- Email: [email protected]
Search for R-parity violating chargino and neutralino decays in e+e− collisions up to s=183 GeV
22 July 1999
Physics Letters B 459 Ž1999. 354–366
Search for R-parity violating chargino and neutralino decays in eqey collisions up to (s s 183 GeV L3 Collaboration M. Acciarri z , P. Achard r, O. Adriani o , M. Aguilar-Benitez y, J. Alcaraz y, G. Alemanni u , J. Allaby p, A. Aloisio ab, M.G. Alviggi ab, G. Ambrosi r, H. Anderhub au , V.P. Andreev f,aj, T. Angelescu l , F. Anselmo i , A. Arefiev aa , T. Azemoon c , T. Aziz j, P. Bagnaia ai , L. Baksay ap, A. Balandras d , R.C. Ball c , S. Banerjee j, Sw. Banerjee j, K. Banicz ar, A. Barczyk au,as , R. Barillere ` p, ai u i af u L. Barone , P. Bartalini , M. Basile , R. Battiston , A. Bay , F. Becattini o , U. Becker n , F. Behner au , J. Berdugo y, P. Berges n , B. Bertucci af , B.L. Betev au , S. Bhattacharya j, M. Biasini af , A. Biland au , J.J. Blaising d , S.C. Blyth ag , G.J. Bobbink b, A. Bohm ¨ a, L. Boldizsar m , B. Borgia p,ai, au r D. Bourilkov , M. Bourquin , S. Braccini r, J.G. Branson al , V. Brigljevic au , F. Brochu d , A. Buffini o , A. Buijs aq , J.D. Burger n , W.J. Burger af , J. Busenitz ap, A. Button c , X.D. Cai n , M. Campanelli au , M. Capell n , G. Cara Romeo i , G. Carlino ab, A.M. Cartacci o , J. Casaus y, G. Castellini o , F. Cavallari ai , N. Cavallo ab, C. Cecchi r, M. Cerrada y, F. Cesaroni v, M. Chamizo r, Y.H. Chang aw, U.K. Chaturvedi q , M. Chemarin x , A. Chen aw, G. Chen g , G.M. Chen g , H.F. Chen s , H.S. Chen g , X. Chereau d , G. Chiefari ab, L. Cifarelli ak , F. Cindolo i , C. Civinini o , I. Clare n , R. Clare n , G. Coignet d , A.P. Colijn b, N. Colino y, S. Costantini h , F. Cotorobai l , B. Cozzoni i , B. de la Cruz y, A. Csilling m , T.S. Dai n , J.A. van Dalen ad , R. D’Alessandro o , R. de Asmundis ab, P. Deglon r, A. Degre´ d , K. Deiters as , D. della Volpe ab, P. Denes ah , F. DeNotaristefani ai , A. De Salvo au , M. Diemoz ai , D. van Dierendonck b, F. Di Lodovico au , C. Dionisi p,ai , M. Dittmar au , A. Dominguez al , A. Doria ab, M.T. Dova q,1, D. Duchesneau d , D. Dufournand d , P. Duinker b, I. Duran am , H. El Mamouni x , A. Engler ag , F.J. Eppling n , F.C. Erne´ b, P. Extermann r, M. Fabre as , R. Faccini ai , M.A. Falagan y, S. Falciano ai , A. Favara o , J. Fay x , O. Fedin aj, M. Felcini au , T. Ferguson ag , F. Ferroni ai , H. Fesefeldt a , E. Fiandrini af , J.H. Field r, F. Filthaut p, P.H. Fisher n , I. Fisk al , G. Forconi n , 0370-2693r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 0 - 2 6 9 3 Ž 9 9 . 0 0 6 8 0 - 2
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L. Fredj r, K. Freudenreich au , C. Furetta z , Yu. Galaktionov aa,n , S.N. Ganguli j, P. Garcia-Abia e, M. Gataullin ae, S.S. Gau k , S. Gentile ai , N. Gheordanescu l , S. Giagu ai , Z.F. Gong s , G. Grenier x , M.W. Gruenewald h , R. van Gulik b, V.K. Gupta ah , A. Gurtu j, L.J. Gutay ar, D. Haas e, A. Hasan ac , D. Hatzifotiadou i , T. Hebbeker h , A. Herve´ p, P. Hidas m , J. Hirschfelder ag , H. Hofer au , G. Holzner au , H. Hoorani ag , S.R. Hou aw, I. Iashvili at , B.N. Jin g , L.W. Jones c , P. de Jong b, I. Josa-Mutuberrıa ´ y, R.A. Khan q , D. Kamrad at, J.S. Kapustinsky w, M. Kaur q,2 , M.N. Kienzle-Focacci r, D. Kim ai , D.H. Kim ao , J.K. Kim ao , S.C. Kim ao , W.W. Kinnison w, J. Kirkby p, D. Kiss m , W. Kittel ad , A. Klimentov n,aa , A.C. Konig ¨ ad, A. Kopp at, I. Korolko aa, V. Koutsenko n,aa , R.W. Kraemer ag , W. Krenz a , A. Kunin n,aa , P. Lacentre at,1,3, P. Ladron de Guevara y, I. Laktineh x , G. Landi o , K. Lassila-Perini au , P. Laurikainen t , A. Lavorato ak , M. Lebeau p, A. Lebedev n , P. Lebrun x , P. Lecomte au , P. Lecoq p, P. Le Coultre au , H.J. Lee h , J.M. Le Goff p, R. Leiste at , E. Leonardi ai , P. Levtchenko aj, C. Li s , C.H. Lin aw, W.T. Lin aw, F.L. Linde b,p, a L. Lista ab, Z.A. Liu g , W. Lohmann at , E. Longo ai , Y.S. Lu g , K. Lubelsmeyer , ¨ C. Luci p,ai , D. Luckey n , L. Lugnier x , L. Luminari ai , W. Lustermann au , W.G. Ma s , M. Maity j, G. Majumder j, L. Malgeri p, A. Malinin aa , C. Mana ˜ y, D. Mangeol ad , P. Marchesini au , G. Marian ap,4 , J.P. Martin x , F. Marzano ai , G.G.G. Massaro b, K. Mazumdar j, R.R. McNeil f , S. Mele p, L. Merola ab, M. Meschini o , W.J. Metzger ad , M. von der Mey a , D. Migani i , A. Mihul l , H. Milcent p, G. Mirabelli ai , J. Mnich p, P. Molnar h , B. Monteleoni o , T. Moulik j, G.S. Muanza x , F. Muheim r, A.J.M. Muijs b, M. Napolitano ab, F. Nessi-Tedaldi au , H. Newman ae, T. Niessen a , A. Nisati ai , H. Nowak at , Y.D. Oh ao , G. Organtini ai , R. Ostonen t , C. Palomares y, D. Pandoulas a , S. Paoletti ai,p, P. Paolucci ab, H.K. Park ag , I.H. Park ao , G. Pascale ai , G. Passaleva p, S. Patricelli ab, T. Paul k , M. Pauluzzi af , C. Paus p, F. Pauss au , D. Peach p, M. Pedace ai , Y.J. Pei a , S. Pensotti z , D. Perret-Gallix d , B. Petersen ad , S. Petrak h , D. Piccolo ab, M. Pieri o , P.A. Piroue´ ah , E. Pistolesi z , V. Plyaskin aa , M. Pohl au , V. Pojidaev aa,o , H. Postema n , J. Pothier p, N. Produit r, D. Prokofiev aj, J. Quartieri ak , G. Rahal-Callot au , N. Raja j, P.G. Rancoita z , G. Raven al , P. Razis ac , D. Ren au , M. Rescigno ai , S. Reucroft k , T. van Rhee aq , S. Riemann at , K. Riles c , A. Robohm au , J. Rodin ap, B.P. Roe c , L. Romero y, S. Rosier-Lees d , J.A. Rubio p, D. Ruschmeier h , H. Rykaczewski au , S. Sakar ai , J. Salicio p, E. Sanchez p, M.P. Sanders ad , M.E. Sarakinos t , C. Schafer ¨ a, V. Schegelsky aj, S. Schmidt-Kaerst a, D. Schmitz a, N. Scholz au , H. Schopper av, D.J. Schotanus ad , J. Schwenke a , G. Schwering a , C. Sciacca ab, D. Sciarrino r, A. Seganti i , L. Servoli af , S. Shevchenko ae,
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N. Shivarov an , V. Shoutko aa , J. Shukla w, E. Shumilov aa , A. Shvorob ae, T. Siedenburg a , D. Son ao , B. Smith ag , P. Spillantini o , M. Steuer n , D.P. Stickland ah , A. Stone f , H. Stone ah , B. Stoyanov an , A. Straessner a , K. Sudhakar j, G. Sultanov q , L.Z. Sun s , H. Suter au , J.D. Swain q , Z. Szillasi ap,4 , X.W. Tang g , L. Tauscher e, L. Taylor k , C. Timmermans ad , Samuel C.C. Ting n , S.M. Ting n , S.C. Tonwar j, J. Toth ´ m , C. Tully ah, K.L. Tung g , Y. Uchida n, J. Ulbricht au , E. Valente ai , G. Vesztergombi m , I. Vetlitsky aa , D. Vicinanza ak , G. Viertel au , S. Villa k , M. Vivargent d , S. Vlachos e, I. Vodopianov aj, H. Vogel ag , H. Vogt at , I. Vorobiev p,aa , A.A. Vorobyov aj, A. Vorvolakos ac , M. Wadhwa e, W. Wallraff a , M. Wang n , X.L. Wang s , Z.M. Wang s , A. Weber a , M. Weber a , P. Wienemann a , H. Wilkens ad , S.X. Wu n , S. Wynhoff a , L. Xia ae, Z.Z. Xu s , B.Z. Yang s , C.G. Yang g , H.J. Yang g , M. Yang g , J.B. Ye s , S.C. Yeh ax , J.M. You ag , An. Zalite aj, Yu. Zalite aj, P. Zemp au , Z.P. Zhang s , a G.Y. Zhu g , R.Y. Zhu ae, A. Zichichi i,p,q , F. Ziegler at , G. Zilizi ap,4 , M. Zoller ¨ a
d
