
M. Garcia-Sciveres,25A. F. Garfinkel,40P. Gatti,36C. Gay,53D.W. Gerdes,28E. Gerstein,9S. Giagu,43P. Giannetti,38 K. Giolo,40M. Giordani,5P. Giromini,15V. Glagolev,11D. Glenzinski,13M. Gold,31N. Goldschmidt,28J. Goldstein,13 G. Gomez,8M. Goncharov,45I. Gorelov,31A. T. Goshaw,12Y. Gotra,39K. Goulianos,42C. Green,40A. Gresele,3G. Grim,5
C. Grosso-Pilcher,10M. Guenther,40G. Guillian,28J. Guimaraes da Costa,18R. M. Haas,14C. Haber,25S. R. Hahn,13 E. Halkiadakis,41C. Hall,18T. Handa,19R. Handler,52F. Happacher,15K. Hara,49A. D. Hardman,40R. M. Harris,13 F. Hartmann,22K. Hatakeyama,42J. Hauser,6J. Heinrich,37A. Heiss,22M. Hennecke,22M. Herndon,21C. Hill,7 A. Hocker,41K. D. Hoffman,10R. Hollebeek,37L. Holloway,20S. Hou,1B. T. Huffman,35R. Hughes,33J. Huston,29 J. Huth,18H. Ikeda,49C. Issever,7J. Incandela,7G. Introzzi,38M. Iori,43A. Ivanov,41J. Iwai,51Y. Iwata,19B. Iyutin,27 E. James,28M. Jones,37U. Joshi,13H. Kambara,16T. Kamon,45T. Kaneko,49J. Kang,28M. Karagoz Unel,32K. Karr,50
S. Kartal,13H. Kasha,53Y. Kato,34T. A. Keaffaber,40K. Kelley,27M. Kelly,28R. D. Kennedy,13R. Kephart,13 D. Khazins,12T. Kikuchi,49B. Kilminster,41B. J. Kim,24D. H. Kim,24H. S. Kim,20M. J. Kim,9S. B. Kim,24 S. H. Kim,49T. H. Kim,27Y. K. Kim,25M. Kirby,12M. Kirk,4L. Kirsch,4S. Klimenko,14P. Koehn,33K. Kondo,51
J. Konigsberg,14A. Korn,27A. Korytov,14K. Kotelnikov,30E. Kovacs,2J. Kroll,37M. Kruse,12V. Krutelyov,45 S. E. Kuhlmann,2K. Kurino,19T. Kuwabara,49N. Kuznetsova,13A. T. Laasanen,40N. Lai,10S. Lami,42S. Lammel,13
J. Lancaster,12K. Lannon,20M. Lancaster,26R. Lander,5A. Lath,44G. Latino,31T. LeCompte,2Y. Le,21J. Lee,41 S.W. Lee,45N. Leonardo,27S. Leone,38J. D. Lewis,13K. Li,53C. S. Lin,13M. Lindgren,6T. M. Liss,20J. B. Liu,41T. Liu,13
Y. C. Liu,1D. O. Litvintsev,13O. Lobban,46N. S. Lockyer,37A. Loginov,30J. Loken,35M. Loreti,36D. Lucchesi,36 P. Lukens,13S. Lusin,52L. Lyons,35J. Lys,25R. Madrak,18K. Maeshima,13P. Maksimovic,21L. Malferrari,3 M. Mangano,38G. Manca,35M. Mariotti,36G. Martignon,36M. Martin,21A. Martin,53V. Martin,32M. Martı´nez,13
J. A. J. Matthews,31P. Mazzanti,3K. S. McFarland,41P. McIntyre,45M. Menguzzato,36A. Menzione,38P. Merkel,13 C. Mesropian,42A. Meyer,13T. Miao,13R. Miller,29J. S. Miller,28H. Minato,49S. Miscetti,15M. Mishina,23 G. Mitselmakher,14Y. Miyazaki,34N. Moggi,3E. Moore,31R. Moore,28Y. Morita,23T. Moulik,40M. Mulhearn,27
A. Mukherjee,13T. Muller,22A. Munar,38P. Murat,13S. Murgia,29J. Nachtman,6V. Nagaslaev,46S. Nahn,53 H. Nakada,49I. Nakano,19R. Napora,21F. Niell,28C. Nelson,13T. Nelson,13C. Neu,33M. S. Neubauer,27D. Neuberger,22 C. Newman-Holmes,13C-Y. P. Ngan,27T. Nigmanov,39H. Niu,4L. Nodulman,2A. Nomerotski,14S. H. Oh,12Y. D. Oh,24
