Search for R-parity breaking sneutrino exchange at LEP

Search for R-Parity Breaking Sneutrino Exchange at LEP M. Acciarri, O. Adriani, M. Aguilar-Benitez, S. Ahlen, J. Alcaraz, G. Alemanni, J. Allaby, A. Aloisio, G. Alverson, M G. Alviggi, et al. To cite this version: M. Acciarri, O. Adriani, M. Aguilar-Benitez, S. Ahlen, J. Alcaraz, et al.. Search for R-Parity Breaking Sneutrino Exchange at LEP. Physics Letters B, Elsevier, 1997, 414, pp.373-381. ฀in2p3-00000070฀ HAL Id: in2p3-00000070 http://hal.in2p3.fr/in2p3-00000070 Submitted on 16 Nov 1998 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-PPE/97-99 28 July 1997 Search for R{Parity Breaking Sneutrino Exchange at LEP The L3 Collaboration Abstract We report on a search for R{parity breaking e ects due to supersymmetric tau{ sneutrino exchange in the reactions e+e ! e+e and e+e ! + at centre{of{ mass energies from 91 GeV to 172 GeV, using the L3 detector at LEP. No evidence for deviations from the Standard Model expectations of the measured cross sections and forward{backward asymmetries for these reactions is found. Upper limits for the couplings 131 and 232 for sneutrino masses up to m   190 GeV are determined from an analysis of the expected e ects due to tau sneutrino exchange. Submitted to Phys. Lett. B Introduction Supersymmetric theories [1] are considered to be the most promising and natural extensions of the Standard Model (SM). The Minimal Supersymmetric Standard Model [2] conserves baryon number B and lepton number L, usually represented as the existence of a multiplicative discrete symmetry called R{parity [3]. R{parity conservation is not ensured by gauge invariance. The most general superpotential even for a minimal supersymmetric model contains interactions violating B and L. The high{x, high{Q2 events observed by the H1 [4] and ZEUS [5] collaborations at HERA have revived the interest in theories with broken R{parity. In this case the superpotential contains L{violating trilinear Yukawa couplings, WR6 lll, of two leptons to scalar sleptons [6,7]: WR6 lll = ijk LiL LjLERk ; (1) where L stands for the left{handed doublets of leptons, E for the right{handed singlets of charged leptons, and ijk are generation indices. At least two di erent generations are coupled in the purely leptonic vertices, since ijk 6= 0 only for i < j . Depending on the value of the couplings  the e ects due to sneutrino exchange in leptonic e+e processes at LEP energies can be large [8]. The e ects of sneutrino exchange manifest themselves as deviations from the Standard Model expectations of the measured cross sections, , and forward{backward asymmetries, Afb, for e+ e and + nal states: e+ e ! e+e ; e+ e ! +  : (2) Mass limits for sneutrinos within the Minimal Supersymmetric Standard Model have been derived from the measurement of the invisible width of the Z: m > 41:8 GeV, if three sneutrino generations are assumed, and m > 37:1 GeV, in case of one sneutrino generation [9]. The search for sneutrinos from models with broken R{parity is extended over the entire centre{of{ mass energy range of LEP1 and LEP2. Measurements of Lepton{Pair Production Measurements of cross sections and forward{backward asymmetries for reactions (2) have been p by the L3 experiment at centre{of{mass energies, s, around the Z peak [10], at p pperformed s = 130:3 and 136.3 GeV [11], and at s = 161:3, 170.3 and 172.3 GeV [12]. The selection procedure and the measurements with their statistical and systematic errors are described in these references [10{12]. The L3 detector and its performance are described in [13]. In except for the e+e ! e+ e cross section measurement the low{statistics results at palls cases = 170:3 GeV are combined with the results at 172.3 GeV to a centre{of{mass energy of 172.1 GeV. For e+ e nal states both leptons have to satisfy 44 <  < 136, where  is the angle between the incoming electron and the outgoing fermion. For muon{pair production around the Z peak the angular acceptance is given by jcos  j < 0:8. At higher energies, the angular range is extended to jcos  j < 0:9. At the Z resonance 72258, and at higher energies 1099 electron{ and muon{pair events have been selected. These correspond to an integrated luminosity of 40.8 pb 1 and 26.2 pb 1 , respectively. 2 For comparison of the measured values with the predictions from the Standard Model or from theories with broken R{parity the measurements are not extrapolated to the full solid angle. The model expectations are computed with the same angular cuts as applied to the data. Exchange of Tau Sneutrinos Lepton{pair production in e+ e collisions is a ected by the R{parity violating exchange of sneutrinos in the s{ and/or t{channel. The corresponding diagrams for e+e nal state events are shown in Figure 1. For muon{ and tau{pair production only the s{channel diagram is present in the Standard Model. Diagrams with sneutrino, j , exchange are present if the couplings ijk are not zero at both vertices. Interactions which violate R{parity will introduce at energies well below the mass of the exchanged sneutrino the usual four{fermion contact interactions. In this letter, the impact of nearby resonances is investigated, which necessitate the inclusion of propagator and non{zero width e ects [8]. Lepton number conservation places severe constraints on the couplings  in the general case. Only if the e ective four{fermion Lagrangian does not violate L conservation, the allowed values reach m ); (3)   0:1  ( 200 GeV where m is the mass of the exchanged sneutrino. This is possible if only some of the operators with a particular generation structure are present in Equation (1). Two main options lead to s{channel sneutrino exchange and potentially large e ects [8]:  case (A): one single Yukawa coupling  is much larger than all the others, so that the latter are negligible; two options are present: the muon sneutrino coupling to electrons 121 is heavily constrained by the R{violating interpretation of the HERA data [8], but the coupling of tau sneutrinos to electrons 131 can be sizeable and is considered further.  case (B): two Yukawa couplings violating the same lepton avour are much larger than the others; for s{channel exchange mainly the case where the coupling of tau sneutrinos to electrons, 131 , and to muons, 232 , are not equal to zero is of interest. The cross section for e+ e nal states in case (A) contains both s{ and t{channel contributions from the exchange of /Z bosons and of tau sneutrinos and anti{sneutrinos. Muon{pair production is not a ected. In tau{pair production only very small e ects due to t{channel electron sneutrino exchange occur. In case (B), the reaction e+e ! + receives large