I. Physikalisches Institut, RWTH, D-52056 Aachen, Germany, and III. Physikalisches Institut, RWTH, D-52056 Aachen, Germany 5 b National Institute for High Energy Physics, NIKHEF, and UniÕersity of Amsterdam, NL-1009 DB Amsterdam, The Netherlands c UniÕersity of Michigan, Ann Arbor, MI 48109, USA Laboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Institute of Physics, UniÕersity of Basel, CH-4056 Basel, Switzerland f Louisiana State UniÕersity, Baton Rouge, LA 70803, USA g Institute of High Energy Physics, IHEP, 100039 Beijing, China 6 h Humboldt UniÕersity, D-10099 Berlin, Germany 5 i UniÕersity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy j Tata Institute of Fundamental Research, Bombay 400 005, India k Northeastern UniÕersity, Boston, MA 02115, USA l Institute of Atomic Physics and UniÕersity of Bucharest, R-76900 Bucharest, Romania m Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary 7 n Massachusetts Institute of Technology, Cambridge, MA 02139, USA o INFN Sezione di Firenze and UniÕersity of Florence, I-50125 Florence, Italy p European Laboratory for Particle Physics, CERN, CH-1211 GeneÕa 23, Switzerland q World Laboratory, FBLJA Project, CH-1211 GeneÕa 23, Switzerland r UniÕersity of GeneÕa, CH-1211 GeneÕa 4, Switzerland s Chinese UniÕersity of Science and Technology, USTC, Hefei, Anhui 230 029, China 6 t SEFT, Research Institute for High Energy Physics, P.O. Box 9, SF-00014 Helsinki, Finland u UniÕersity of Lausanne, CH-1015 Lausanne, Switzerland v INFN-Sezione di Lecce and UniÕersita´ Degli Studi di Lecce, I-73100 Lecce, Italy w Los Alamos National Laboratory, Los Alamos, NM 87544, USA x Institut de Physique Nucleaire de Lyon, IN2P3-CNRS, UniÕersite´ Claude Bernard, F-69622 Villeurbanne, France ´ y Centro de InÕestigaciones Energeticas, Medioambientales y Tecnologıcas, CIEMAT, E-28040 Madrid, Spain 8 ´ ´ z INFN-Sezione di Milano, I-20133 Milan, Italy aa Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia ab INFN-Sezione di Napoli and UniÕersity of Naples, I-80125 Naples, Italy ac Department of Natural Sciences, UniÕersity of Cyprus, Nicosia, Cyprus ad UniÕersity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands ae California Institute of Technology, Pasadena, CA 91125, USA af INFN-Sezione di Perugia and UniÕersita´ Degli Studi di Perugia, I-06100 Perugia, Italy ag Carnegie Mellon UniÕersity, Pittsburgh, PA 15213, USA ah Princeton UniÕersity, Princeton, NJ 08544, USA ai INFN-Sezione di Roma and UniÕersity of Rome, ‘‘La Sapienza’’, I-00185 Rome, Italy aj Nuclear Physics Institute, St. Petersburg, Russia
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ak
UniÕersity and INFN, Salerno, I-84100 Salerno, Italy UniÕersity of California, San Diego, CA 92093, USA am Dept. de Fisica de Particulas Elementales, UniÕ. de Santiago, E-15706 Santiago de Compostela, Spain Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria ao Center for High Energy Physics, AdÕ. Inst. of Sciences and Technology, 305-701 Taejon, South Korea ap UniÕersity of Alabama, Tuscaloosa, AL 35486, USA aq Utrecht UniÕersity and NIKHEF, NL-3584 CB Utrecht, The Netherlands ar Purdue UniÕersity, West Lafayette, IN 47907, USA as Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland at DESY-Institut fur ¨ Hochenergiephysik, D-15738 Zeuthen, Germany au Eidgenossische Technische Hochschule, ETH Zurich, CH-8093 Zurich, Switzerland ¨ ¨ ¨ av UniÕersity of Hamburg, D-22761 Hamburg, Germany aw National Central UniÕersity, Chung-Li, Taiwan, ROC ax Department of Physics, National Tsing Hua UniÕersity, Taiwan, ROC al
an
Received 19 February 1999 Editor: K. Winter
Abstract A search for chargino and neutralino pair production in eqey collisions at center-of-mass energies between 161 GeV and 183 GeV is performed under the assumptions that R-parity is not conserved and that only purely leptonic or hadronic R-parity violating decays are allowed. No signal is found in the data. Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived. q 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction The most general superpotential of the Minimal Supersymmetric Standard Model ŽMSSM. w1x, which describes a supersymmetric, renormalizable and gauge invariant theory, with minimal particle content, includes the term WR w2x: WR s l i jk L i L j E k q lXi jk L i Q j D k q lXXi jkUi Dj Dk ,
Ž 1. 1 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina. 2 Also supported by Panjab University, Chandigarh-160014, India. 3 Supported by Deutscher Akademischer Austauschdienst. 4 Also supported by the Hungarian OTKA fund under contract numbers T22238 and T026178. 5 Supported by the German Bundesministerium fur ¨ Bildung, Wissenschaft, Forschung und Technologie. 6 Supported by the National Natural Science Foundation of China. 7 Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T024011. 8 Supported also by the Comision ´ Interministerial de Ciencia y Tecnologıa. ´
where l i jk , lXi jk and lXXi jk are the Yukawa couplings and i, j and k are the generation indices; L i and Q i are the left-handed lepton- and quark-doublet superfields; E i , Di and Ui are the right-handed singlet superfields for charged leptons, down- and up-type quarks, respectively. In order to prevent the simultaneous presence of identical fermionic fields, the following antisymmetry relations are required: l i jk s yl ji k and lXXi jk s ylXXi k j , so that there are in total 9 q 27 q 9 independent Yukawa couplings. The l i jk and lXi jk couplings violate the leptonic quantum number L, while the lXXi jk couplings violate the baryonic quantum number B. Their simultaneous presence would lead to a fast proton decay w3x, which is experimentally excluded w4x. This can be avoided by imposing the conservation of R-parity, a multiplicative quantum number defined as: R s Ž y1 .
3BqLq2S
,
Ž 2.
where S is the spin. R is q1 for all ordinary particles, and y1 for their supersymmetric partners. As a consequence, if R-parity is conserved, super-
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Table 1 Allowed R-parity violating direct decays of the neutralino and chargino. Charged conjugate states are implied Coupling
l i jk lXXi jk
Neutralino decays
l
y i nj
l
q k ,
uid jdk
q y j k
ni l l
Chargino decays q q y ninj l q k , li lj lk did jdk, uiu jdk, uid juk
symmetric particles can be produced only in pairs and they decay in cascade to the lightest supersymmetric particle ŽLSP., which is stable. However, the absence of either the B-violating or the L-violating terms, or of a subset of them, is enough to prevent a fast proton decay, while allowing the LSP to decay into Standard Model particles via scalar lepton or quark exchange. In this paper, we search for pair-produced neutralinos Žeq ey™ x˜ 10 x˜ 10 , eq ey™ x˜ i0x˜ 10 ,i G 2. and . with subsequent R-parity charginos Žeq ey™ x˜q ˜y 1 x 1 violating decays, assuming that one of the l i jk or lXXi jk coupling constants is non-negligible. The lXi jk couplings are not considered. In all processes studied, the lightest neutralino is assumed to decay into three fermions according to the dominant l i jk or lXXi jk interaction term, as detailed in Table 1. When the chargino is the LSP or when the R-parity violating coupling is strong enough, direct decays of charginos into three fermions can also occur via the dominant R-parity violating term, as listed in Table 1. If the x˜ 10 is the LSP, charginos can also decay indirectly into x˜ 10 W ) , or into x˜ 02 W ) for chargino heavier than the next-to-lightest neutralino. Similarly the heavier neutralinos Ž x˜ 0i ,i G 2. can decay indirectly into Z )x˜ 10 or directly into fermions. When the lightest scalar lepton is the LSP, the process x˜ 10 ™ l l˜ is taken into account for the l i jk analysis. In the present analysis, the dominant coupling is assumed to be greater than 10y5 w5x, corresponding to decay lengths smaller than 1 cm. Searches for R-parity violating decays have also been performed by other LEP experiments w6x.
2. Data and Monte Carlo samples The data used corresponds to an integrated luminosity of 55.3 pby1 collected by the L3 detector w7x at the center-of-mass energy Ž's . of 182.7 GeV,
hereafter referred to as 183 GeV. For the indirect l i jk analysis, 21.1 pby1 of data collected at 161– 172 GeV are also used. The signal events are generated with SUSYGEN w8x for different values of neutralino and chargino masses, for all possible choices of the generation indices. The following Monte Carlo generators are used to simulate Standard Model processes: PYTHIA w9x for eq ey™ qq, eq ey™ Z eq ey and eq ey™ ZZ, BHAGENE3 w10x for eq ey™ eq ey, KORALZ w11x for eq ey™ mq my and eq ey™ tq ty, PHOJET w12x for eq ey ™ eq ey qq, DIAG36 w13x for eq ey ™ eq ey l q l y Ž l s e,m,t . and KORALW w14x for eq ey ™ WqWy. The number of simulated events corresponds to at least 100 times the luminosity of the data, except for Bhabha and two-photon processes, where the Monte Carlo samples correspond to approximately 5 times the luminosity. The detector response is simulated using the GEANT package w15x. It takes into account effects of energy loss, multiple scattering and showering in the detector materials. Hadronic interactions are simulated with the GHEISHA program w16x. Time dependent inefficiencies of the different subdetectors are also taken into account in the simulation procedure.
