Article
Reference
Search for High-Mass Resonances Decaying to eμ in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We describe a general search for resonances decaying to a neutral eμ final state in pp collisions at a center-of-mass energy of 1.96 TeV. Using a data sample representing 344 pb−1 of integrated luminosity recorded by the Collider Detector at Fermilab II experiment, we compare standard model predictions with the number of observed events for invariant masses between 50 and 800 GeV/c2. Finding no significant excess (5 events observed vs 7.7±0.8 expected for Meμ>100 GeV/c2), we set limits on sneutrino and Z′ masses as functions of lepton family number violating couplings.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for High-Mass Resonances Decaying to eμ in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2006, vol. 96, no.
21, p. 211802
DOI : 10.1103/PhysRevLett.96.211802
Available at:
http://archive-ouverte.unige.ch/unige:38339
Disclaimer: layout of this document may differ from the published version.
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Search for High-Mass Resonances Decaying to e in p p Collisions at p s
1:96 TeV
A. Abulencia,23D. Acosta,17J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,16D. Ambrose,16S. Amerio,43 D. Amidei,34A. Anastassov,52K. Anikeev,16A. Annovi,18J. Antos,1M. Aoki,55G. Apollinari,16J.-F. Arguin,33 T. Arisawa,57A. Artikov,14W. Ashmanskas,16A. Attal,8F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43 H. Bachacou,28W. Badgett,16A. Barbaro-Galtieri,28V. E. Barnes,48B. A. Barnett,24S. Baroiant,7V. Bartsch,30G. Bauer,32
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C. Bromberg,35E. Brubaker,13J. Budagov,14H. S. Budd,49S. Budd,23K. Burkett,16G. Busetto,43P. Bussey,20 K. L. Byrum,2S. Cabrera,15M. Campanelli,19M. Campbell,34F. Canelli,8A. Canepa,48D. Carlsmith,59R. Carosi,46 S. Carron,15M. Casarsa,54A. Castro,5P. Catastini,46D. Cauz,54M. Cavalli-Sforza,3A. Cerri,28L. Cerrito,42S. H. Chang,27 J. Chapman,34Y. C. Chen,1M. Chertok,7G. Chiarelli,46G. Chlachidze,14F. Chlebana,16I. Cho,27K. Cho,27D. Chokheli,14
J. P. Chou,21P. H. Chu,23S. H. Chuang,59K. Chung,12W. H. Chung,59Y. S. Chung,49M. Ciljak,46C. I. Ciobanu,23 M. A. Ciocci,46A. Clark,19D. Clark,6M. Coca,15G. Compostella,43M. E. Convery,50J. Conway,7B. Cooper,30 K. Copic,34M. Cordelli,18G. Cortiana,43F. Cresciolo,46A. Cruz,17C. Cuenca Almenar,7J. Cuevas,11R. Culbertson,16
D. Cyr,59S. DaRonco,43S. D’Auria,20M. D’Onofrio,3D. Dagenhart,6P. de Barbaro,49S. De Cecco,51A. Deisher,28 G. De Lentdecker,49M. Dell’Orso,46F. Delli Paoli,43S. Demers,49L. Demortier,50J. Deng,15M. Deninno,5D. De Pedis,51
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E. Lytken,48P. Mack,25D. MacQueen,33R. Madrak,16K. Maeshima,16T. Maki,22P. Maksimovic,24S. Malde,42 G. Manca,29F. Margaroli,5R. Marginean,16C. Marino,23A. Martin,60V. Martin,38M. Martı´nez,3T. Maruyama,55 H. Matsunaga,55M. E. Mattson,58R. Mazini,33P. Mazzanti,5K. S. McFarland,49P. McIntyre,53R. McNulty,29A. Mehta,29 S. Menzemer,11A. Menzione,46P. Merkel,48C. Mesropian,50A. Messina,51M. von der Mey,8T. Miao,16N. Miladinovic,6 J. Miles,32R. Miller,35J. S. Miller,34C. Mills,10M. Milnik,25R. Miquel,28A. Mitra,1G. Mitselmakher,17A. Miyamoto,26 N. Moggi,5B. Mohr,8R. Moore,16M. Morello,46P. Movilla Fernandez,28J. Mu¨lmensta¨dt,28A. Mukherjee,16Th. Muller,25 R. Mumford,24P. Murat,16J. Nachtman,16J. Naganoma,57S. Nahn,32I. Nakano,40A. Napier,56D. Naumov,37V. Necula,17 C. Neu,45M. S. Neubauer,9J. Nielsen,28T. Nigmanov,47L. Nodulman,2O. Norniella,3E. Nurse,30T. Ogawa,57S. H. Oh,15