additional contribution from tau sneutrino exchange in the s{channel and e ects similar to those in e+ e ! e+ e are obtained. Deviations from the Standard Model predictions are computed with a program provided by the authors of [8]. Initial{state radiation (ISR) changes the e ective centre{of{mass energy in a large fraction of the observed events. These e ects are taken into account by computing the rst order exponentiated cross sections and asymmetries following [14]. Other QED corrections give smaller e ects and are neglected. Examples of calculations before and after the inclusion of initial{state radiation are shown in Figure 2a for e+e ! e+e and in Figure 2b for e+e ! + . Large e ects are observed, 3 especially in case of a radiative return to a nearby resonance. The typical interference curve in e+ e nal state events is smeared after the inclusion of ISR, due to the integration over areas with constructive and destructive interference. Analysis and Results For each centre{of{mass energy the deviations from the Standard Model resulting from tau sneutrino exchange are computed, as a function of the tau sneutrino mass and coupling strength . This is done on a two{dimensional grid which contains 150 values for m  between 60 and 210 GeV, and 100 values for  between 0.01 and 0.20. Above the Z peak, the events at high e ective centre{of{mass energy are used. They do not include the radiative return to the Z and have the largest sensitivity to a nearby resonance. For e+ e nal states the programs ALIBABA [15] and TOPAZ0 [16] are used at the Z peak and at higher energies, respectively. For muon{pair production the program ZFITTER [17] is used throughout. The error on a deviation consists of three parts, which are combined in quadrature: the statistical, the systematic and the theoretical error. The systematic error for cross sections includes the luminosity error of our measurements ranging from 0.25% to 0.6%. Thus the cross section measurements depend on the normalisation. For Afb measurements there is no such dependence, and the statistical errors above the Z peak are still dominant. The measurements of  and Afb are independent and are combined. No statistically signi cant deviations from the Standard Model predictions are observed, see [10{12]. The maximum likelihood method is used to derive a one sided upper limit on the coupling strength  at the 90% con dence level for a given value of m  . The likelihood function, L, is constructed by combining the cross section and Afb measurements at the di erent centre{of{mass energies: 0 21 n ( (SM; ; m )  meas)2 (Afb (SM; ; m ) Ameas ) X fb   A + ln L = @ 2 2 2
 2
  Afb i=1 i   = error((SM; ; m  ) meas) ; Afb = error(Afb(SM; ; m  ) Ameas fb ) ;  (4) (5) where (SM; ; m  ) and Afb(SM; ; m  ) are the expectations for the cross section and the forward{backward asymmetry from the Standard Model combined with the additional e ect of sneutrino exchange as a function of the mass and the coupling strength, and meas and Ameas fb are the measured quantities. The index i runs over all centre{of{mass energy points. After proper normalisation the likelihood function gives the probability for ijk < lim for any value of lim in the physically allowed region. Finally, the results for the di erent sneutrino masses are combined in a single exclusion plot in the (m  ; ) plane. The results for e+ e nal states are shown in Figure 3. The LEP high energy data has larger sensitivity to new physics of this type compared to the high precision measurements at the Z peak. This is due to the fact that the absolute, and not the relative, error is crucial in this search. The results are compared with limits on R{parity breaking interactions from processes at lower energies [7]. The strongest limit on the coupling 131 comes from precise measurements of the ratio R = ( ! e )= ( !  ). Using the latest data [18], a 90% con dence level upper limit is derived and also shown in Figure 3. 4 For muon{pair production the results are shown in Figure 4, together with the 90% con dence level constraints derived from the lower energy measurements. In this case the strongest limit on 232 comes from precise measurements of the ratio R = ( ! e )= ( ! e ). In order to simplify the presentation of the limit in two dimensions, it is assumed that 131 = 232 . In both cases, large and previously unexplored areas in the (m  ; 131 ) and (m  ; 131 = 232 ) planes are excluded. Acknowledgements We wish to express our gratitude to the CERN accelerator divisions for the excellent performance of the LEP machine. We acknowledge the contributions of all the engineers and technicians who have participated in the construction and maintenance of this experiment. We are grateful to J. Kalinowski, R. Ruckl, H. Spiesberger and P. Zerwas for providing us with their code and for clarifying discussions. 5 The L3 Collaboration: M.Acciarri,29 O.Adriani,18 M.Aguilar-Benitez,28 S.Ahlen,12 J.Alcaraz,28 G.Alemanni,24 J.Allaby,19 A.Aloisio,31 G.Alverson,13 M.G.Alviggi,31 G.Ambrosi,21 H.Anderhub,51 V.P.Andreev,7 40 T.Angelescu,14 F.Anselmo,10 A.Are ev,30 T.Azemoon,3 T.Aziz,11 P.Bagnaia,39 L.Baksay,46 S.Banerjee,11 Sw.Banerjee,11 K.Banicz,48 A.Barczyk,51 49 R.Barillere,19 L.Barone,39 P.Bartalini,36 A.Baschirotto,29 M.Basile,10 R.Battiston,36 A.Bay,24 F.Becattini,18 U.Becker,17 F.Behner,51 J.Berdugo,28 P.Berges,17 B.Bertucci,36 B.L.Betev,51 S.Bhattacharya,11 M.Biasini,19 A.Biland,51 G.M.Bilei36 J.J.Blaising,4 S.C.Blyth,37 G.J.Bobbink,2 R.Bock,1 A.Bohm,1 L.Boldizsar,15 B.Borgia,39 D.Bourilkov,51 M.Bourquin,21 S.Braccini,21 J.G.Branson,42 V.Brigljevic,51 I.C.Brock,37 A.Buni,18 A.Buijs,47 J.D.Burger,17 W.J.Burger,21 J.Busenitz,46 A.Button,3 X.D.Cai,17 M.Campanelli,51 M.Capell,17 G.Cara Romeo,10 G.Carlino,31 A.M.Cartacci,18 J.Casaus,28 G.Castellini,18 F.Cavallari,39 N.Cavallo,31 C.Cecchi,21 M.Cerrada,28 F.Cesaroni,25 M.Chamizo,28 Y.H.Chang,53 U.K.Chaturvedi,20 S.V.Chekanov,33 M.Chemarin,27 A.Chen,53 G.Chen,8 G.M.Chen,8 H.F.Chen,22 H.S.Chen,8 X.Chereau,4 G.Chiefari,31 C.Y.Chien,5 L.Cifarelli,41 F.Cindolo,10 C.Civinini,18 I.Clare,17 R.Clare,17 H.O.Cohn,34 G.Coignet,4 A.P.Colijn,2 N.Colino,28 V.Commichau,1 S.Costantini,9 F.Cotorobai,14 B.de la Cruz,28 A.Csilling,15 T.S.Dai,17 R.D'Alessandro,18 R.de Asmundis,31 A.Degre,4 K.Deiters,49 D.della Volpe,31 P.Denes,38 F.DeNotaristefani,39 D.DiBitonto,46 M.Diemoz,39 D.van Dierendonck,2 F.Di Lodovico,51 C.Dionisi,39 M.Dittmar,51 A.Dominguez,42 A.Doria,31 M.T.Dova,20 D.Duchesneau,4 P.Duinker,2 I.Duran,43 S.Dutta,11 S.Easo,36 Yu.Efremenko,34 H.El Mamouni,27 A.Engler,37 F.J.Eppling,17 F.C.Erne2, J.P.Ernenwein,27 P.Extermann,21 M.Fabre,49 R.Faccini,39 S.Falciano,39 A.Favara,18 J.Fay,27 O.Fedin,40 