3. Analysis procedure 3.1. l i jk : topology and preselection When the l i jk couplings dominate, the LSP decays into three leptons. The possible topologies arising from the different final states are listed in Table 2. Selections for the different topologies are developed. The decay products of the x˜ 10 pair are four charged leptons and two neutrinos. If the l 133 coupling dominates, each neutralino can decay into netq ty, nt eq ty, nety tq or nt ey tq, and the final state contains at least two t leptons, which are selected with lower efficiency with respect to electrons and muons. If the neutralino mass is high Ž Mx˜ 0 G 50 GeV. the event topology consists of four 1 charged leptons, isotropically distributed, and missing energy Ž Eu .. On the contrary, for low neutralino
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Table 2 Processes and topologies considered in the l i jk coupling analysis Process
Topology
eq ey™ x˜ 10 x˜ 10 ™ l l l l nn
4 l q Eu
eq ey™ x˜ i0x˜ 10 ™ x˜ 10 x˜ 10 Z ) ™ l l l l nn qq Ž i G 2. l l l l nn l l l l l l nn nn
4 l q 2 jets q Eu 6 l q Eu 4 l q Eu
eq ey™ x˜q ˜y ˜ 10 x˜ 10 W ) W ) ™ l l l l nn qq’qq’ 1 x 1 ™ x l l l l nn qq’ l n l l l l nn l n l n
4 l q 4 jets q Eu 5 l q 2 jets q Eu 6 l q Eu
eq ey™ x˜q ˜y 1 x 1 ™ l l l l l l l l l l nn l l nnnn
6 l 4 l q Eu 2 l q Eu
masses Ž Mx˜ 10 F 20 GeV. the events consist of two back-to-back lepton pairs and missing energy. In this case, there is a large background from Standard Model lepton-pair processes. In the case of the process eqey™ x˜ i0x˜ 10 ™ 0 ) 0 x˜ 1 Z x˜ 1 , three possible final states are present, corresponding to the Z ) decays into hadrons, charged leptons or neutrinos. As is shown in Table 2, the topologies arising from this process include leptons and missing energy or leptons plus hadronic jets and missing energy. These topologies are covered by combining the neutralino and chargino pair selections, described below. In the case of chargino production, the signal topologies depend not only on the values of the chargino and neutralino masses, but also on the chargino decay modes. For small values of the mass difference D M s Mx˜ 1" y Mx˜ 10 Ž D M F 10 GeV. the event energy is mainly carried by the neutralino decay products, almost independently of the different chargino decay channels. Therefore, a common selection is developed for all chargino decay modes. On the contrary, for medium and large D M values Ž D M ) 10 GeV., different selections are developed for each possible configuration. As the process eqey ™ qq and hadronic Wq Wy decays are the main background for the hadronic mode, a relatively low multiplicity is required. In the case of chargino R-parity violating direct decays, three possible topologies can occur. When
both charginos decay into three charged leptons, the final state is almost background free. If the decay products are two charged leptons and four neutrinos, the main background contributions come from leptonic W and t decays and two-photon interactions. The last topology, four charged leptons plus missing energy, is already taken into account by the neutralino pair selection. Events are preselected by requiring at least 3 charged tracks and 4 calorimetric clusters in order to remove eqey™ eqey,mqmy and purely leptonic tqty and WqWy decays. Events have to contain at least two charged leptons. The visible energy has to be smaller than 90% of 's in order to remove high multiplicity qq events and hadronic Wq Wy and ZZ decays. The missing momentum vector is required to be in the polar angle range between 58 and 1758. In order to reject cosmic ray interactions, at least one time of flight measurement is required to be consistent with the beam crossing. Tagged two-photon interactions are rejected by requiring the sum of the energies measured in the small angle calorimeters between 1.58 and 9.08 to be less than 10 GeV. In addition, the visible energy must be greater than 15% of 's . Background from two-photon collisions is further reduced by requiring the transverse missing momentum to be greater than 5 GeV. After the preselection is applied, 337 events are selected in the data sample and 334 events are
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jet resolution parameter ym n is defined as the ycut value at which the event configuration changes from n to m jets, when using the DURHAM w17x clustering scheme. The data are in good agreement with the Monte Carlo expectations. 3.2. lXXi jk : topology and preselection
Fig. 1. Distributions of a. the number of tracks, b. the number of leptons, c. the normalised visible energy and d. lnŽ y 34 . after the l i jk preselection. The solid histograms show the normalised sum of Standard Model processes. The dotted and dashed histograms show two examples of signal, with dominant coupling l133 . The dotted histograms represent the process eq ey ™ x˜ 10 x˜ 10 , for Mx˜ 10 s91 GeV, corresponding to one hundred times the expected cross section. The dashed ones represent eq ey ™ x˜q ˜y 1x 1 , with Mx˜ 1" s 91 GeV and D M s 50 GeV, corresponding to four times the expected cross section.
expected in total from Standard Model processes. The signal preselection efficiency for eqey™ x˜q ˜y1 1 x is about 85% for Mx˜ 1" s 91 GeV, at 's s 183 GeV. For eqey™ x˜ 10 x˜ 10 the efficiency is greater than 80% for Mx˜ 10 G 50 GeV and between 35% and 60% for 5 GeV F Mx˜ 10 F 20 GeV. Fig. 1 shows the number of tracks, number of leptons, normalised visible energy and lnŽ y 34 . distributions after the preselection. The
If lXXi jk couplings dominate, the LSP decays into three quarks, with a flavour composition given by the generation indices of the dominant lXXi jk coupling. The possible topologies arising from the different final states are summarized in Table 3. In the case of x˜ 10 pair production, the final state contains six quarks, and three different topologies can occur, depending on the value of Mx˜ 10 . For low and medium neutralino masses Ž Mx˜ 10 F 50 GeV. not all the six jets in the event can be resolved, while for high masses Ž Mx˜ 10 ) 50 GeV. they can be well separated. Therefore, the signal topology is two-jet like for small values of the neutralino mass. For intermediate neutralino masses the events are more similar to four jets, while for large neutralino masses the events can contain six isolated jets, isotropically distributed. For small neutralino masses the main background contribution comes from eq ey™ qq. When the signal topology is four-jet like, qqgg and hadronic WqWy decays contribute. For high neutralino masses, the most important background source is WqWy events. The same multi-jet topology occurs when charginos decay directly into three quarks, or when the mass difference D M is sufficiently small, so that
Table 3 XX Processes and topologies considered in the l i jk coupling analyses for events containing two, four or six resolved jets Process q y
x 10 x 10
e e ™ ˜ ˜
or
y xq 1 x1
˜ ˜ ™ qqqqqq
Topology 2 jets 4 jets 6 jets
eq ey™ x˜ i0x˜ 10 ™ x˜ 10 x˜ 10 Z ) ™ qqqqqq ff Ž i G 2.
4 jets or 6 jets Ž4 or 6. jets q leptons Ž4 or 6. jets q Eu
eq ey™ x˜q ˜y ˜ 10 x˜ 10 W ) W ) ™ qqqqqq ff’f’’f’’’ 1 x 1 ™ x
4 jets or 6 jets Ž4 or 6. jets q leptonŽs. q Eu
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the decay products of the W ) pair carry only a negligible fraction of the event energy. Since charginos of masses up to 45 GeV are excluded by Z lineshape measurements w18x, the two-jet topology is not addressed in this case. Analogous considerations apply for the processes x˜ 02 x˜ 10 and x˜ 03 x˜ 10 . When D M is large, leptons and neutrinos from W ) decays can carry a relevant fraction of the event energy, leading to lower selection efficiencies. In this analysis no attempt is made to identify quark flavours. However, the efficiency is found to be slightly higher for events containing b-quarks than for events with light quarks. Therefore, only the results obtained by the choice lXXi jk s lXX112 will be quoted in the next sections. The preselection of the lXXi jk analysis aims at selecting well balanced hadronic events. Low multiplicity events, like leptonic Z and W decays, are rejected by requiring at least 13 calorimetric clusters. At least one charged track has to be present. The visible energy has to be greater than 50% of 's . The energy imbalances, parallel and perpendicular to the beam direction, are required to be smaller than 20% of the visible energy. Above the Z peak a large fraction of background events contains a hard initial state radiation ŽISR. photon. Unbalanced events with an ISR photon in the beam pipe are removed by means of the requirement on the parallel energy imbalance. In order to reject events with an ISR photon seen in the detector, the invariant mass of the hadronic system has to be greater than 80% of 's . In order to remove background contributions from two-photon interactions, the energy in a cone of 128 half opening angle around the beam axis must not exceed 30% of the total visible energy. Furthermore, the thrust axis is required to be well contained in the detector with a polar angle between 88 and 1728. After the preselection is applied, 1953 events are selected in the data sample and 1949 are expected from Standard Model processes, mainly coming from qq and Wq Wy events. The signal preselection efficiency for eqey™ x˜ 10 x˜ 10 is about 85% and for 's s eq ey™ x˜q ˜y 1 x 1 is between 60% and 90%, at Ž . 183 GeV. Fig. 2 shows the ln y 34 , thrust, lnŽ y45 . and narrow jet broadening Ž BN . w19x distributions after the preselection. There is a good agreement between data and Monte Carlo expectations.
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Fig. 2. Distributions of a. lnŽ y 34 ., b. thrust, c. lnŽ y45 . and d. the XX narrow jet broadening after the l i jk preselection. The solid histograms show Standard Model processes at 's s183 GeV. The dashed and dotted histograms show two examples of signal, with XX dominant coupling l112 . The dashed histograms represent the q y 0 0 process e e ™ x˜ 1 x˜ 1 , with Mx˜ 10 s90 GeV, corresponding to five hundred times the expected cross section. The dotted ones represent the same process, with Mx˜ 10 s 30 GeV, corresponding to twenty times the expected cross section.