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T. Rodrigo,11E. Rogers,23S. Rolli,56R. Roser,16M. Rossi,54R. Rossin,17C. Rott,48A. Ruiz,11J. Russ,12V. Rusu,13 H. Saarikko,22S. Sabik,33A. Safonov,53W. K. Sakumoto,49G. Salamanna,51O. Salto´,3D. Saltzberg,8C. Sanchez,3
L. Santi,54S. Sarkar,51L. Sartori,46K. Sato,55P. Savard,33A. Savoy-Navarro,44T. Scheidle,25P. Schlabach,16 E. E. Schmidt,16M. P. Schmidt,60M. Schmitt,38T. Schwarz,34L. Scodellaro,11A. L. Scott,10A. Scribano,46F. Scuri,46
A. Sedov,48S. Seidel,37Y. Seiya,41A. Semenov,14L. Sexton-Kennedy,16I. Sfiligoi,18M. D. Shapiro,28T. Shears,29 P. F. Shepard,47D. Sherman,21M. Shimojima,55M. Shochet,13Y. Shon,59I. Shreyber,36A. Sidoti,44P. Sinervo,33
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Y. Takeuchi,55K. Takikawa,55M. Tanaka,2R. Tanaka,40N. Tanimoto,40M. Tecchio,34P. K. Teng,1K. Terashi,50 S. Tether,32J. Thom,16A. S. Thompson,20E. Thomson,45P. Tipton,49V. Tiwari,12S. Tkaczyk,16D. Toback,53S. Tokar,14
K. Tollefson,35T. Tomura,55D. Tonelli,46M. To¨nnesmann,35S. Torre,18D. Torretta,16S. Tourneur,44W. Trischuk,33 R. Tsuchiya,57S. Tsuno,40N. Turini,46F. Ukegawa,55T. Unverhau,20S. Uozumi,55D. Usynin,45A. Vaiciulis,49 S. Vallecorsa,19A. Varganov,34E. Vataga,37G. Velev,16G. Veramendi,23V. Veszpremi,48R. Vidal,16I. Vila,11R. Vilar,11
T. Vine,30I. Vollrath,33I. Volobouev,28G. Volpi,46F. Wu¨rthwein,9P. Wagner,53R. G. Wagner,2R. L. Wagner,16 W. Wagner,25R. Wallny,8T. Walter,25Z. Wan,52S. M. Wang,1A. Warburton,33S. Waschke,20D. Waters,30 W. C. Wester III,16B. Whitehouse,56D. Whiteson,45A. B. Wicklund,2E. Wicklund,16G. Williams,33H. H. Williams,45
P. Wilson,16B. L. Winer,39P. Wittich,16S. Wolbers,16C. Wolfe,13T. Wright,34X. Wu,19S. M. Wynne,29A. Yagil,16 K. Yamamoto,41J. Yamaoka,52T. Yamashita,40C. Yang,60U. K. Yang,13Y. C. Yang,27W. M. Yao,28G. P. Yeh,16J. Yoh,16
K. Yorita,13T. Yoshida,41G. B. Yu,49I. Yu,27S. S. Yu,16J. C. Yun,16L. Zanello,51A. Zanetti,54I. Zaw,21F. Zetti,46 X. Zhang,23J. Zhou,52and S. Zucchelli5
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
17University 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, Helsinki, Finland and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
23University of Illinois, Urbana, Illinois 61801, USA
211802-2
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center 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
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
37University of New Mexico, Albuquerque, New Mexico 87131, USA
38Northwestern University, Evanston, Illinois 60208, USA
39The Ohio State University, Columbus, Ohio 43210, USA
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
48Purdue University, West Lafayette, Indiana 47907, USA
49University of Rochester, Rochester, New York 14627, USA
50The Rockefeller University, New York, New York 10021, USA
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155, USA
57Waseda University, Tokyo 169, Japan
58Wayne State University, Detroit, Michigan 48201, USA
59University of Wisconsin, Madison, Wisconsin 53706, USA
60Yale University, New Haven, Connecticut 06520, USA (Received 7 March 2006; published 30 May 2006)