T. Ohmoto,19T. Ohsugi,19R. Oishi,49T. Okusawa,34J. Olsen,52W. Orejudos,25C. Pagliarone,38F. Palmonari,38 R. Paoletti,38V. Papadimitriou,46D. Partos,4J. Patrick,13G. Pauletta,48M. Paulini,9T. Pauly,35C. Paus,27D. Pellett,5 A. Penzo,48L. Pescara,36T. J. Phillips,12G. Piacentino,38J. Piedra,8K. T. Pitts,20A. Pomposˇ,40L. Pondrom,52G. Pope,39
T. Pratt,35F. Prokoshin,11J. Proudfoot,2F. Ptohos,15O. Pukhov,11G. Punzi,38J. Rademacker,35A. Rakitine,27
F. Ratnikov,44H. Ray,28D. Reher,25A. Reichold,35P. Renton,35M. Rescigno,43A. Ribon,36W. Riegler,18F. Rimondi,3 L. Ristori,38M. Riveline,47W. J. Robertson,12T. Rodrigo,8S. Rolli,50L. Rosenson,27R. Roser,13R. Rossin,36C. Rott,40 A. Roy,40A. Ruiz,8D. Ryan,50A. Safonov,5R. St. Denis,17W. K. Sakumoto,41D. Saltzberg,6C. Sanchez,33A. Sansoni,15 L. Santi,48S. Sarkar,43H. Sato,49P. Savard,47A. Savoy-Navarro,13P. Schlabach,13E. E. Schmidt,13M. P. Schmidt,53 M. Schmitt,32L. Scodellaro,36A. Scott,6A. Scribano,38A. Sedov,40S. Seidel,31Y. Seiya,49A. Semenov,11F. Semeria,3 T. Shah,27M. D. Shapiro,25P. F. Shepard,39T. Shibayama,49M. Shimojima,49M. Shochet,10A. Sidoti,36J. Siegrist,25 A. Sill,46P. Sinervo,47P. Singh,20A. J. Slaughter,53K. Sliwa,50F. D. Snider,13R. Snihur,26A. Solodsky,42T. Speer,16
M. Spezziga,46P. Sphicas,27F. Spinella,38M. Spiropulu,10L. Spiegel,13J. Steele,52A. Stefanini,38J. Strologas,20 F. Strumia,16D. Stuart,7A. Sukhanov,14K. Sumorok,27T. Suzuki,49T. Takano,34R. Takashima,19K. Takikawa,49 P. Tamburello,12M. Tanaka,49B. Tannenbaum,6M. Tecchio,28R. J. Tesarek,13P. K. Teng,1K. Terashi,42S. Tether,27 J. Thom,13A. 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
F. Ukegawa,49T. Unverhau,17T. Vaiciulis,41J. Valls,44 A. Varganov,28E. Vataga,38S. Vejcik III,13G. Velev,13 G. Veramendi,25R. Vidal,13I. Vila,8R.Vilar,8I. Volobouev,25M. von der Mey,6D.Vucinic,27R. G. Wagner,2R. L. Wagner,13 W. Wagner,22N. B. Wallace,44Z. Wan,44C. Wang,12M. J. Wang,1S. M. Wang,14B. Ward,17S. Waschke,17T. Watanabe,49 D. Waters,26T. Watts,44M. Weber,25H. Wenzel,22W. C. Wester III,13B. Whitehouse,50A. B. Wicklund,2E. Wicklund,13
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. Zucchelli31
(The 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
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
43Instituto Nazionale de 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 20 June 2003; published 22 October 2003)
We report on a search for a high mass, narrow width particle that decays directly toe,e, or. We use approximately110 pb1of data collected with the Collider Detector at Fermilab from 1992 to
1995. No evidence of lepton flavor violating decays is found. Limits are set on the production and decay of sneutrinos withR-parity violating interactions.
