Search for R-parity violating decays of supersymmetric particles in e⁺e⁻ collisions at √s= 189 GeV

Article

Reference

Search for R-parity violating decays of supersymmetric particles in e

e

collisions at √s= 189 GeV

L3 Collaboration

ACHARD, Pablo (Collab.), et al .

Abstract

A search for chargino, neutralino and scalar lepton pair-production in e+e− collisions at the centre-of-mass energy of 189 GeV is performed under the assumptions that R-parity is not conserved in decays and only one of the coupling constants λijk,λ′ijk or λ′′ijk is non-negligible.

No signal is found in a data sample corresponding to an integrated luminosity of 176.4 pb −1 . Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived.

L3 Collaboration, ACHARD, Pablo (Collab.), et al . Search for R-parity violating decays of supersymmetric particles in e

e

collisions at √s= 189 GeV. The European Physical Journal.

C, Particles and Fields , 2001, vol. 19, no. 3, p. 397-414

DOI : 10.1007/s100520100608

Available at:

http://archive-ouverte.unige.ch/unige:44039

Disclaimer: layout of this document may differ from the published version.

1 / 1

Digital Object Identifier (DOI) 10.1007/s100520100608

T ^HE E ^UROPEAN

P HYSICAL J OURNAL C

c Societ`a Italiana di Fisica Springer-Verlag 2001

Search for R-parity violating decays of supersymmetric particles in e ⁺ e ⁻ collisions at √

s = 189 GeV

The L3 Collaboration

M. Acciarri²⁶, P. Achard¹⁹, O. Adriani¹⁶, M. Aguilar-Benitez²⁵, J. Alcaraz²⁵, G. Alemanni²², J. Allaby¹⁷, A. Aloisio²⁷, M.G. Alviggi²⁷, G. Ambrosi¹⁹, H. Anderhub⁴⁸, V.P. Andreev^6,36, T. Angelescu¹², F. Anselmo⁹, A. Arefiev²⁷, T. Azemoon³, T. Aziz¹⁰, P. Bagnaia³⁵, A. Bajo²⁵, L. Baksay⁴³, A. Balandras⁴, S.V. Baldew²,

S. Banerjee¹⁰, Sw. Banerjee¹⁰, A. Barczyk^46,48, R. Barill`ere¹⁷, P. Bartalini²², M. Basile⁹, N. Batalova⁴⁵, R. Battiston³², A. Bay²², F. Becattini¹⁶, U. Becker¹⁴, F. Behner⁴⁸, L. Bellucci¹⁶, R. Berbeco³, J. Berdugo²⁵, P. Berges¹⁴,

B. Bertucci³², B.L. Betev⁴⁸, S. Bhattacharya¹⁰, M. Biasini³², A. Biland⁴⁸, J.J. Blaising⁴, S.C. Blyth³³, G.J. Bobbink², A. B¨ohm¹, L. Boldizsar¹³, B. Borgia³⁵, D. Bourilkov⁴⁸, M. Bourquin¹⁹, S. Braccini¹⁹, J.G. Branson³⁹, F. Brochu⁴, A. Buffini¹⁶, A. Buijs⁴⁴, J.D. Burger¹⁴, W.J. Burger³², X.D. Cai¹⁴, M. Capell¹⁴, G. Cara Romeo⁹, G. Carlino²⁷, A.M. Cartacci¹⁶, J. Casaus²⁵, G. Castellini¹⁶, F. Cavallari³⁵, N. Cavallo³⁷, C. Cecchi³², M. Cerrada²⁵, F. Cesaroni²³, M. Chamizo¹⁹, Y.H. Chang⁵⁰, U.K. Chaturvedi¹⁸, M. Chemarin²⁴, A. Chen⁵⁰, G. Chen⁷, G.M. Chen⁷, H.F. Chen²⁰, H.S. Chen⁷, G. Chiefari²⁷, L. Cifarelli³⁸, F. Cindolo⁹, C. Civinini¹⁶, I. Clare¹⁴, R. Clare¹⁴, G. Coignet⁴, N. Colino²⁵, S. Costantini⁵, F. Cotorobai¹², B. de la Cruz²⁵, A. Csilling¹³, S. Cucciarelli³², T.S. Dai¹⁴, J.A. van Dalen³⁰, R. D’Alessandro¹⁶, R. de Asmundis²⁷, P. D´eglon¹⁹, A. Degr´e⁴, K. Deiters⁴⁶, D. della Volpe²⁷, E. Delmeire¹⁹, P. Denes³⁴, F. DeNotaristefani³⁵, A. De Salvo⁴⁸, M. Diemoz³⁵, M. Dierckxsens², D. van Dierendonck², C. Dionisi³⁵, M. Dittmar⁴⁸, A. Dominguez³⁹, A. Doria²⁷, M.T. Dova^18,e, D. Duchesneau⁴, D. Dufournaud⁴, P. Duinker²,