We describe a general search for resonances decaying to a neutralefinal state inppcollisions at a center-of-mass energy of 1.96 TeV. Using a data sample representing344 pb1of integrated luminosity recorded by the Collider Detector at Fermilab II experiment, we compare standard model predictions with the number of observed events for invariant masses between 50 and800 GeV=c2. Finding no significant excess (5 events observed vs7:70:8expected forMe>100 GeV=c2), we set limits on sneutrino and Z0masses as functions of lepton family number violating couplings.
DOI:10.1103/PhysRevLett.96.211802 PACS numbers: 13.85.Rm, 12.60.Cn, 12.60.Jv, 14.70.Pw
An observed excess of high-mass opposite sign electron- muon pairs at the Tevatron would provide evidence of physics beyond the standard model (SM). Nearly every extension of the SM predicts flavor changing neutral cur- rent effects. General supersymmetric (SUSY) models, for instance, permit the violation ofR-parity symmetry (RPV) and describe the lepton family number violating (LFV) production and decay of sneutrinos (~), the scalar super- partners of SM neutrinos [1]. Models with additional gauge symmetry can also accommodate anesignature through
LFV decays of a new heavy neutral gauge boson, the Z0 [2,3]. Signals from such processes may be easily detected at the Tevatron. SM backgrounds are small and character- ized by invariant mass spectra that lie well below the range presumed for new physics.
We search for a general signature of high-mass opposite sign epairs and are sensitive to both resonant and non- resonant production mechanisms. The CDF collaboration investigated the high-mass echannel in a Run I search for the direct RPV production and decay of the tau sneu-
trino (~) [4,5]. That analysis excluded sneutrino masses below360 GeV=c2 for a particular set of RPV coupling values. This Letter describes our continued investigation of this channel using data from Run II of the Tevatron taken with the upgraded Collider Detector at Fermilab (CDF II).
The data sample we use represents an integrated luminosity of 34421 pb1, approximately 3 times that used in Run I. We search for an e resonance by examining invariant masses (Me) between 100 and800 GeV=c2 for an excess of events. We find event counts that are consis- tent with SM predictions and use our result to constrain models of LFV~andZ0 decay by excludingecoupling values as functions of the new particle masses. In contrast to searches that utilize low-energy data, where the effects of new heavy particles may be masked by cancellations of various contributions, the interpretation of our results is robust.
The CDF II detector is an azimuthally and forward- backward symmetric apparatus designed to study pp re- actions at the Tevatron. The detector has a charged particle tracking system immersed in a 1.4 T magnetic field aligned coaxially with thepp beams. A 3.1 m long open-cell drift chamber, the Central Outer Tracker [6], covers the radial range from 40 to 137 cm and provides coverage for the pseudorapidity rangejj&1[7]. Segmented electromag- netic and hadronic sampling calorimeters surround the tracking system and measure the energies of interacting particles in the range jj<3:6 [8–10]. A set of drift chambers located outside the central hadron calorimeters and another set behind a 60 cm thick iron shield track muons with jj 0:6 [11]. Additional drift chambers and scintillation counters detect muons in the region0:6 jj 1:0. Gas Cˇ erenkov counters located in the 3:7<
jj<4:7 region [12] measure the average number of inelastic pp collisions per bunch crossing and thereby determine the beam luminosity.