3.3. Analysis optimization After the preselection level, dedicated selections are performed to maximize the l i jk and lXXi jk analysis sensitivities according to the topologies arising from those couplings. In the l i jk case, the best sensitivity is achieved performing different selections according to the neutralino mass values and decay products. Chargino selections depend also on W ) decay modes. In total thirteen selections are performed for which the cut values of the following variables are optimized simultaneously: acollinearity and acoplanarity angles, thrust, y 34 and polar angle of the missing momentum. The acollinearity and acoplanarity angles are calculated by forcing hadronic and leptonic objects in every event into exactly two jets with the DURHAM algorithm. Fourteen selections are performed for the lXXi jk analysis. The sum of the di-jet masses is required not to be consistent with WqWy pair production. For the lXXi jk neutralino selection, the optimization proce-
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Table 4 ' Efficiency ranges of x˜ 10 x˜ 10 and x˜q ˜y 1 x 1 selections for direct R-parity violating neutralino and chargino decays, at s s 183 GeV. In case of the process x˜q ˜y1 , the lowest mass value considered is Mx˜ 1" s 15 GeV for l133 and 45 GeV for lXX112 1 x Coupling
Process
Mx˜ s 5–25 GeV
Mx˜ s 25–55 GeV
Mx˜ s 55–91 GeV
l133 l133 lXX112
x 10 x 10 y xq 1 x1 0 0 x1 x1 ,
3%–10% 10%–18%
10%–33% 21%–40%
39%–55% 43%–54%
19%–22%
23%–24%
21%–32%
˜ ˜ ˜ ˜ ˜ ˜
x˜q ˜y 1 x 1
dure includes the following variables: thrust, wide jet broadening variable, y 34 and y45 . For the lXXi jk chargino selection, the cut values of the following variables are optimized: thrust, BN , y 34 and y56 . The optimization procedure uses Monte Carlo signal and background events and is described in Ref. w20x. For two-photon interactions, which represent an important background source for the l i jk analysis, the optimization and the background estimation are performed on two independent samples to avoid potential biases from statistical fluctuations. 3.4. Efficiencies Here we discuss only the results obtained for l i jk s l133 and lXXi jk s lXX112 , since these choices of the generation indices give the lowest selection efficiencies. In the following, only the efficiencies at 's s 183 GeV are quoted for simplicity. The efficiencies of the processes eqey™ x˜ 10 x˜ 10 , eqey™ x˜q ˜y1 and eqey™ x˜ i0x˜ 10 are summarized in Tables 1 x 4 and 5 for direct and indirect R-parity violating decays. In the case of direct R-parity violating decays, the efficiencies are estimated for different lightest neutralino or chargino masses. For both l133 and lXX112
mediated decays, the efficiencies increase with increasing lightest neutralino or chargino mass. At high masses, six fermions are expected to be isotropically produced and can be disentangled from W-pair production background events. For low masses, the signal signatures look like back-to-back jet events and the selection efficiencies are smaller due to the dominant background coming from the two-fermion processes. In addition, for the l133 selection the efficiencies obtained for low masses Žbelow 50 GeV. are higher for chargino than for neutralino due to the contribution of the six charged lepton final state. In the case of indirect R-parity violating decays for both charginos and next-to-lightest neutralinos, the efficiencies are estimated for different masses and D M ranges. For a chargino mass of 91 GeV and assuming x˜ 1" ™ x˜ 10 W ) or x˜ 1" ™ x˜ 20 W ) ™ x˜ 10 Z ) W ) , the efficiencies obtained for l133 and lXX112 mediated decays decrease with increasing D M. At high D M, the signal signatures are very similar to those of W-pair production. For Mx˜ i0 q Mx˜ 10 s 180 GeV, the efficiencies of the process eqey™ x˜ 0i x˜ 10 Ž i s 2,3,4. decrease slightly with increasing D M for the l133 and lXX112 analyses. In the latter case, the efficiencies are smaller compared to those obtained for charginos due to the invisible or purely leptonic Z ) decays.
Table 5 ' Efficiency ranges of x˜ i0x˜ 10 and x˜q ˜y 1 x 1 selections for indirect R-parity violating neutralino and chargino decays, at s s 183 GeV. The chargino selection efficiencies correspond to Mx˜ 1" s 91 GeV. For the process x˜ i0x˜ 10 the efficiencies correspond to Mx˜ i0 q Mx˜ 10 s 180 GeV Coupling Process D M s 5–20 GeV D M s 20–50 GeV D M s 50–80 GeV
l133 l 133 lXX112 lXX112
x˜ i0x˜ 10 x˜q ˜y 1 x 1 0 0 x˜ i x˜ 1 x˜q ˜y 1 x 1
47%–54% 47%–56% 31%–32% 30%–43%
42%–47% 28%–47% 28%–30% 45%–51%
26%–37% 17%–26% 28%–24% 10%–43%
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4. Results The summary of the searches is given in Table 6, showing the number of candidates and expected background events. We do not observe any excess of events. Therefore we set upper limits on the neutralino and chargino production cross sections assuming direct or indirect R-parity violating decays. We also derive limits on the masses of these particles in the framework of the MSSM. Exclusion limits at 95% C.L. are derived taking into account the estimated background contamination. The predicted contribution from the two-photon processes, due to the large statistical error, is not included, such that data events consistent with this background are conservatively considered as signal. Systematic errors on the signal efficiency are evaluated as in Ref. w20x. The typical relative error is 5% and is dominated by Monte Carlo statistics. It is taken into account in the calculations of the signal upper limits w21x.
Fig. 3. 95% C.L. upper limits on: a. the neutralino production cross section as a function of the neutralino mass, b. the chargino production cross section as a function of the chargino mass, in the direct decay mode and c. the chargino production cross section as a function of D M, for Mx˜ 1" s91 GeV, in the indirect decay mode. The solid lines show the limits obtained by the l i jk s l133 XX analysis, and the dashed lines show those obtained by the l i jk s lXX112 analysis.
4.1. Upper limits on neutralino and chargino production cross sections The 95% C.L. upper limits on neutralino and chargino pair-production cross sections at 183 GeV, both for l i jk and lXXi jk , are shown in Fig. 3. For the indirect l i jk analysis the data collected at 183 GeV are combined with those at the lower center of mass energies. Hence this upper limit should be interpreted as a limit on the luminosity-weighted average y1 cross section Ž L TOT ÝE 's s ŽE 's . L ŽE 's ... In the case of dominant l i jk coupling, the neutralino pair-production cross section is below 0.2 pb at 95% C.L. for Mx˜ 10 G 40 GeV and below 0.1 pb for
Table 6 Number of expected background and observed data events for the different selections Coupling
Process
N background
Ndata
l i jk l i jk l i jk lXXi jk lXXi jk
x 10 x 10 y xq 1 x1 q y x1 x1 x 10 x 10 y xq 1 x1
1.3 " 0.1 1.6 " 0.1 10.4 " 0.5 62 " 2 46 " 1
0 0 10 52 40
˜ ˜ ˜ ˜ ˜
˜ ˜ ˜ ˜ ˜
Žindirect. Ždirect.
Mx˜ 10 G 60 GeV. The chargino cross section is below 0.4 pb for a chargino mass greater than 45 GeV in the direct decay mode and is below 0.2 pb for Mx˜ 1" s 91 GeV and D M F 50 GeV in the indirect decay mode. In the case of dominant lXXi jk coupling, the neutralino cross section is below 0.6 pb at 95% C.L. for any value of the neutralino mass. The chargino cross section is below 0.6 pb for a chargino mass greater than 45 GeV in the direct decay mode and is below 0.4 pb for Mx˜ 1" s 91 GeV and D M F 60 GeV in the indirect decay mode. 4.2. Interpretation in the MSSM The results are also interpreted as excluded regions in the MSSM parameter space. In the MSSM framework neutralino and chargino masses, couplings and cross sections depend on the gaugino mass parameter, M2 , the higgsino mass mixing parameter, m , the ratio of the vacuum expectation values of the two Higgs doublets, tan b , and the common mass of the scalar fermions at the GUT
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scale, m 0 . Therefore the exclusion regions can be expressed in the M2 y m plane for a given value of m 0 and tan b . The results presented here hold for l i jk and lXXi jk ) 10y5 and for 0 F M2 F 2000 GeV, y500 GeV F m F 500 GeV. They do not depend on the value of the trilinear coupling in the Higgs sector, A. In addition to the limits obtained with this analysis, we take into account the constraints from L3 cross section measurements at the Z pole. A point in the MSSM parameter space is excluded at 95% C.L. by Z lineshape measurements if:
ž
s SUSY s SM
/
G Z ) G LIM ,
Ž 3.
where s SUSY is the sum of the pair-production cross sections of supersymmetric particles at 's s 91 GeV, calculated with SUSYGEN w8x, and s SM is the total Z cross section predicted by the Standard Model. G Z and G LIM s 24 MeV are the measured total Z width and the 95% C.L. upper limit on possible non-Standard Model contributions to the total Z width w18x. Fig. 4 shows the exclusion regions at 95% C.L. in the M2 y m plane for tan b s '2 and for m 0 s 60 GeV or 500 GeV, for both l i jk and lXXi jk . The excluded regions with the present results are dominated by the chargino analyses. Moreover some regions beyond the chargino kinematic limit are excluded at large m 0 and low tan b values by the x˜ 20 x˜ 10 and x˜ 03 x˜ 10 analyses and at low m 0 by the x˜ 10 x˜ 10 analyses. Lower limits on the masses of the supersymmetric particles are reported for the following two regions of the parameter space where: 1. the x˜ 10 is the LSP Ž50 GeV F m 0 F 500 GeV and any tan b or any m 0 and 2 F tan b F 40.; 2. the lightest scalar lepton can be the LSP Ž0 F m 0 - 50 GeV and 1 F tan b - 2.. In region 2., in the presence of dominant l i jk coupling the decay chain x˜ 10 ™ l l˜™ l l n leads to the same final states arising from neutralino direct R-parity violating decays, so that the analysis is efficient also when the lightest scalar lepton is the LSP. The additional contribution of the process x˜ 03 x˜ 10 ™ l l˜x˜ 10 is taken into account for the region in which M l˜- Mx˜ 30 . When the lXXi jk coupling dominate, the decay x˜ 10 ™ l l˜™ l l qqq occurs. Since there is
Fig. 4. Exclusion regions at 95% confidence level for l i jk s l133 Ža–b. and lXXi jk s lXX112 Žc–d., for tan b s'2 and m 0 s60 GeV Ža–c. or 500 GeV Žb–d.. The light grey region is excluded by the Z lineshape measurements and the dark grey region by the present analyses. The black solid lines indicate the chargino kinematic limit. The grey region beyond the kinematic limit is excluded by neutralino analyses.