DOI: 10.1103/PhysRevLett.91.171602 PACS numbers: 13.85.Rm, 11.30.Hv, 12.60.Jv, 14.80.Ly
Particles that decay toe,e, oroccur in a num-ber of extensions to the standard model. Examples in-clude Higgs bosons in models with multiple Higgs doublets [1– 3], sneutrinos in supersymmetric models with R-parity violation (RPV) [4 – 6], horizontal gauge bosons [7], and Z0 bosons [8]. In this Letter, we report results from a search based on final states containinge. We are sensitive toeand modes through ! and !e, respectively. We analyze data from pp collisions at center of mass energy ps1:8 TeV re-corded with the Collider Detector at Fermilab (CDF) during the 1992 to 1995 Tevatron run. The integrated luminosity is approximately110 pb1.
The CDF detector has been described in detail else-where [9]. This analysis makes use of several detector subsystems. The position ofppcollisions along the beam line is measured in the vertex time projection chambers (VTX). The central tracking chamber (CTC), located within the 1.4 T magnetic field of a superconducting solenoid, measures the momentum of charged particles. The transverse momentum resolution for muon and charged hadron tracks in the pseudorapidity interval
jj<1:1 that are constrained to originate at the beam line is better than 0:1%pT, where pT is measured in
GeV=cand is the momentum component transverse to the beam line. Sampling calorimeters surround the solenoid. The central electromagnetic calorimeter (CEM) covers
jj<1:1 and measures the energy of electromagnetic showers with a resolution of about 13:7= ET
p
2%, where means addition in quadrature and the transverse energyET is measured in GeV. The transverse energy is
defined as Esin, with E being the shower energy measured in the calorimeter and the polar angle of the energy flow, from the pp interaction vertex to the calorimeter deposition. Within the CEM, proportional chambers (CES) measure the transverse shape of showers. Drift chambers located outside the calorimeters detect muons in the region jj<1. A measure of the energy carried by neutrinos escaping the detector is the missing transverse energyE6 T, calculated from the vector sum of the energy depositions in the calorimeters and the mo-mentum of muon tracks.
Events containing an electron and a muon are recorded by an assortment of single lepton, dilepton, and jet trig-gers [10]. We select events that have an electron withET >
20 GeV, a muon withpT >20 GeV=c, and a primarypp
interaction vertex within 60 cm of the center of the detector. The electron and muon must have opposite charges. We identify electrons and muons using criteria that retain efficiency for very high momentum particles [11]. Electrons must have a shower contained within the sensitive region of the CEM and have a CTC track with pT >13 GeV=c that matches the position of the shower in the CES. The CEM determines the electron energy. Requirements on the pT of the associated CTC track are kept loose because electrons can lose energy in the tracking volume due to bremsstrahlung. In cases in which an electron hasET <100 GeVorpT<25 GeV=c,
than four. Electrons that appear to be one leg of a photon conversion — because the second leg is found or because the CTC track is not confirmed by hits in the VTX — are removed. Muons must have a track in the CTC that originates from the primary vertex and matches a track segment in the muon system. The energy along the muon path through the calorimeters must be consistent with a minimum ionizing particle. Electrons and muons are both required to be isolated from other energy deposition in the calorimeter. The combination of triggers is, within 1 standard deviation, fully efficient for events that satisfy the offline selection.
Backgrounds with two isolated highpT leptons arise
from standard model production ofZ=,WW, WZ,ZZ, and tt. The cross sections for the first four of these processes are calculated at next-to-leading order using MCFMv1.0 [12] and MRST99 parton distribution func-tions [13]. Thettproduction cross section is taken from the parametrized formula of Ref. [14] evaluated at the Tevatron average top quark mass of174:3 GeV=c2 [15].