We use data collected with high-PT triggers that re- quire central (jj&1:0) lepton candidates. We select events that contain at least one reconstructed electron of ET>20 GeV and one reconstructed muon with PT >
20 GeV=c. Energy deposited by the leptons in the central electromagnetic calorimeters must be of an isolated nature and should also satisfy the standard set of ‘‘tight’’ CDF lepton identification (ID) criteria [13].
We select oppositely chargedepairs and ensure that the lepton tracks come from a commonpp interaction. We reject events containing cosmic-ray muons and photon- conversion electrons by imposing additional event topol- ogy requirements [13]. In order to maximize acceptance to a broad range of new physics signatures, we do not impose requirements on missing transverse energy or jets.
SMebackgrounds include qq !Z= ! (Drell- Yan), top (tt) and diboson (WW, WZ, andZZ) production.
We determine geometric and kinematic acceptances for these processes using the PYTHIA Monte Carlo generator
[14] and a GEANT3 [15] based simulation of the CDF II detector. These simulations employ the CTEQ5L [16]
parton distribution functions (PDF’s) to model the momen- tum distribution of the initial-state partons. We define the acceptance for each background process,i, as the fraction of generated events that satisfy the lepton ID and event topology requirements. We correct the i by multiplying with trigger efficiencies (trg) measured fromW !eand Z! data and factors (freco, fID) that account for differences in lepton reconstruction and ID efficiency be- tween simulation and data [13]. We refer to the combined correction factor,trgfrecofID, as.
The expected contribution of each SM background is given by the product of i with the corresponding cross section and the integrated luminosity of our data sample. We estimate top and diboson background contri- butions using next-to-leading order (NLO) cross sections:
6.1 pb for tt[17], 12.1 pb for WW, 3.7 pb forWZ, and 1.4 pb forZZ[18]. We use a next-to-next-to-leading order continuum (Mll>30 GeV=c2) cross section for the Drell- Yan process, 337.7 pb, which we calculate by scaling the
PYTHIAleading order cross section by the ratio of NNLO to LO predictions obtained fromPHOZPRcalculations [19].
Jets that are misidentified as leptons account for ap- proximately 5% of the total background. We determine the misidentification probability (‘‘fake rate’’) [13] by examining separate data samples collected with various jet ET triggers for the occurrence of leptons. The real leptonic content of these samples is assumed to be negli- gible. We obtain fake rates from the ratios of misidentified leptons and the number of jets in these samples. We then estimate the background from this source by applying the measured fake rates, parametrized as functions of ET, to the jets in events in the high-PT sample that contain a single lepton candidate.
We present the number of observed and predicted back- ground events for twoMeregions in Table I. The domi- nant uncertainty on the background predictions arises from a 6% uncertainty in the luminosity measurement [20].
Additional uncertainty contributions include those associ- ated with the SM cross sections, fake rate measurements, lepton ID and reconstruction scale factors, and PDF model.
TABLE I. Expected and observed event totals for low- and high-massMeregions. The uncertainties shown are the combi- nation of statistical and systematic errors.
Channel 50< Me<100 GeV=c2 Me>100 GeV=c2 Z=?! 38:82:9 0:60:0
WW, WZ, ZZ 6:60:5 3:50:2
tt 3:60:5 3:20:5
Fake Lepton 2:91:7 0:40:60:4
Prediction 51:93:4 7:70:8
Observation 56 5
211802-4
Overall, we find that uncertainties on our signal acceptance and background expectations have little impact on the limits we set.
The 50Me100 GeV=c2 region listed in Table I represents an invariant mass range that is rich in back- ground. Finding good agreement between our observation and predicted background in this region, we next consider theMerange above100 GeV=c2. Here, too, we find good agreement. Figure 1 shows the observed and predicted background Me distributions over a portion of the full 50–800 GeV=c2 invariant mass range.