no event generator available for this process, we quote two sets of mass limits for lXXi jk coupling, for region 1. and region 2., respectively. Fig. 5 shows the 95% C.L. lower limits on neutralino and chargino masses as a function of tan b . In region 1., we derive the following lower limits at 95% C.L. on the neutralino and chargino masses: Mx˜ 10 ) 26.8 GeV, Mx˜ 20 ) 44.3 GeV, Mx˜ 1" ) 91.1 GeV Ž l i jk analysis., Mx˜ 1" ) 90.9 GeV Ž lXXi jk analysis.. In region 2., the 95% C.L. lower limits on the neutralino and chargino masses set by the lXXi jk analysis are: Mx˜ 10 ) 26.8 GeV, Mx˜ 20 ) 34.8 GeV, Mx˜ 1" ) 76.9 GeV. From the exclusion contours in the M2 y m 0 plane we set indirect lower limits on the scalar lepton masses. Fig. 6 shows the 95% C.L. lower limits on
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Fig. 5. 95% C.L. lower limits on the masses of x˜ 10 Ža., x˜ 20 Žb. and x˜ 1" Žc., as a function of tan b , for 0 F m 0 F 500 GeV, 0 F M2 F 2000 GeV and y500 GeV F m F 500 GeV. The solid lines show the limit obtained by the l133 analysis, the dashed XX lines by the l112 analysis in region 1. and the dotted lines by the XX l112 analysis in region 2..
the mass of the supersymmetric partner of the righthanded electron Ž Me˜ R . for tan b s 2. These limits hold also for tan b ) 2. Fig. 7 shows the 95% C.L. lower limit on the scalar electron mass as a function
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Fig. 7. 95% C.L. lower limit on the mass of the scalar electron as a function of tan b , for any value of m , M2 and m 0 .
of tan b . In region 1., we obtain the following lower limits: Me˜ R ) 79.3 GeV Ž l i jk analysis. and Me˜ R ) 73.0 GeV Ž lXXi jk analysis., while in region 2. the 95% C.L. limits are: Me˜ R ) 61.8 GeV Ž l i jk analysis. and Me˜ R ) 29.5 GeV Ž lXXi jk analysis.. The supersymmetric partner of the right-handed electron is predicted to be the lightest scalar lepton 9 , hence tighter bounds are obtained for all other scalar leptons. The search for neutralino and chargino R-parity violating decays reaches at least the same sensitivity as in the R-parity conserving case w22x. Therefore, the supersymmetry limits obtained at LEP are independent of R-parity conservation assumptions. Acknowledgements We wish to express our gratitude to the CERN accelerator division for the excellent performance of the LEP machine. We acknowledge the effort of the engineers and technicians who have participated in the construction and maintenance of this experiment. References
Fig. 6. 95% C.L. exclusion contours in the M2 y m 0 plane, for tan b s 2. The lines labelled with the corresponding value in GeV represent the contours of constant scalar electron mass. The solid and dotted curves show the 95% C.L. lower limits on M2 as a function of m 0 , from which we derive the limits on the scalar electron mass.
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Search for R-parity violating chargino and neutralino decays in eqey collisions up to (s s 183 GeV L3 Collaboration M. Acciarri z , P. Achard r, O. Adriani o , M. Aguilar-Benitez y, J. Alcaraz y, G. Alemanni u , J. Allaby p, A. Aloisio ab, M.G. Alviggi ab, G. Ambrosi r, H. Anderhub au , V.P. Andreev f,aj, T. Angelescu l , F. Anselmo i , A. Arefiev aa , T. Azemoon c , T. Aziz j, P. Bagnaia ai , L. Baksay ap, A. Balandras d , R.C. Ball c , S. Banerjee j, Sw. Banerjee j, K. Banicz ar, A. Barczyk au,as , R. Barillere ` p, ai u i af u L. Barone , P. Bartalini , M. Basile , R. Battiston , A. Bay , F. Becattini o , U. Becker n , F. Behner au , J. Berdugo y, P. Berges n , B. Bertucci af , B.L. Betev au , S. Bhattacharya j, M. Biasini af , A. Biland au , J.J. Blaising d , S.C. Blyth ag , G.J. Bobbink b, A. Bohm ¨ a, L. Boldizsar m , B. Borgia p,ai, au r D. Bourilkov , M. Bourquin , S. Braccini r, J.G. Branson al , V. Brigljevic au , F. Brochu d , A. Buffini o , A. Buijs aq , J.D. Burger n , W.J. Burger af , J. Busenitz ap, A. Button c , X.D. Cai n , M. Campanelli au , M. Capell n , G. Cara Romeo i , G. Carlino ab, A.M. Cartacci o , J. Casaus y, G. Castellini o , F. Cavallari ai , N. Cavallo ab, C. Cecchi r, M. Cerrada y, F. Cesaroni v, M. Chamizo r, Y.H. Chang aw, U.K. Chaturvedi q , M. Chemarin x , A. Chen aw, G. Chen g , G.M. Chen g , H.F. Chen s , H.S. Chen g , X. Chereau d , G. Chiefari ab, L. Cifarelli ak , F. Cindolo i , C. Civinini o , I. Clare n , R. Clare n , G. Coignet d , A.P. Colijn b, N. Colino y, S. Costantini h , F. Cotorobai l , B. Cozzoni i , B. de la Cruz y, A. Csilling m , T.S. Dai n , J.A. van Dalen ad , R. D’Alessandro o , R. de Asmundis ab, P. Deglon r, A. Degre´ d , K. Deiters as , D. della Volpe ab, P. Denes ah , F. DeNotaristefani ai , A. De Salvo au , M. Diemoz ai , D. van Dierendonck b, F. Di Lodovico au , C. Dionisi p,ai , M. Dittmar au , A. Dominguez al , A. Doria ab, M.T. Dova q,1, D. Duchesneau d , D. Dufournand d , P. Duinker b, I. Duran am , H. El Mamouni x , A. Engler ag , F.J. Eppling n , F.C. Erne´ b, P. Extermann r, M. Fabre as , R. Faccini ai , M.A. Falagan y, S. Falciano ai , A. Favara o , J. Fay x , O. Fedin aj, M. Felcini au , T. Ferguson ag , F. Ferroni ai , H. Fesefeldt a , E. Fiandrini af , J.H. Field r, F. Filthaut p, P.H. Fisher n , I. Fisk al , G. Forconi n , 0370-2693r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 0 - 2 6 9 3 Ž 9 9 . 0 0 6 8 0 - 2
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d
I. Physikalisches Institut, RWTH, D-52056 Aachen, Germany, and III. Physikalisches Institut, RWTH, D-52056 Aachen, Germany 5 b National Institute for High Energy Physics, NIKHEF, and UniÕersity of Amsterdam, NL-1009 DB Amsterdam, The Netherlands c UniÕersity of Michigan, Ann Arbor, MI 48109, USA Laboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Institute of Physics, UniÕersity of Basel, CH-4056 Basel, Switzerland f Louisiana State UniÕersity, Baton Rouge, LA 70803, USA g Institute of High Energy Physics, IHEP, 100039 Beijing, China 6 h Humboldt UniÕersity, D-10099 Berlin, Germany 5 i UniÕersity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy j Tata Institute of Fundamental Research, Bombay 400 005, India k Northeastern UniÕersity, Boston, MA 02115, USA l Institute of Atomic Physics and UniÕersity of Bucharest, R-76900 Bucharest, Romania m Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary 7 n Massachusetts Institute of Technology, Cambridge, MA 02139, USA o INFN Sezione di Firenze and UniÕersity of Florence, I-50125 Florence, Italy p European Laboratory for Particle Physics, CERN, CH-1211 GeneÕa 23, Switzerland q World Laboratory, FBLJA Project, CH-1211 GeneÕa 23, Switzerland r UniÕersity of GeneÕa, CH-1211 GeneÕa 4, Switzerland s Chinese UniÕersity of Science and Technology, USTC, Hefei, Anhui 230 029, China 6 t SEFT, Research Institute for High Energy Physics, P.O. Box 9, SF-00014 Helsinki, Finland u UniÕersity of Lausanne, CH-1015 Lausanne, Switzerland v INFN-Sezione di Lecce and UniÕersita´ Degli Studi di Lecce, I-73100 Lecce, Italy w Los Alamos National Laboratory, Los Alamos, NM 87544, USA x Institut de Physique Nucleaire de Lyon, IN2P3-CNRS, UniÕersite´ Claude Bernard, F-69622 Villeurbanne, France ´ y Centro de InÕestigaciones Energeticas, Medioambientales y Tecnologıcas, CIEMAT, E-28040 Madrid, Spain 8 ´ ´ z INFN-Sezione di Milano, I-20133 Milan, Italy aa Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia ab INFN-Sezione di Napoli and UniÕersity of Naples, I-80125 Naples, Italy ac Department of Natural Sciences, UniÕersity of Cyprus, Nicosia, Cyprus ad UniÕersity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands ae California Institute of Technology, Pasadena, CA 91125, USA af INFN-Sezione di Perugia and UniÕersita´ Degli Studi di Perugia, I-06100 Perugia, Italy ag Carnegie Mellon UniÕersity, Pittsburgh, PA 15213, USA ah Princeton UniÕersity, Princeton, NJ 08544, USA ai INFN-Sezione di Roma and UniÕersity of Rome, ‘‘La Sapienza’’, I-00185 Rome, Italy aj Nuclear Physics Institute, St. Petersburg, Russia
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ak
UniÕersity and INFN, Salerno, I-84100 Salerno, Italy UniÕersity of California, San Diego, CA 92093, USA am Dept. de Fisica de Particulas Elementales, UniÕ. de Santiago, E-15706 Santiago de Compostela, Spain Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria ao Center for High Energy Physics, AdÕ. Inst. of Sciences and Technology, 305-701 Taejon, South Korea ap UniÕersity of Alabama, Tuscaloosa, AL 35486, USA aq Utrecht UniÕersity and NIKHEF, NL-3584 CB Utrecht, The Netherlands ar Purdue UniÕersity, West Lafayette, IN 47907, USA as Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland at DESY-Institut fur ¨ Hochenergiephysik, D-15738 Zeuthen, Germany au Eidgenossische Technische Hochschule, ETH Zurich, CH-8093 Zurich, Switzerland ¨ ¨ ¨ av UniÕersity of Hamburg, D-22761 Hamburg, Germany aw National Central UniÕersity, Chung-Li, Taiwan, ROC ax Department of Physics, National Tsing Hua UniÕersity, Taiwan, ROC al
an
Received 19 February 1999 Editor: K. Winter
Abstract A search for chargino and neutralino pair production in eqey collisions at center-of-mass energies between 161 GeV and 183 GeV is performed under the assumptions that R-parity is not conserved and that only purely leptonic or hadronic R-parity violating decays are allowed. No signal is found in the data. Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived. q 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction The most general superpotential of the Minimal Supersymmetric Standard Model ŽMSSM. w1x, which describes a supersymmetric, renormalizable and gauge invariant theory, with minimal particle content, includes the term WR w2x: WR s l i jk L i L j E k q lXi jk L i Q j D k q lXXi jkUi Dj Dk ,
Ž 1. 1 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina. 2 Also supported by Panjab University, Chandigarh-160014, India. 3 Supported by Deutscher Akademischer Austauschdienst. 4 Also supported by the Hungarian OTKA fund under contract numbers T22238 and T026178. 5 Supported by the German Bundesministerium fur ¨ Bildung, Wissenschaft, Forschung und Technologie. 6 Supported by the National Natural Science Foundation of China. 7 Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T024011. 8 Supported also by the Comision ´ Interministerial de Ciencia y Tecnologıa. ´
where l i jk , lXi jk and lXXi jk are the Yukawa couplings and i, j and k are the generation indices; L i and Q i are the left-handed lepton- and quark-doublet superfields; E i , Di and Ui are the right-handed singlet superfields for charged leptons, down- and up-type quarks, respectively. In order to prevent the simultaneous presence of identical fermionic fields, the following antisymmetry relations are required: l i jk s yl ji k and lXXi jk s ylXXi k j , so that there are in total 9 q 27 q 9 independent Yukawa couplings. The l i jk and lXi jk couplings violate the leptonic quantum number L, while the lXXi jk couplings violate the baryonic quantum number B. Their simultaneous presence would lead to a fast proton decay w3x, which is experimentally excluded w4x. This can be avoided by imposing the conservation of R-parity, a multiplicative quantum number defined as: R s Ž y1 .