M.Felcini,51 B.Fenyi,46 T.Ferguson,37 F.Ferroni,39 H.Fesefeldt,1 E.Fiandrini,36 J.H.Field,21 F.Filthaut,37 P.H.Fisher,17 I.Fisk,42 G.Forconi,17 L.Fredj,21 K.Freudenreich,51 C.Furetta,29 Yu.Galaktionov,30 17 S.N.Ganguli,11 P.Garcia-Abia,50 S.S.Gau,13 S.Gentile,39 N.Gheordanescu,14 S.Giagu,39 S.Goldfarb,24 J.Goldstein,12 Z.F.Gong,22 A.Gougas,5 G.Gratta,35 M.W.Gruenewald,9 V.K.Gupta,38 A.Gurtu,11 L.J.Gutay,48 B.Hartmann,1 A.Hasan,32 D.Hatzifotiadou,10 T.Hebbeker,9 A.Herve,19 W.C.van Hoek,33 H.Hofer,51 S.J.Hong,45 H.Hoorani,37 S.R.Hou,53 G.Hu,5 V.Innocente,19 K.Jenkes,1 B.N.Jin,8 L.W.Jones,3 P.de Jong,19 I.Josa-Mutuberria,28 A.Kasser,24 R.A.Khan,20 D.Kamrad,50 Yu.Kamyshkov,34 J.S.Kapustinsky,26 Y.Karyotakis,4 M.Kaur,20 } M.N.Kienzle-Focacci,21 D.Kim,39 D.H.Kim,45 J.K.Kim,45 S.C.Kim,45 Y.G.Kim,45 W.W.Kinnison,26 A.Kirkby,35 D.Kirkby,35 J.Kirkby,19 D.Kiss,15 W.Kittel,33 A.Klimentov,17 30 A.C.Konig,33 A.Kopp,50 I.Korolko,30 V.Koutsenko,17 30 R.W.Kraemer,37 W.Krenz,1 A.Kunin,17 30 P.Ladron de Guevara,28 I.Laktineh,27 G.Landi,18 C.Lapoint,17 K.Lassila-Perini,51 P.Laurikainen,23 M.Lebeau,19 A.Lebedev,17 P.Lebrun,27 P.Lecomte,51 P.Lecoq,19 P.Le Coultre,51 H.J.Lee,9 J.M.Le Go ,19 R.Leiste,50 E.Leonardi,39 P.Levtchenko,40 C.Li,22 C.H.Lin,53 W.T.Lin,53 F.L.Linde,2 19 L.Lista,31 Z.A.Liu,8 W.Lohmann,50 E.Longo,39 W.Lu,35 Y.S.Lu,8 K.Lubelsmeyer,1 C.Luci,39 D.Luckey,17 L.Luminari,39 W.Lustermann,49 W.G.Ma,22 M.Maity,11 G.Majumder,11 L.Malgeri,39 A.Malinin,30 C.Ma~na,28 D.Mangeol,33 S.Mangla,11 P.Marchesini,51 A.Marin,12 J.P.Martin,27 F.Marzano,39 G.G.G.Massaro,2 D.McNally,19 R.R.McNeil,7 S.Mele,31 L.Merola,31 M.Meschini,18 W.J.Metzger,33 M.von der Mey,1 Y.Mi,24 A.Mihul,14 A.J.W.van Mil,33 H.Milcent,19 G.Mirabelli,39 J.Mnich,19 P.Molnar,9 B.Monteleoni,18 R.Moore,3 S.Morganti,39 T.Moulik,11 R.Mount,35 S.Muller,1 F.Muheim,21 A.J.M.Muijs,2 S.Nahn,17 M.Napolitano,31 F.Nessi-Tedaldi,51 H.Newman,35 T.Niessen,1 A.Nippe,1 A.Nisati,39 H.Nowak,50 Y.D.Oh,45 H.Opitz,1 G.Organtini,39 R.Ostonen,23 C.Palomares,28 D.Pandoulas,1 S.Paoletti,39 P.Paolucci,31 H.K.Park,37 I.H.Park,45 G.Pascale,39 G.Passaleva,19 S.Patricelli,31 T.Paul,13 M.Pauluzzi,36 C.Paus,19 F.Pauss,51 D.Peach,19 Y.J.Pei,1 S.Pensotti,29 D.Perret-Gallix,4 B.Petersen,33 S.Petrak,9 A.Pevsner,5 D.Piccolo,31 M.Pieri,18 P.A.Piroue,38 E.Pistolesi,29 V.Plyaskin,30 M.Pohl,51 V.Pojidaev,30 18 H.Postema,17 N.Produit,21 D.Proko ev,40 G.Rahal-Callot,51 N.Raja,11 P.G.Rancoita,29 M.Rattaggi,29 G.Raven,42 P.Razis,32 K.Read,34 D.Ren,51 M.Rescigno,39 S.Reucroft,13 T.van Rhee,47 S.Riemann,50 K.Riles,3 A.Robohm,51 J.Rodin,17 B.P.Roe,3 L.Romero,28 S.Rosier-Lees,4 Ph.Rosselet,24 W.van Rossum,47 S.Roth,1 J.A.Rubio,19 D.Ruschmeier,9 H.Rykaczewski,51 J.Salicio,19 E.Sanchez,28 M.P.Sanders,33 M.E.Sarakinos,23 S.Sarkar,11 M.Sassowsky,1 C.Schafer,1 V.Schegelsky,40 S.Schmidt-Kaerst,1 D.Schmitz,1 P.Schmitz,1 N.Scholz,51 H.Schopper,52 D.J.Schotanus,33 J.Schwenke,1 G.Schwering,1 C.Sciacca,31 D.Sciarrino,21 L.Servoli,36 S.Shevchenko,35 N.Shivarov,44 V.Shoutko,30 J.Shukla,26 E.Shumilov,30 A.Shvorob,35 T.Siedenburg,1 D.Son,45 A.Sopczak,50 B.Smith,17 P.Spillantini,18 M.Steuer,17 D.P.Stickland,38 A.Stone,7 H.Stone,38 B.Stoyanov,44 A.Straessner,1 K.Strauch,16 K.Sudhakar,11 G.Sultanov,20 L.Z.Sun,22 G.F.Susinno,21 H.Suter,51 J.D.Swain,20 X.W.Tang,8 L.Tauscher,6 L.Taylor,13 Samuel C.C.Ting,17 S.M.Ting,17 