We quantify the consistency between theMedistribu- tions presented in Fig. 1 by performing a 2 test, using variable-width Me bins to achieve sufficient predicted background occupancies for the Gaussian approximation of Poisson statistics. The test results in a total reduced2 statistic (2=Ndof) of 14:0=111:27, under the assump- tion that systematic uncertainties in each bin are com- pletely correlated. The statistic implies a p value, the probability of finding a larger 2, of 23%. This value provides a statistical basis for accepting our results as consistent with SM expectations.
Finding no evidence of new physics in theMe spec- trum, we turn from the model-independent search to con- straints on specific models; RPV sneutrino and LFV Z0 decay. In RPV SUSY models, the s-channel dd!
~
i~i !eprocess is governed by two LFV couplings.
0
i11 determines the sneutrino production cross section fromdanddwhile 1i2gives the sneutrino branching ratio toe[21]. The indexirefers to the lepton generation of the sneutrino. We do not consider initial states that include up-type quarks since RPV sneutrino hadro-production oc- curs only through aLiQjDkterm in the superpotential [21].
The final state for this process consists ofeonly, i.e., it does not contain additional neutrinos or a SUSY lightest supersymmetric particle.
Since strong limits exist for~e and~ couplings [22], we focus instead on the third generation sneutrino,~. We assume the ‘‘single coupling dominance’’ hypothesis [22]
and set all and 0couplings but 0311and 132to zero so that contributions to theechannel originate from the~ only. Previous limits on 0311 and 132 from low-energy experiments are 0.10 and 0.05 [23], assuming squark and slepton masses of100 GeV=c2.
We show an example NLO BR [24] that corre- sponds to 0311 0:10and 132 0:05in Fig. 2. We use such curves for different 132 and 0311 values and a mass- dependent calculated from PYTHIA Monte Carlo simulations of the dd!~ ~ !e process to obtain signal expectations over the 100 to 800 GeV=c2 range.
Assuming coupling values of 132 0:05 and 0311 0:1, for example, we find that a hypothetical300 GeV=c2
~
signal consists of16:21:3events.
We calculate an upper limitBRfordd!~ ~ ! e using the numbers of observed and predicted back- ground events in bins ofMe. We scan theMe range in steps of10 GeV=c2and use the simulated mass resolution at each step to weight all data and background events between 100 and 800 GeV=c2. A Bayesian algorithm [25] is used with a uniform prior to translate the weighted event totals to a 95% confidence level (C.L.) upper limit on the number of signal events in data. We divide this limit by the integrated luminosity and signal to obtain a 95% C.L. upper limit on the BR for the process.
The for a generic spin-0 particle decaying toeis mass dependent, increasing slowly from 10% at
2) (GeV/c
µ
Me
100 200 300 400 500 600 700 800
) (pb)µ BR(e×σ95% CL
0.02 0.04 0.06 0.08 0.1
0.12 95% CL Limit (Obs.)
µ
→e ν∼τ
→ d N.L.O. d
=0.05 λ132
=0.10,
311
λ’
FIG. 2 (color online). Observed 95% C.L. upper limit on BRfordd!~ ~ !e(solid line) and the NLO prediction (dashed line) as a function of e invariant mass. Their inter- section gives a530 GeV=c2~mass limit for the values of 0311 and 132 indicated. Because of small differences in the signal acceptances, the 95% C.L. upper limit on the Z0 BR(not shown) is larger than that of the sneutrino,0:02 pbgreater at 200 GeV=c2 and 0:003 pb greater at 600 GeV=c2, for ex- ample.
2) (GeV/c
µ
Me
60 80 100 120 140 160 180 200 220 240
2Events/ 5 GeV/c
10-1
1 10
2) (GeV/c
µ
Me
60 80 100 120 140 160 180 200 220 240
2Events/ 5 GeV/c
10-1
1 10
Data τ τ
→ Z Fakes
t t Diboson
FIG. 1 (color online). Observed and predictedMeDistribu- tions. The observedeinvariant mass spectrum agrees well with that of the combined SM and fake lepton backgrounds. No events are observed in data beyond159 GeV=c2.