For each of the five processes, the fraction of events expected to pass the offline selection (the acceptance) is calculated by Monte Carlo using PYTHIAv5.7 [16], the CDF detector simulation, and the same analysis software used to select events in the data. The efficiency of the lepton identification requirements in the simulation is calibrated using electrons and muons from Z decays. For each background process, the expected number of events is estimated to be BABnee=eeAee, where B
is the cross section for the background process,AB is the acceptance for the background process,neeis the number ofee events observed in the data with a mass in the region of theZpeak,ee is theZ= cross section times branching fraction toee, andAeeis the acceptance for ee. By normalizing to ee data, we eliminate un-certainties in the background level due to the integrated luminosity and the Z= cross section times branching fraction to leptons, and we reduce the uncertainty from lepton efficiencies. The tt, WW,WZ, and ZZ cross sec-tions are assigned uncertainties of25%,11%,10%, and
10%, respectively. Other uncertainties are from the de-tector model (6%), Monte Carlo statistics (2%–4%),Z!
eestatistics (2%), particle identification efficiencies (2%), and trigger efficiencies (1%). The remaining backgrounds arise from particles produced in jets. These include in-strumental fakes and real leptons from b- and c-quark decays. All of them are denoted false leptons. The false lepton backgrounds are estimated using a method similar to that of Ref. [17], in which the probability for a false lepton to appear isolated is measured in a control sample of dijet data.
In a WW or tt event, the electron and the muon, originating from differentW bosons, have largely inde-pendent directions, spin correlations not withstanding. To reduce these backgrounds, we require that the angle be-tween the electron and muon in the plane transverse to the beam be at least 120. This is almost fully efficient for
two-body decays of a particle as heavy as theZboson. For theechannel, there are no further requirements. Ane event is additionally classified as aneevent if the angle between the 6ET and the muon, ;6ET, is less than
60. This eliminates events that are not consistent with !, since the tau is energetic enough that its decay products are nearly collinear. Likewise, an eevent is additionally classified as aevent ife;6ET<60.
The distribution ofe;6ETversusE6 Tis shown in Fig. 1.
Since the electrons and muons are nearly back-to-back in ,;6ETis approximatelye;6ET 180.
There are19ecandidates in the data. Four of these are alsoecandidates, and 12 are alsocandidates. The contributions of the various backgrounds to each channel are listed in Table I. The dominant source of background isZ=!where one tau decays toeand the other to . The small contribution from Z=! is from cases in which one muon is mistaken for an electron after radiating an energetic photon.
The distributions of the lepton pair masses mll0 are
shown in Fig. 2. For the eand hypotheses, mll0 is
calculated assuming that the tau momentum compo-nents are p
x plx 6Ex, pyply 6Ey, and pz plz
1 6ET=plT, whereplis the momentum of the electron or
muon to which the tau is assumed to have decayed [2,3]. The data show no indication of a signal peak. The dis-tribution of observed events is compared to the expected
FIG. 1. The angle in the plane transverse to the beam line between the direction of the missing energy and the electron versus the missing transverse energy: data (upper left); simu-lated Z=!!e, the dominant background (upper right); and a hypothetical signal ~!!e for a sneu-trino mass of100 GeV=c2(lower left) and200 GeV=c2(lower
background shape using a Kolmogorov test. The test statistic probability is 19%, 65%, and 74% for e, e, and , respectively, consistent with the absence of a signal.