I. Duran⁴⁰, H. El Mamouni²⁴, A. Engler³³, F.J. Eppling¹⁴, F.C. Ern´e², P. Extermann¹⁹, M. Fabre⁴⁶, M.A. Falagan²⁵, S. Falciano^35,17, A. Favara¹⁷, J. Fay²⁴, O. Fedin³⁶, M. Felcini⁴⁸, T. Ferguson³³, H. Fesefeldt¹, E. Fiandrini³²,

J.H. Field¹⁹, F. Filthaut¹⁷, P.H. Fisher¹⁴, I. Fisk³⁹, G. Forconi¹⁴, K. Freudenreich⁴⁸, C. Furetta²⁶, Yu. Galaktionov^27,14, S.N. Ganguli¹⁰, P. Garcia-Abia⁵, M. Gataullin³¹, S.S. Gau¹¹, S. Gentile^35,17, N. Gheordanescu¹², S. Giagu³⁵,

Z.F. Gong²⁰, G. Grenier²⁴, O. Grimm⁴⁸, M.W. Gruenewald⁸, M. Guida³⁸, R. van Gulik², V.K. Gupta³⁴, A. Gurtu¹⁰, L.J. Gutay⁴⁵, D. Haas⁵, A. Hasan²⁹, D. Hatzifotiadou⁹, T. Hebbeker⁸, A. Herv´e¹⁷, P. Hidas¹³, J. Hirschfelder³³, H. Hofer⁴⁸, G. Holzner⁴⁸, H. Hoorani³³, S.R. Hou⁵⁰, Y. Hu³⁰, I. Iashvili⁴⁷, B.N. Jin⁷, L.W. Jones³, P. de Jong², I. Josa-Mutuberr´ıa²⁵, R.A. Khan¹⁸, M. Kaur^18,f, M.N. Kienzle-Focacci¹⁹, D. Kim³⁵, J.K. Kim⁴², J. Kirkby¹⁷, D. Kiss¹³, W. Kittel³⁰, A. Klimentov^14,27, A.C. K¨onig³⁰, M. Kopal⁴⁵, A. Kopp⁴⁷, V. Koutsenko^14,27, M. Kr¨aber⁴⁸, R.W. Kraemer³³, W. Krenz¹, A. Kr¨uger⁴⁷, A. Kunin^14,27, P. Ladron de Guevara²⁵, I. Laktineh²⁴, G. Landi¹⁶, M. Lebeau¹⁷, A. Lebedev¹⁴, P. Lebrun²⁴, P. Lecomte⁴⁸, P. Lecoq¹⁷, P. Le Coultre⁴⁸, H.J. Lee⁸, J.M. Le Goff¹⁷, R. Leiste⁴⁷, P. Levtchenko³⁶, C. Li²⁰, S. Likhoded⁴⁷, C.H. Lin⁵⁰, W.T. Lin⁵⁰, F.L. Linde², L. Lista²⁷, Z.A. Liu⁷, W. Lohmann⁴⁷, E. Longo³⁵, Y.S. Lu⁷, K. L¨ubelsmeyer¹, C. Luci^17,35, D. Luckey¹⁴, L. Lugnier²⁴, L. Luminari³⁵, W. Lustermann⁴⁸, W.G. Ma²⁰, M. Maity¹⁰, L. Malgeri¹⁷, A. Malinin¹⁷, C. Ma˜na²⁵, D. Mangeol³⁰, J. Mans³⁴, G. Marian¹⁵, J.P. Martin²⁴, F. Marzano³⁵, K. Mazumdar¹⁰, R.R. McNeil⁶, S. Mele¹⁷, L. Merola²⁷, M. Meschini¹⁶, W.J. Metzger³⁰, M. von der Mey¹, A. Mihul¹², H. Milcent¹⁷, G. Mirabelli³⁵, J. Mnich¹, G.B. Mohanty¹⁰, T. Moulik¹⁰, G.S. Muanza²⁴, A.J.M. Muijs², B. Musicar³⁹, M. Musy³⁵, M. Napolitano²⁷, F. Nessi-Tedaldi⁴⁸, H. Newman³¹, T. Niessen¹, A. Nisati³⁵, H. Nowak⁴⁷, R. Ofierzynski⁴⁸, G. Organtini³⁵, A. Oulianov²⁷, C. Palomares²⁵, D. Pandoulas¹, S. Paoletti^35,17, P. Paolucci²⁷, R. Paramatti³⁵, H.K. Park³³, I.H. Park⁴², G. Passaleva¹⁷, S. Patricelli²⁷, T. Paul¹¹, M. Pauluzzi³², C. Paus¹⁷, F. Pauss⁴⁸, M. Pedace³⁵, S. Pensotti²⁶, D. Perret-Gallix⁴, B. Petersen³⁰, D. Piccolo²⁷, F. Pierella⁹, M. Pieri¹⁶, P.A. Pirou´e³⁴, E. Pistolesi²⁶, V. Plyaskin²⁷, M. Pohl¹⁹, V. Pojidaev^27,16, H. Postema¹⁴, J. Pothier¹⁷, D.O. Prokofiev⁴⁵, D. Prokofiev³⁶, J. Quartieri³⁸, G. Rahal-Callot^48,17, M.A. Rahaman¹⁰, P. Raics¹⁵, N. Raja¹⁰, R. Ramelli⁴⁸, P.G. Rancoita²⁶, R. Ranieri¹⁶, A. Raspereza⁴⁷, G. Raven³⁹, P. Razis²⁹, D. Ren⁴⁸,