M.Tonutti,1 S.C.Tonwar,11 J.Toth,15 C.Tully,38 H.Tuchscherer,46 K.L.Tung,8 Y.Uchida,17 J.Ulbricht,51 U.Uwer,19 E.Valente,39 R.T.Van de Walle,33 G.Vesztergombi,15 I.Vetlitsky,30 G.Viertel,51 M.Vivargent,4 R.Volkert,50 H.Vogel,37 H.Vogt,50 I.Vorobiev,19 30 A.A.Vorobyov,40 A.Vorvolakos,32 M.Wadhwa,6 W.Wallra 1, J.C.Wang,17 X.L.Wang,22 Z.M.Wang,22 A.Weber,1 F.Wittgenstein,19 S.X.Wu,20 S.Wynho ,1 J.Xu,12 Z.Z.Xu,22 B.Z.Yang,22 C.G.Yang,8 X.Y.Yao,8 J.B.Ye,22 S.C.Yeh,53 J.M.You,37 An.Zalite,40 Yu.Zalite,40 P.Zemp,51 Y.Zeng,1 Z.Zhang,8 Z.P.Zhang,22 B.Zhou,12 G.Y.Zhu,8 R.Y.Zhu,35 A.Zichichi,10 19 20 F.Ziegler.50 ; ; ;] ; ; ; ; ; ; ; ; ; 6 ; 1 I. Physikalisches Institut, RWTH, D-52056 Aachen, FRGx III. Physikalisches Institut, RWTH, D-52056 Aachen, FRGx 2 National Institute for High Energy Physics, NIKHEF, and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands 3 University of Michigan, Ann Arbor, MI 48109, USA 4 Laboratoire d'Annecy-le-Vieux de Physique des Particules, LAPP,IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France 5 Johns Hopkins University, Baltimore, MD 21218, USA 6 Institute of Physics, University of Basel, CH-4056 Basel, Switzerland 7 Louisiana State University, Baton Rouge, LA 70803, USA 8 Institute of High Energy Physics, IHEP, 100039 Beijing, China4 9 Humboldt University, D-10099 Berlin, FRGx 10 University of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy 11 Tata Institute of Fundamental Research, Bombay 400 005, India 12 Boston University, Boston, MA 02215, USA 13 Northeastern University, Boston, MA 02115, USA 14 Institute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania 15 Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungaryz 16 Harvard University, Cambridge, MA 02139, USA 17 Massachusetts Institute of Technology, Cambridge, MA 02139, USA 18 INFN Sezione di Firenze and University of Florence, I-50125 Florence, Italy 19 European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland 20 World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland 21 University of Geneva, CH-1211 Geneva 4, Switzerland 22 Chinese University of Science and Technology, USTC, Hefei, Anhui 230 029, China4 23 SEFT, Research Institute for High Energy Physics, P.O. Box 9, SF-00014 Helsinki, Finland 24 University of Lausanne, CH-1015 Lausanne, Switzerland 25 INFN-Sezione di Lecce and Universita Degli Studi di Lecce, I-73100 Lecce, Italy 26 Los Alamos National Laboratory, Los Alamos, NM 87544, USA 27 Institut de Physique Nucleaire de Lyon, IN2P3-CNRS,Universite Claude Bernard, F-69622 Villeurbanne, France 28 Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain[ 29 INFN-Sezione di Milano, I-20133 Milan, Italy 30 Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia 31 INFN-Sezione di Napoli and University of Naples, I-80125 Naples, Italy 32 Department of Natural Sciences, University of Cyprus, Nicosia, Cyprus 33 University of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands 34 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 35 California Institute of Technology, Pasadena, CA 91125, USA 36 INFN-Sezione di Perugia and Universita Degli Studi di Perugia, I-06100 Perugia, Italy 37 Carnegie Mellon University, Pittsburgh, PA 15213, USA 38 Princeton University, Princeton, NJ 08544, USA 39 INFN-Sezione di Roma and University of Rome, \La Sapienza", I-00185 Rome, Italy 