3BqLq2S
,
Ž 2.
where S is the spin. R is q1 for all ordinary particles, and y1 for their supersymmetric partners. As a consequence, if R-parity is conserved, super-
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Table 1 Allowed R-parity violating direct decays of the neutralino and chargino. Charged conjugate states are implied Coupling
l i jk lXXi jk
Neutralino decays
l
y i nj
l
q k ,
uid jdk
q y j k
ni l l
Chargino decays q q y ninj l q k , li lj lk did jdk, uiu jdk, uid juk
symmetric particles can be produced only in pairs and they decay in cascade to the lightest supersymmetric particle ŽLSP., which is stable. However, the absence of either the B-violating or the L-violating terms, or of a subset of them, is enough to prevent a fast proton decay, while allowing the LSP to decay into Standard Model particles via scalar lepton or quark exchange. In this paper, we search for pair-produced neutralinos Žeq ey™ x˜ 10 x˜ 10 , eq ey™ x˜ i0x˜ 10 ,i G 2. and . with subsequent R-parity charginos Žeq ey™ x˜q ˜y 1 x 1 violating decays, assuming that one of the l i jk or lXXi jk coupling constants is non-negligible. The lXi jk couplings are not considered. In all processes studied, the lightest neutralino is assumed to decay into three fermions according to the dominant l i jk or lXXi jk interaction term, as detailed in Table 1. When the chargino is the LSP or when the R-parity violating coupling is strong enough, direct decays of charginos into three fermions can also occur via the dominant R-parity violating term, as listed in Table 1. If the x˜ 10 is the LSP, charginos can also decay indirectly into x˜ 10 W ) , or into x˜ 02 W ) for chargino heavier than the next-to-lightest neutralino. Similarly the heavier neutralinos Ž x˜ 0i ,i G 2. can decay indirectly into Z )x˜ 10 or directly into fermions. When the lightest scalar lepton is the LSP, the process x˜ 10 ™ l l˜ is taken into account for the l i jk analysis. In the present analysis, the dominant coupling is assumed to be greater than 10y5 w5x, corresponding to decay lengths smaller than 1 cm. Searches for R-parity violating decays have also been performed by other LEP experiments w6x.
2. Data and Monte Carlo samples The data used corresponds to an integrated luminosity of 55.3 pby1 collected by the L3 detector w7x at the center-of-mass energy Ž's . of 182.7 GeV,
hereafter referred to as 183 GeV. For the indirect l i jk analysis, 21.1 pby1 of data collected at 161– 172 GeV are also used. The signal events are generated with SUSYGEN w8x for different values of neutralino and chargino masses, for all possible choices of the generation indices. The following Monte Carlo generators are used to simulate Standard Model processes: PYTHIA w9x for eq ey™ qq, eq ey™ Z eq ey and eq ey™ ZZ, BHAGENE3 w10x for eq ey™ eq ey, KORALZ w11x for eq ey™ mq my and eq ey™ tq ty, PHOJET w12x for eq ey ™ eq ey qq, DIAG36 w13x for eq ey ™ eq ey l q l y Ž l s e,m,t . and KORALW w14x for eq ey ™ WqWy. The number of simulated events corresponds to at least 100 times the luminosity of the data, except for Bhabha and two-photon processes, where the Monte Carlo samples correspond to approximately 5 times the luminosity. The detector response is simulated using the GEANT package w15x. It takes into account effects of energy loss, multiple scattering and showering in the detector materials. Hadronic interactions are simulated with the GHEISHA program w16x. Time dependent inefficiencies of the different subdetectors are also taken into account in the simulation procedure.
3. Analysis procedure 3.1. l i jk : topology and preselection When the l i jk couplings dominate, the LSP decays into three leptons. The possible topologies arising from the different final states are listed in Table 2. Selections for the different topologies are developed. The decay products of the x˜ 10 pair are four charged leptons and two neutrinos. If the l 133 coupling dominates, each neutralino can decay into netq ty, nt eq ty, nety tq or nt ey tq, and the final state contains at least two t leptons, which are selected with lower efficiency with respect to electrons and muons. If the neutralino mass is high Ž Mx˜ 0 G 50 GeV. the event topology consists of four 1 charged leptons, isotropically distributed, and missing energy Ž Eu .. On the contrary, for low neutralino
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Table 2 Processes and topologies considered in the l i jk coupling analysis Process
Topology
eq ey™ x˜ 10 x˜ 10 ™ l l l l nn
4 l q Eu
eq ey™ x˜ i0x˜ 10 ™ x˜ 10 x˜ 10 Z ) ™ l l l l nn qq Ž i G 2. l l l l nn l l l l l l nn nn
4 l q 2 jets q Eu 6 l q Eu 4 l q Eu
eq ey™ x˜q ˜y ˜ 10 x˜ 10 W ) W ) ™ l l l l nn qq’qq’ 1 x 1 ™ x l l l l nn qq’ l n l l l l nn l n l n
4 l q 4 jets q Eu 5 l q 2 jets q Eu 6 l q Eu
eq ey™ x˜q ˜y 1 x 1 ™ l l l l l l l l l l nn l l nnnn
6 l 4 l q Eu 2 l q Eu
masses Ž Mx˜ 10 F 20 GeV. the events consist of two back-to-back lepton pairs and missing energy. In this case, there is a large background from Standard Model lepton-pair processes. In the case of the process eqey™ x˜ i0x˜ 10 ™ 0 ) 0 x˜ 1 Z x˜ 1 , three possible final states are present, corresponding to the Z ) decays into hadrons, charged leptons or neutrinos. As is shown in Table 2, the topologies arising from this process include leptons and missing energy or leptons plus hadronic jets and missing energy. These topologies are covered by combining the neutralino and chargino pair selections, described below. In the case of chargino production, the signal topologies depend not only on the values of the chargino and neutralino masses, but also on the chargino decay modes. For small values of the mass difference D M s Mx˜ 1" y Mx˜ 10 Ž D M F 10 GeV. the event energy is mainly carried by the neutralino decay products, almost independently of the different chargino decay channels. Therefore, a common selection is developed for all chargino decay modes. On the contrary, for medium and large D M values Ž D M ) 10 GeV., different selections are developed for each possible configuration. As the process eqey ™ qq and hadronic Wq Wy decays are the main background for the hadronic mode, a relatively low multiplicity is required. In the case of chargino R-parity violating direct decays, three possible topologies can occur. When
both charginos decay into three charged leptons, the final state is almost background free. If the decay products are two charged leptons and four neutrinos, the main background contributions come from leptonic W and t decays and two-photon interactions. The last topology, four charged leptons plus missing energy, is already taken into account by the neutralino pair selection. Events are preselected by requiring at least 3 charged tracks and 4 calorimetric clusters in order to remove eqey™ eqey,mqmy and purely leptonic tqty and WqWy decays. Events have to contain at least two charged leptons. The visible energy has to be smaller than 90% of 's in order to remove high multiplicity qq events and hadronic Wq Wy and ZZ decays. The missing momentum vector is required to be in the polar angle range between 58 and 1758. In order to reject cosmic ray interactions, at least one time of flight measurement is required to be consistent with the beam crossing. Tagged two-photon interactions are rejected by requiring the sum of the energies measured in the small angle calorimeters between 1.58 and 9.08 to be less than 10 GeV. In addition, the visible energy must be greater than 15% of 's . Background from two-photon collisions is further reduced by requiring the transverse missing momentum to be greater than 5 GeV. After the preselection is applied, 337 events are selected in the data sample and 334 events are
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jet resolution parameter ym n is defined as the ycut value at which the event configuration changes from n to m jets, when using the DURHAM w17x clustering scheme. The data are in good agreement with the Monte Carlo expectations. 3.2. lXXi jk : topology and preselection
Fig. 1. Distributions of a. the number of tracks, b. the number of leptons, c. the normalised visible energy and d. lnŽ y 34 . after the l i jk preselection. The solid histograms show the normalised sum of Standard Model processes. The dotted and dashed histograms show two examples of signal, with dominant coupling l133 . The dotted histograms represent the process eq ey ™ x˜ 10 x˜ 10 , for Mx˜ 10 s91 GeV, corresponding to one hundred times the expected cross section. The dashed ones represent eq ey ™ x˜q ˜y 1x 1 , with Mx˜ 1" s 91 GeV and D M s 50 GeV, corresponding to four times the expected cross section.