M. Rescigno³⁵, S. Reucroft¹¹, S. Riemann⁴⁷, K. Riles³, J. Rodin⁴³, B.P. Roe³, L. Romero²⁵, A. Rosca⁸, S. Rosier-Lees⁴, J.A. Rubio¹⁷, G. Ruggiero¹⁶, H. Rykaczewski⁴⁸, S. Saremi⁶, S. Sarkar³⁵, J. Salicio¹⁷, E. Sanchez¹⁷, M.P. Sanders³⁰, C. Sch¨afer¹⁷, V. Schegelsky³⁶, S. Schmidt-Kaerst¹, D. Schmitz¹, H. Schopper⁴⁹, D.J. Schotanus³⁰, G. Schwering¹, C. Sciacca²⁷, A. Seganti⁹, L. Servoli³², S. Shevchenko³¹, N. Shivarov⁴¹, V. Shoutko²⁷, E. Shumilov²⁷, A. Shvorob³¹, T. Siedenburg¹, D. Son⁴², B. Smith³³, P. Spillantini¹⁶, M. Steuer¹⁴, D.P. Stickland³⁴, A. Stone⁶, B. Stoyanov⁴¹, A. Straessner¹, K. Sudhakar¹⁰, G. Sultanov¹⁸, L.Z. Sun²⁰, S. Sushkov⁸, H. Suter⁴⁸, J.D. Swain¹⁸, Z. Szillasi^43,c, T. Sztaricskai^43,c, X.W. Tang⁷, L. Tauscher⁵, L. Taylor¹¹, B. Tellili²⁴, C. Timmermans³⁰, Samuel C.C. Ting¹⁴, S.M. Ting¹⁴, S.C. Tonwar¹⁰, J. T´oth¹³, C. Tully¹⁷, K.L. Tung⁷, Y. Uchida¹⁴, J. Ulbricht⁴⁸, E. Valente³⁵,

G. Vesztergombi¹³, I. Vetlitsky²⁷, D. Vicinanza³⁸, G. Viertel⁴⁸, S. Villa¹¹, M. Vivargent⁴, S. Vlachos⁵, I. Vodopianov³⁶,