40 Nuclear Physics Institute, St. Petersburg, Russia 41 University and INFN, Salerno, I-84100 Salerno, Italy 42 University of California, San Diego, CA 92093, USA 43 Dept. de Fisica de Particulas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain 44 Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 So a, Bulgaria 45 Center for High Energy Physics, Korea Adv. Inst. of Sciences and Technology, 305-701 Taejon, Republic of Korea 46 University of Alabama, Tuscaloosa, AL 35486, USA 47 Utrecht University and NIKHEF, NL-3584 CB Utrecht, The Netherlands 48 Purdue University, West Lafayette, IN 47907, USA 49 Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland 50 DESY-Institut fur Hochenergiephysik, D-15738 Zeuthen, FRG 51 Eidgenossische Technische Hochschule, ETH Zurich, CH-8093 Zurich, Switzerland 52 University of Hamburg, D-22761 Hamburg, FRG 53 High Energy Physics Group, Taiwan, China x Supported by the German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie z Supported by the Hungarian OTKA fund under contract numbers T14459 and T24011. 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[15] W. Beenakker, F.B. Berends and S.C. van der Marck, Nucl. Phys. B 349 (1991) 323. [16] G. Montagna, O. Nicrosini, G. Passarino, F. Piccinini and R. Pittau, Nucl. Phys. B 401 (1993) 3. [17] D. Bardin et al., FORTRAN package ZFITTER, and preprint CERN{TH. 6443/92; D. Bardin et al., Z. Phys. C 44 (1989) 493; D. Bardin et al., Nucl. Phys. B 351 (1991) 1; D. Bardin et al., Phys. Lett. B 255 (1991) 290.. 8 [18] R. Barnett et al., Phys. Rev. D 54 (1996) 1. 9 e =Z e e e =Z e e e e e e e e e ~j ~j e e Figure 1: Feynman diagrams for the reaction e+e s{ and t{channel with 1j1 6= 0. 10 ! e+ e e , including the exchange of j in the + − + − e e →e e 150 e+e− → µ+µ− ISR no ISR ISR no ISR ∆ σ [pb] ∆ σ [pb] 100 50 100 50 0 0 140 0.2 + − 160  180 √s [GeV] 200 140 + − + − e e →e e 160  180 200 180 200 √s [GeV] + − e e →µ µ 0 0 ∆ Afb ∆ Afb -0.2 -0.2 -0.4 -0.6 -0.4 ISR no ISR 140 160  180 √s [GeV] 200 -0.6 ISR no ISR 140 160  √s [GeV] Figure 2: Deviations of the cross section,
, and the forward{backward asymmetry,
Afb from the SM expectations due to sneutrino exchange as a function of the centre{of{mass energy: on the left for e+e ! e+e and on the right for e+e ! + . The solid line shows the results with and the dashed line without inclusion of ISR. The parameter values for these calculations are m  = 165:3 GeV,   = 1 GeV, 131 = 0:08 and 232 = 0:08. 11 Excluded by low energy measurements coupling λ131 0.08 Excluded by L3 0.06 0.04 0.02 L3 75 90% CL 100 125 mτ-sneutrino 150 175 [GeV] Figure 3: Upper limits on the coupling strength 131 as a function of m  derived from the measurements of the reaction e+e ! e+e . The shaded area is excluded by lower energy measurements at 90% con dence level. The jagged curve is the 90% con dence level upper limit from this analysis. 12 Excluded by low energy measurements coupling λ131=λ232 0.08 Excluded by L3 0.06 0.04 0.02 L3 75 90% CL 100 125 mτ-sneutrino 150 175 [GeV] Figure 4: Upper limits on the coupling strength 232 (assumed to be equal to 131 ) as a function of m  derived from the measurements of muon{pair production. The shaded area is excluded by lower energy measurements at 90% con dence level. The jagged curve is the 90% con dence level upper limit from this analysis. 13