expected in total from Standard Model processes. The signal preselection efficiency for eqey™ x˜q ˜y1 1 x is about 85% for Mx˜ 1" s 91 GeV, at 's s 183 GeV. For eqey™ x˜ 10 x˜ 10 the efficiency is greater than 80% for Mx˜ 10 G 50 GeV and between 35% and 60% for 5 GeV F Mx˜ 10 F 20 GeV. Fig. 1 shows the number of tracks, number of leptons, normalised visible energy and lnŽ y 34 . distributions after the preselection. The
If lXXi jk couplings dominate, the LSP decays into three quarks, with a flavour composition given by the generation indices of the dominant lXXi jk coupling. The possible topologies arising from the different final states are summarized in Table 3. In the case of x˜ 10 pair production, the final state contains six quarks, and three different topologies can occur, depending on the value of Mx˜ 10 . For low and medium neutralino masses Ž Mx˜ 10 F 50 GeV. not all the six jets in the event can be resolved, while for high masses Ž Mx˜ 10 ) 50 GeV. they can be well separated. Therefore, the signal topology is two-jet like for small values of the neutralino mass. For intermediate neutralino masses the events are more similar to four jets, while for large neutralino masses the events can contain six isolated jets, isotropically distributed. For small neutralino masses the main background contribution comes from eq ey™ qq. When the signal topology is four-jet like, qqgg and hadronic WqWy decays contribute. For high neutralino masses, the most important background source is WqWy events. The same multi-jet topology occurs when charginos decay directly into three quarks, or when the mass difference D M is sufficiently small, so that
Table 3 XX Processes and topologies considered in the l i jk coupling analyses for events containing two, four or six resolved jets Process q y
x 10 x 10
e e ™ ˜ ˜
or
y xq 1 x1
˜ ˜ ™ qqqqqq
Topology 2 jets 4 jets 6 jets
eq ey™ x˜ i0x˜ 10 ™ x˜ 10 x˜ 10 Z ) ™ qqqqqq ff Ž i G 2.
4 jets or 6 jets Ž4 or 6. jets q leptons Ž4 or 6. jets q Eu
eq ey™ x˜q ˜y ˜ 10 x˜ 10 W ) W ) ™ qqqqqq ff’f’’f’’’ 1 x 1 ™ x
4 jets or 6 jets Ž4 or 6. jets q leptonŽs. q Eu
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the decay products of the W ) pair carry only a negligible fraction of the event energy. Since charginos of masses up to 45 GeV are excluded by Z lineshape measurements w18x, the two-jet topology is not addressed in this case. Analogous considerations apply for the processes x˜ 02 x˜ 10 and x˜ 03 x˜ 10 . When D M is large, leptons and neutrinos from W ) decays can carry a relevant fraction of the event energy, leading to lower selection efficiencies. In this analysis no attempt is made to identify quark flavours. However, the efficiency is found to be slightly higher for events containing b-quarks than for events with light quarks. Therefore, only the results obtained by the choice lXXi jk s lXX112 will be quoted in the next sections. The preselection of the lXXi jk analysis aims at selecting well balanced hadronic events. Low multiplicity events, like leptonic Z and W decays, are rejected by requiring at least 13 calorimetric clusters. At least one charged track has to be present. The visible energy has to be greater than 50% of 's . The energy imbalances, parallel and perpendicular to the beam direction, are required to be smaller than 20% of the visible energy. Above the Z peak a large fraction of background events contains a hard initial state radiation ŽISR. photon. Unbalanced events with an ISR photon in the beam pipe are removed by means of the requirement on the parallel energy imbalance. In order to reject events with an ISR photon seen in the detector, the invariant mass of the hadronic system has to be greater than 80% of 's . In order to remove background contributions from two-photon interactions, the energy in a cone of 128 half opening angle around the beam axis must not exceed 30% of the total visible energy. Furthermore, the thrust axis is required to be well contained in the detector with a polar angle between 88 and 1728. After the preselection is applied, 1953 events are selected in the data sample and 1949 are expected from Standard Model processes, mainly coming from qq and Wq Wy events. The signal preselection efficiency for eqey™ x˜ 10 x˜ 10 is about 85% and for 's s eq ey™ x˜q ˜y 1 x 1 is between 60% and 90%, at Ž . 183 GeV. Fig. 2 shows the ln y 34 , thrust, lnŽ y45 . and narrow jet broadening Ž BN . w19x distributions after the preselection. There is a good agreement between data and Monte Carlo expectations.
361
Fig. 2. Distributions of a. lnŽ y 34 ., b. thrust, c. lnŽ y45 . and d. the XX narrow jet broadening after the l i jk preselection. The solid histograms show Standard Model processes at 's s183 GeV. The dashed and dotted histograms show two examples of signal, with XX dominant coupling l112 . The dashed histograms represent the q y 0 0 process e e ™ x˜ 1 x˜ 1 , with Mx˜ 10 s90 GeV, corresponding to five hundred times the expected cross section. The dotted ones represent the same process, with Mx˜ 10 s 30 GeV, corresponding to twenty times the expected cross section.
3.3. Analysis optimization After the preselection level, dedicated selections are performed to maximize the l i jk and lXXi jk analysis sensitivities according to the topologies arising from those couplings. In the l i jk case, the best sensitivity is achieved performing different selections according to the neutralino mass values and decay products. Chargino selections depend also on W ) decay modes. In total thirteen selections are performed for which the cut values of the following variables are optimized simultaneously: acollinearity and acoplanarity angles, thrust, y 34 and polar angle of the missing momentum. The acollinearity and acoplanarity angles are calculated by forcing hadronic and leptonic objects in every event into exactly two jets with the DURHAM algorithm. Fourteen selections are performed for the lXXi jk analysis. The sum of the di-jet masses is required not to be consistent with WqWy pair production. For the lXXi jk neutralino selection, the optimization proce-
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Table 4 ' Efficiency ranges of x˜ 10 x˜ 10 and x˜q ˜y 1 x 1 selections for direct R-parity violating neutralino and chargino decays, at s s 183 GeV. In case of the process x˜q ˜y1 , the lowest mass value considered is Mx˜ 1" s 15 GeV for l133 and 45 GeV for lXX112 1 x Coupling
Process
Mx˜ s 5–25 GeV
Mx˜ s 25–55 GeV
Mx˜ s 55–91 GeV
l133 l133 lXX112
x 10 x 10 y xq 1 x1 0 0 x1 x1 ,
3%–10% 10%–18%
10%–33% 21%–40%
39%–55% 43%–54%
19%–22%
23%–24%
21%–32%
˜ ˜ ˜ ˜ ˜ ˜
x˜q ˜y 1 x 1
dure includes the following variables: thrust, wide jet broadening variable, y 34 and y45 . For the lXXi jk chargino selection, the cut values of the following variables are optimized: thrust, BN , y 34 and y56 . The optimization procedure uses Monte Carlo signal and background events and is described in Ref. w20x. For two-photon interactions, which represent an important background source for the l i jk analysis, the optimization and the background estimation are performed on two independent samples to avoid potential biases from statistical fluctuations. 3.4. Efficiencies Here we discuss only the results obtained for l i jk s l133 and lXXi jk s lXX112 , since these choices of the generation indices give the lowest selection efficiencies. In the following, only the efficiencies at 's s 183 GeV are quoted for simplicity. The efficiencies of the processes eqey™ x˜ 10 x˜ 10 , eqey™ x˜q ˜y1 and eqey™ x˜ i0x˜ 10 are summarized in Tables 1 x 4 and 5 for direct and indirect R-parity violating decays. In the case of direct R-parity violating decays, the efficiencies are estimated for different lightest neutralino or chargino masses. For both l133 and lXX112
mediated decays, the efficiencies increase with increasing lightest neutralino or chargino mass. At high masses, six fermions are expected to be isotropically produced and can be disentangled from W-pair production background events. For low masses, the signal signatures look like back-to-back jet events and the selection efficiencies are smaller due to the dominant background coming from the two-fermion processes. In addition, for the l133 selection the efficiencies obtained for low masses Žbelow 50 GeV. are higher for chargino than for neutralino due to the contribution of the six charged lepton final state. In the case of indirect R-parity violating decays for both charginos and next-to-lightest neutralinos, the efficiencies are estimated for different masses and D M ranges. For a chargino mass of 91 GeV and assuming x˜ 1" ™ x˜ 10 W ) or x˜ 1" ™ x˜ 20 W ) ™ x˜ 10 Z ) W ) , the efficiencies obtained for l133 and lXX112 mediated decays decrease with increasing D M. At high D M, the signal signatures are very similar to those of W-pair production. For Mx˜ i0 q Mx˜ 10 s 180 GeV, the efficiencies of the process eqey™ x˜ 0i x˜ 10 Ž i s 2,3,4. decrease slightly with increasing D M for the l133 and lXX112 analyses. In the latter case, the efficiencies are smaller compared to those obtained for charginos due to the invisible or purely leptonic Z ) decays.