H. Vogel³³, H. Vogt⁴⁷, I. Vorobiev³³, A.A. Vorobyov³⁶, A. Vorvolakos²⁹, M. Wadhwa⁵, W. Wallraff¹, M. Wang¹⁴, X.L. Wang²⁰, Z.M. Wang²⁰, A. Weber¹, M. Weber¹, P. Wienemann¹, H. Wilkens³⁰, S.X. Wu¹⁴, S. Wynhoff¹⁷, L. Xia³¹, Z.Z. Xu²⁰, J. Yamamoto³, B.Z. Yang²⁰, C.G. Yang⁷, H.J. Yang⁷, M. Yang⁷, J.B. Ye²⁰, S.C. Yeh⁵¹, An. Zalite³⁶, Yu. Zalite³⁶, Z.P. Zhang²⁰, G.Y. Zhu⁷, R.Y. Zhu³¹, A. Zichichi^9,17,18, G. Zilizi^43,c, B. Zimmermann⁴⁸, M. Z¨oller¹

1 I. Physikalisches Institut, RWTH, 52056 Aachen, Germany^a III. Physikalisches Institut, RWTH, 52056 Aachen, Germany^a

2 National Institute for High Energy Physics, NIKHEF, and University of Amsterdam, 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, 74941 Annecy-le-Vieux CEDEX, France