100 GeV=c2 to 27% at 800 GeV=c2. We account for uncertainties on the signal and background expectations in limit calculation although the final results are largely insensitive to these numbers.
The intersection of the observed upper limit and a NLO BRcurve defines a~mass limit for specific values of the 0311 and 132 couplings, as shown in Fig. 2. We con- struct exclusion regions in the 0311Me plane by plot- ting the mass limit as a function of both RPV couplings.
Figure 3 shows the exclusion region parametrized by five assumed values of 132. The plot indicates, for example, that we exclude at 95% C.L. 0311values above0:01for a sample~mass of300 GeV=c2 and 132*0:02.
Our search is also sensitive to LFVZ0decay. Thepp ! Z0!e process proceeds through diagonal U 10 cou- plings at the initial-state vertex and an off-diagonal LFV U 10coupling,Ql12, that determines theZ0branching ratio (BR) to e [2]. We set 95% C.L. limits on Ql12 as a function of theZ0mass using NLOBR’s from a group ofE6-inspired models. AlthoughE6 models do not incor- porate the LFVQl12coupling by construction, we use these models because they provide a theoretical reference by specifying initial-state Z0-quark coupling and a NLO Z0 production cross section. We utilize NLO cross sections provided by the, , secluded, and E6models [26] and extend these models to include the Ql12 coupling [27], which introducesZ0!edecays.
We set limits on Ql12 as a function of the Z0 mass following the procedure previously described for RPV~ decay. Thewe use in calculating the upper limit on BRis obtained fromPYTHIAMonte Carlo simulations in which the Z0 is required to couple to a left-handed leptonic LFV current, as may be favored inE6-motivated U 10 models that incorporate LFV [27]. Z0 acceptance (8% at 100 GeV=c2, and increasing to 20% at
800 GeV=c2) is smaller than that for the scalar sneutrino due to the spin-1 nature of the particle.
TheQl12Meregions that we exclude for the modified , , secluded, and models are shown in Fig. 4.
Assuming initial-state Z0 couplings specified by the model, for example, we exclude Ql12 values above 0:01–0:1for Z0 masses between 200 and 700 GeV=c2. Because the LFV Ql12 coupling is not intrinsic to E6 models, our limits should not be interpreted as constraints on the models themselves.
In summary, we searched in344 pb1of CDF II data for high-masseevents and found anMe distribution con- sistent with SM predictions. With no evidence for new physics, we set correlated 95% C.L. limits on the mass and 0311 and 132RPV couplings of the ~. We achieve a 170 GeV=c2 improvement in the~mass limit for spe- cific RPV coupling values of 03110:1and 1320:05 [28] and, more generally, exclude ~masses over a range of 0311 and 132 values. We also place limits on potential LFVQl12couplings of theZ0boson as a function of its mass usingE6-like models of U 10symmetry.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
We thank S. Mahji for providing us with NLO~production cross sections and B. Dobrescu, J. Kang and P. Langacker for guidance on incorporating LFVZ0decay inE6models.
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 Founda- tion; the A. P. Sloan Foundation; the Bundesministerium fuer Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research
2) (GeV/c
µ
Me
100 200 300 400 500 600 700 800
12l Q
10-3
10-2
10-1
Models : Modified E6
χψ Secluded η
FIG. 4 (color online). Ql12Meexclusion regions depicting 95% C.L. upper limits on the LFVQl12coupling in extended , ,, and secludedE6 models.
2) (GeV/c
µ
Me
100 200 300 400 500 600 700 800
311
’ λ
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
132 : λ
0.05 0.04 0.03 0.02 0.01
FIG. 3 (color online). 0311Meexclusion regions. Regions to the left of the five curves shown represent ranges of 0311 values that we exclude at 95% C.L. as a function of~ mass.
Each region corresponds to a fixed value of 132.
211802-6
Foundation; the Particle Physics and Astronomy Re- search Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comision Inter- ministerial de Ciencia y Tecnologia, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT-20002, Probe for New Physics; by the Research Fund of Istanbul University Project No. 1755/21122001; and by the Research Corporation.
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