As a specific signal model, we consider the process dd!~!ll0 mediated by RPV interactions [4 –6]. The RPV sneutrino couplings allowed in the super-symmetric Lagrangian are "ijk~jLek
ReiL~iLe j RekL
"0ijk~i Ld
k Rd
j
L H:c:, where the indices i, j, and k label
generations; "ijk is nonzero only for i < j; and mix-ing is ignored [4,18]. Constraints on the couplmix-ing strengths have been derived from measurements of low
energy processes [18,19]. The upper bound on the coupling that mediates dd!~ is "0311<0:11 mdd~R=100 GeV. The limit is slightly stronger for "0211 and much stronger for "0111. Of the couplings that contribute to ~; !ll0, those with the loosest bounds are "132<0:062 m~R=100 GeV, "231<
0:070 mee~R=100 GeV,"233<0:070 m~R=100 GeV,
and"122<0:049 m~R=100 GeV. Limits on–e
con-version in nuclei severely restrict the ""0 products that contribute to the e channel, for example, j"231"0311j<
4:1109 assuming sparticle masses of 100 GeV=c2
[20]. Searches at CERNeecollider rule out sneutrino massesm~<86 GeV=c2if any one (but only one) of the "constants is nonzero [21]. Previous CDF searches have examined scenarios with"0121 [22] and"0333[23] nonzero. We simulate the signal process by generating events using the heavy Higgs (H0) process in PYTHIA. The H0 decay table is modified to include each of thee,e, and modes in turn while all other decay channels are switched off. All initial states except forddare inhibited. Events are generated for nine particle massesm~between 50 and 800 GeV=c2. The events are passed through the
CDF detector simulation and the analysis software used for the data. For eachm~and each decay mode, the mean and rmsmll0 is computed. The rms widths of thee,e,
and mass distributions are 3, 6, and 7 GeV=c2,
re-spectively, form~100 GeV=c2. Form~400 GeV=c2
they are 30, 14, and60 GeV=c2. The broadening withm ~
is due to the muonpT resolution and, to a lesser degree,
the electron energy resolution. At low m~, the resolution
for theeandmodes is poorer than foredue to the inclusion of 6ET. Theeresolution degrades more slowly
than the others because the muons from tau decays have lower momenta.
To study the full range ofm~without gaps in which a signal might hide, we count events within 3 standard deviations of the mean mll0 for a sequence ofm
~
values
starting at 50 GeV=c2 with step size from theith to the
(i1)th value equal to one-tenth of the rms of themll0
distribution at theith point. The mean and rms of themll0 TABLE I. The expected number of background events and the observed number of
candi-dates in each channel.
Source e e
Z=! 13:910:99 5:200:43 6:480:59 Z=! 0:270:16 0 0:270:16 WW 2:370:32 0:420:07 0:450:08 WZ 0:130:02 0:030:01 0:040:01 ZZ 0:0240:004 0:0070:001 0:0070:002
tt 1:340:36 0:400:11 0:350:10
Falsee 0:850:44 0:040:23 0:960:32
False 0:980:27 0:060:11 1:070:25
Total background 19:881:42 5:950:55 9:620:81
Data 19 4 12
FIG. 2. The reconstructed mass of the lepton pairs. The points show the nineteen ecandidates (upper), the four e
distribution for eachm~is linearly interpolated between
values at the generatedm~points. A 3 standard deviation window provides good statistical sensitivity while incur-ring little dependence on any possible systematic errors in the width or mean.
For each decay mode, we derive a95%C.L. upper limit [24] on B using the number of events, expected background, and acceptance in the mass window corre-sponding to eachm~point. Figure 3 shows the limits as a
function ofm~. The limits onB~!e and B~!are higher than the limit onB~!e
because of the tau branching ratio to leptons and, par-ticularly at lowm~, because the leptons from tau decays
tend to fall below the20 GeV=c pT threshold.
As a benchmark, Fig. 3 also shows the theoretical cross section times branching fraction for dd!~!e plus dd!~!e as a function of m~ in the case "0311 0:1and"132 0:05. The curve is obtained using next-to-leading order values ofdd!~for"0 0:01
[25,26], which we scale by "0=0:012. B~!e is
calculated assuming that weak decays of the sneutrino are kinematically forbidden and that the only nonzero RPVcouplings are"0311and"132.
In conclusion, we find no evidence for new particles with lepton flavor violating decays. We set limits on the cross section for single sneutrino production times the branching fractionsB~!e, B~!e, and B~!
as a function of the sneutrino mass. For a sneutrino mass of200 GeV=c2, the95%C.L. upper limits onB are 0.14, 1.2, and 1.9 pb for the e,e, and modes, respectively.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions. We are grateful to Debajyoti Choudhury and Swapan Majhi for providing NLO sneutrino cross sections for
s
p
1:8 TeV. This work was supported by the U.S. Department of Energy and National Science Foun-dation; 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 Foun-dation; the Bundesministerium fuer Bildung und Forschung, Germany; the Korea Science and Engineer-ing Foundation (KoSEF); the Korea Research Foun-dation; and the Comision Interministerial de Ciencia y Tecnologia, Spain.
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