5 Institute of Physics, University of Basel, 4056 Basel, Switzerland

6 Louisiana State University, Baton Rouge, LA 70803, USA

7 Institute of HighEnergy Physics, IHEP, 100039 Beijing, P.R. China^g

8 Humboldt University, 10099 Berlin, Germany^a

9 University of Bologna and INFN-Sezione di Bologna, 40126 Bologna, Italy

10 Tata Institute of Fundamental Research, Bombay 400 005, India

11 Northeastern University, Boston, MA 02115, USA

12 Institute of Atomic Physics and University of Bucharest, 76900 Bucharest, Romania

13 Central ResearchInstitute for Physics of the Hungarian Academy of Sciences, 1525 Budapest 114, Hungary^b

14 Massachusetts Institute of Technology, Cambridge, MA 02139, USA

15 KLTE-ATOMKI, 4010 Debrecen, Hungary^c

16 INFN Sezione di Firenze and University of Florence, 50125 Florence, Italy

17 European Laboratory for Particle Physics, CERN, 1211 Geneva 23, Switzerland

18 World Laboratory, FBLJA Project, 1211 Geneva 23, Switzerland

19 University of Geneva, 1211 Geneva 4, Switzerland

20 Chinese University of Science and Technology, USTC, Hefei, Anhui 230 029, P.R. China^g

22 University of Lausanne, 1015 Lausanne, Switzerland

23 INFN-Sezione di Lecce and Universit`a Degli Studi di Lecce, 73100 Lecce, Italy

24 Institut de Physique Nucl´eaire de Lyon, IN2P3-CNRS,Universit´e Claude Bernard, 69622 Villeurbanne, France

25 Centro de Investigaciones Energ´eticas, Medioambientales y Tecnolog´ıcas, CIEMAT, 28040 Madrid, Spain^d

26 INFN-Sezione di Milano, 20133 Milan, Italy

27 Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia

28 INFN-Sezione di Napoli and University of Naples, 80125 Naples, Italy

29 Department of Natural Sciences, University of Cyprus, Nicosia, Cyprus

30 University of Nijmegen and NIKHEF, 6525 ED Nijmegen, The Netherlands

31 California Institute of Technology, Pasadena, CA 91125, USA

32 INFN-Sezione di Perugia and Universit`a Degli Studi di Perugia, 06100 Perugia, Italy

33 Carnegie Mellon University, Pittsburgh, PA 15213, USA

34 Princeton University, Princeton, NJ 08544, USA

35 INFN-Sezione di Roma and University of Rome, “La Sapienza”, 00185 Rome, Italy

36 Nuclear Physics Institute, St. Petersburg, Russia

37 INFN-Sezione di Napoli and University of Potenza, 85100 Potenza, Italy

38 University and INFN, Salerno, 84100 Salerno, Italy

39 University of California, San Diego, CA 92093, USA

40 Dept. de Fisica de Particulas Elementales, Univ. de Santiago, 15706 Santiago de Compostela, Spain

41 Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, 1113 Sofia, Bulgaria

42 Laboratory of HighEnergy Physics, Kyungpook National University, 702-701 Taegu, Republic of Korea

43 University of Alabama, Tuscaloosa, AL 35486, USA

44 Utrecht University and NIKHEF, 3584 CB Utrecht, The Netherlands

45 Purdue University, West Lafayette, IN 47907, USA

46 Paul Scherrer Institut, PSI, 5232 Villigen, Switzerland

47 DESY, 15738 Zeuthen, Germany

48 Eidgen¨ossische Technische Hochschule, ETH Z¨urich, 8093 Z¨urich, Switzerland

49 University of Hamburg, 22761 Hamburg, Germany

50 National Central University, Chung-Li, Taiwan, P.R. China

51 Department of Physics, National Tsing Hua University, Taiwan, P.R. China

Received: 11 October 2000 / Published online: 23 March 2001 – cSpringer-Verlag 2001

Abstract. A searchfor chargino, neutralino and scalar lepton pair-production in e⁺e⁻ collisions at the centre-of-mass energy of 189 GeV is performed under the assumptions that R-parity is not conserved in

decays and only one of the coupling constantsλijk,λ_ijkorλ_ijkis non-negligible. No signal is found in a data sample corresponding to an integrated luminosity of 176.4 pb⁻¹. Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived.

1 Introduction

The most general superpotential of the Minimal Super- symmetric Standard Model (MSSM) [1], which describes a supersymmetric, renormalizable and gauge invariant the- ory, with minimal particle content, includesthe term W_R [2,3]:

WR=λijkLiLjEk +λ_ijkLiQjDk +λ_ijkUiDjDk, (1) where λijk, λ_ijk and λ_ijk are the Yukawa couplingsand i, jand k the generation indices; L_i and Q_i are the left- handed lepton- and quark-doublet superfields, E_i, D_i and Uiare the right-handed singlet superfields for charged lep- tons, down- and up-type quarks, respectively. In order to prevent the simultaneous presence of identical fermionic fields, the following antisymmetry relations are required:

λ_ijk = −λ_jik and λ_ijk = −λ_ikj, reducing to 9 + 27 + 9 the total number of independent Yukawa couplings. The LiLjEk and LiQjDk termsviolate the leptonic quantum number L, while the UiDjDk termsviolate the baryonic quantum number B.

The simultaneous presence of L- and B-violating terms would lead to a fast proton decay¹ [4]. Therefore, exist- ing limits[5] on the proton lifetime require either of two possibilities.

The first one is to impose R-parity conservation, which forbidsall termsin (1). R-parity isa multiplicative quan- tum number defined as:

R = (−1)^3B+L+2S, (2)

where S isthe spin. R is+1 for all ordinary particles, and

−1 for their supersymmetric partners. If R-parity is con- served, supersymmetric particles can be produced only in pairs and they decay in cascade to the lightest supersym- metric particle (LSP), which isstable [6].

The second possibility is considered in this paper. Since the absence of either the B-violating or the L-violating

a Supported by the German Bundesministerium f¨ur Bildung, Wissenschaft, Forschung und Technologie

b Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T024011

cAlso supported by the Hungarian OTKA fund under contract numbers T22238 and T026178

dSupported also by the Comisi´on Interministerial de Ciencia y Tecnolog´ıa

eAlso supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina

f Also supported by Panjab University, Chandigarh-160014, India

g Supported by the National Natural Science Foundation of China

1 Withcontributions at the tree level fromλ_11kλ_11k, and at one-loop level from any productλijkλ_lmnorλ_ijkλ_lmn

termsisenough to prevent a fast proton decay, there is no need to impose a priori R-parity conservation. As a consequence, two new kinds of processes are allowed: sin- gle production of supersymmetric particles, or LSP decays into Standard Model particlesvia scalar lepton or quark exchange. In the latter case, the MSSM production mech- anismsare unaltered by the operatorsin (1).

In this paper, we describe the search for pair-produced neutralinos² (e⁺e⁻ → χ˜⁰_mχ˜⁰_n, with m = 1,2 and n = 1, ..,4), charginos(e⁺e⁻ → χ˜⁺₁χ˜⁻₁) and scalar leptons (e⁺e⁻ → ˜⁺_R˜⁻_R, e⁺e⁻ → ν˜ν˜) with subsequent R-parity violating decays, assuming that only one of the coupling constantsλ_ijk,λ_ijkorλ_ijkisnon-negligible. Only ˜_R(su- persymmetric partners of the right-handed charged lep- tons) are considered in this analysis, since they are ex- pected to be lighter than the corresponding left-handed ones.