Table 5 ' Efficiency ranges of x˜ i0x˜ 10 and x˜q ˜y 1 x 1 selections for indirect R-parity violating neutralino and chargino decays, at s s 183 GeV. The chargino selection efficiencies correspond to Mx˜ 1" s 91 GeV. For the process x˜ i0x˜ 10 the efficiencies correspond to Mx˜ i0 q Mx˜ 10 s 180 GeV Coupling Process D M s 5–20 GeV D M s 20–50 GeV D M s 50–80 GeV
l133 l 133 lXX112 lXX112
x˜ i0x˜ 10 x˜q ˜y 1 x 1 0 0 x˜ i x˜ 1 x˜q ˜y 1 x 1
47%–54% 47%–56% 31%–32% 30%–43%
42%–47% 28%–47% 28%–30% 45%–51%
26%–37% 17%–26% 28%–24% 10%–43%
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4. Results The summary of the searches is given in Table 6, showing the number of candidates and expected background events. We do not observe any excess of events. Therefore we set upper limits on the neutralino and chargino production cross sections assuming direct or indirect R-parity violating decays. We also derive limits on the masses of these particles in the framework of the MSSM. Exclusion limits at 95% C.L. are derived taking into account the estimated background contamination. The predicted contribution from the two-photon processes, due to the large statistical error, is not included, such that data events consistent with this background are conservatively considered as signal. Systematic errors on the signal efficiency are evaluated as in Ref. w20x. The typical relative error is 5% and is dominated by Monte Carlo statistics. It is taken into account in the calculations of the signal upper limits w21x.
Fig. 3. 95% C.L. upper limits on: a. the neutralino production cross section as a function of the neutralino mass, b. the chargino production cross section as a function of the chargino mass, in the direct decay mode and c. the chargino production cross section as a function of D M, for Mx˜ 1" s91 GeV, in the indirect decay mode. The solid lines show the limits obtained by the l i jk s l133 XX analysis, and the dashed lines show those obtained by the l i jk s lXX112 analysis.
4.1. Upper limits on neutralino and chargino production cross sections The 95% C.L. upper limits on neutralino and chargino pair-production cross sections at 183 GeV, both for l i jk and lXXi jk , are shown in Fig. 3. For the indirect l i jk analysis the data collected at 183 GeV are combined with those at the lower center of mass energies. Hence this upper limit should be interpreted as a limit on the luminosity-weighted average y1 cross section Ž L TOT ÝE 's s ŽE 's . L ŽE 's ... In the case of dominant l i jk coupling, the neutralino pair-production cross section is below 0.2 pb at 95% C.L. for Mx˜ 10 G 40 GeV and below 0.1 pb for
Table 6 Number of expected background and observed data events for the different selections Coupling
Process
N background
Ndata
l i jk l i jk l i jk lXXi jk lXXi jk
x 10 x 10 y xq 1 x1 q y x1 x1 x 10 x 10 y xq 1 x1
1.3 " 0.1 1.6 " 0.1 10.4 " 0.5 62 " 2 46 " 1
0 0 10 52 40
˜ ˜ ˜ ˜ ˜
˜ ˜ ˜ ˜ ˜
Žindirect. Ždirect.
Mx˜ 10 G 60 GeV. The chargino cross section is below 0.4 pb for a chargino mass greater than 45 GeV in the direct decay mode and is below 0.2 pb for Mx˜ 1" s 91 GeV and D M F 50 GeV in the indirect decay mode. In the case of dominant lXXi jk coupling, the neutralino cross section is below 0.6 pb at 95% C.L. for any value of the neutralino mass. The chargino cross section is below 0.6 pb for a chargino mass greater than 45 GeV in the direct decay mode and is below 0.4 pb for Mx˜ 1" s 91 GeV and D M F 60 GeV in the indirect decay mode. 4.2. Interpretation in the MSSM The results are also interpreted as excluded regions in the MSSM parameter space. In the MSSM framework neutralino and chargino masses, couplings and cross sections depend on the gaugino mass parameter, M2 , the higgsino mass mixing parameter, m , the ratio of the vacuum expectation values of the two Higgs doublets, tan b , and the common mass of the scalar fermions at the GUT
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scale, m 0 . Therefore the exclusion regions can be expressed in the M2 y m plane for a given value of m 0 and tan b . The results presented here hold for l i jk and lXXi jk ) 10y5 and for 0 F M2 F 2000 GeV, y500 GeV F m F 500 GeV. They do not depend on the value of the trilinear coupling in the Higgs sector, A. In addition to the limits obtained with this analysis, we take into account the constraints from L3 cross section measurements at the Z pole. A point in the MSSM parameter space is excluded at 95% C.L. by Z lineshape measurements if:
ž
s SUSY s SM
/
G Z ) G LIM ,
Ž 3.
where s SUSY is the sum of the pair-production cross sections of supersymmetric particles at 's s 91 GeV, calculated with SUSYGEN w8x, and s SM is the total Z cross section predicted by the Standard Model. G Z and G LIM s 24 MeV are the measured total Z width and the 95% C.L. upper limit on possible non-Standard Model contributions to the total Z width w18x. Fig. 4 shows the exclusion regions at 95% C.L. in the M2 y m plane for tan b s '2 and for m 0 s 60 GeV or 500 GeV, for both l i jk and lXXi jk . The excluded regions with the present results are dominated by the chargino analyses. Moreover some regions beyond the chargino kinematic limit are excluded at large m 0 and low tan b values by the x˜ 20 x˜ 10 and x˜ 03 x˜ 10 analyses and at low m 0 by the x˜ 10 x˜ 10 analyses. Lower limits on the masses of the supersymmetric particles are reported for the following two regions of the parameter space where: 1. the x˜ 10 is the LSP Ž50 GeV F m 0 F 500 GeV and any tan b or any m 0 and 2 F tan b F 40.; 2. the lightest scalar lepton can be the LSP Ž0 F m 0 - 50 GeV and 1 F tan b - 2.. In region 2., in the presence of dominant l i jk coupling the decay chain x˜ 10 ™ l l˜™ l l n leads to the same final states arising from neutralino direct R-parity violating decays, so that the analysis is efficient also when the lightest scalar lepton is the LSP. The additional contribution of the process x˜ 03 x˜ 10 ™ l l˜x˜ 10 is taken into account for the region in which M l˜- Mx˜ 30 . When the lXXi jk coupling dominate, the decay x˜ 10 ™ l l˜™ l l qqq occurs. Since there is
Fig. 4. Exclusion regions at 95% confidence level for l i jk s l133 Ža–b. and lXXi jk s lXX112 Žc–d., for tan b s'2 and m 0 s60 GeV Ža–c. or 500 GeV Žb–d.. The light grey region is excluded by the Z lineshape measurements and the dark grey region by the present analyses. The black solid lines indicate the chargino kinematic limit. The grey region beyond the kinematic limit is excluded by neutralino analyses.
no event generator available for this process, we quote two sets of mass limits for lXXi jk coupling, for region 1. and region 2., respectively. Fig. 5 shows the 95% C.L. lower limits on neutralino and chargino masses as a function of tan b . In region 1., we derive the following lower limits at 95% C.L. on the neutralino and chargino masses: Mx˜ 10 ) 26.8 GeV, Mx˜ 20 ) 44.3 GeV, Mx˜ 1" ) 91.1 GeV Ž l i jk analysis., Mx˜ 1" ) 90.9 GeV Ž lXXi jk analysis.. In region 2., the 95% C.L. lower limits on the neutralino and chargino masses set by the lXXi jk analysis are: Mx˜ 10 ) 26.8 GeV, Mx˜ 20 ) 34.8 GeV, Mx˜ 1" ) 76.9 GeV. From the exclusion contours in the M2 y m 0 plane we set indirect lower limits on the scalar lepton masses. Fig. 6 shows the 95% C.L. lower limits on
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Fig. 5. 95% C.L. lower limits on the masses of x˜ 10 Ža., x˜ 20 Žb. and x˜ 1" Žc., as a function of tan b , for 0 F m 0 F 500 GeV, 0 F M2 F 2000 GeV and y500 GeV F m F 500 GeV. The solid lines show the limit obtained by the l133 analysis, the dashed XX lines by the l112 analysis in region 1. and the dotted lines by the XX l112 analysis in region 2..
the mass of the supersymmetric partner of the righthanded electron Ž Me˜ R . for tan b s 2. These limits hold also for tan b ) 2. Fig. 7 shows the 95% C.L. lower limit on the scalar electron mass as a function
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Fig. 7. 95% C.L. lower limit on the mass of the scalar electron as a function of tan b , for any value of m , M2 and m 0 .
of tan b . In region 1., we obtain the following lower limits: Me˜ R ) 79.3 GeV Ž l i jk analysis. and Me˜ R ) 73.0 GeV Ž lXXi jk analysis., while in region 2. the 95% C.L. limits are: Me˜ R ) 61.8 GeV Ž l i jk analysis. and Me˜ R ) 29.5 GeV Ž lXXi jk analysis.. The supersymmetric partner of the right-handed electron is predicted to be the lightest scalar lepton 9 , hence tighter bounds are obtained for all other scalar leptons. The search for neutralino and chargino R-parity violating decays reaches at least the same sensitivity as in the R-parity conserving case w22x. Therefore, the supersymmetry limits obtained at LEP are independent of R-parity conservation assumptions. Acknowledgements We wish to express our gratitude to the CERN accelerator division for the excellent performance of the LEP machine. We acknowledge the effort of the engineers and technicians who have participated in the construction and maintenance of this experiment. References
Fig. 6. 95% C.L. exclusion contours in the M2 y m 0 plane, for tan b s 2. The lines labelled with the corresponding value in GeV represent the contours of constant scalar electron mass. The solid and dotted curves show the 95% C.L. lower limits on M2 as a function of m 0 , from which we derive the limits on the scalar electron mass.
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