Supersymmetric particles can decay directly into two or three fermionsaccording to the dominant interaction term, asdetailed in Table 1. Indirect decaysvia the LSP can occur aswell. Four-body decaysof the lightest scalar lepton are also taken into account in the case ofλ_ijk and λ_ijk. In the present analysis, the dominant coupling is assumed to be greater than 10⁻⁵ [8], corresponding to decay lengthslessthan 1 cm. PreviousL3 resultsonλijk

andλ_ijk Yukawa couplingscan be found in [9]. Searches for R-parity violating decays of supersymmetric particles were also performed by other LEP experiments [10].

2 Data andMonte Carlo samples

The data used correspond to an integrated luminosity of 176.4 pb⁻¹collected by the L3 detector [11] at the centre- of-mass energy (√

s) of 189 GeV.

The signal events are generated with the program SUSYGEN [12] for different mass values and for all possi- ble choicesof the generation indices.

The following Monte Carlo generators are used to sim- ulate Standard Model background processes:PYTHIA [13]

for e⁺e⁻→q¯q, e⁺e⁻→Z e⁺e⁻ and e⁺e⁻→ZZ,BHWIDE [14] for e⁺e⁻→e⁺e⁻,KORALZ[15] for e⁺e⁻→µ⁺µ⁻ and e⁺e⁻→τ⁺τ⁻,PHOJET[16] andPYTHIAfor e⁺e⁻→e⁺e⁻ hadrons,DIAG36[17] for e⁺e⁻ →e⁺e⁻⁺⁻ (= e, µ, τ), KORALW [18] for e⁺e⁻ →W⁺W⁻ and EXCALIBUR[19] for e⁺e⁻ → W^±∓ν. The number of simulated events cor- responds to at least 50 times the luminosity of the data, except for Bhabha and two-photon processes, where the Monte Carlo samples correspond to 2 to 6 times the lumi- nosity.

2 Single production of supersymmetric particles is not con- sidered in this paper

Table 1. R-parity violating decays of the supersymmetric particles considered in this analysis.

Charged conjugate states are implied. Indirect decays via scalar leptons are relevant only for neutralinos when the scalar lepton is the LSP. Only supersymmetric partners of the right-handed charged leptons are taken into account. Decays to more than three fermions are not listed. Z^∗ and W^∗indicate virtual Z and W

Particle Direct decays Indirect decays

λijk λ_ijk λ_ijk via ˜χ⁰₁ via ˜

˜

χ⁰₁ ⁻_i νj⁺_k,νi⁺_j⁻_k ⁻_iuj¯dk,νidj¯dk ¯ui¯dj¯dk − ˜

˜

χ⁰_n(n≥2) ⁻_i νj⁺_k,νi⁺_j⁻_k ⁻_iuj¯dk,νidj¯dk ¯ui¯dj¯dk Z^∗χ˜⁰₁, Z^∗χ˜⁰_m(m<n), ˜ W^∗χ˜^±₁

˜

χ⁺₁ νiνj⁺_k,⁺_i⁺_j⁻_k νiuj¯dk,⁺_i ¯djdk ¯di¯dj¯dk, uiujdk, W^∗χ˜⁰₁, W^∗χ˜⁰₂ uidjuk

˜⁻_kR νi⁻_j,νj⁻_i − − ⁻_kχ˜⁰₁ −

˜

νi, ˜νj ⁻_j⁺_k,⁻_i⁺_k − − νiχ˜⁰₁,νjχ˜⁰₁

The detector response is simulated using the GEANT package [20]. It takesinto account effectsof energy loss, multiple scattering and showering in the detector materi- als. Hadronic interactions are simulated with theGHEISHA program [21]. Time dependent inefficienciesof the differ- ent subdetectors are also taken into account in the simu- lation procedure.

3 Event reconstruction

Leptons(= e,µ, τ) and hadronic jetsare reconstructed asfollows. An electron isdefined asan electromagnetic shower, with energy greater than 1 GeV, matched with a track in the central chamber. Muon identification requires a track in the muon chambersmatched with a track in the central chamber. Hadronic tau decaysare reconstructed from narrow isolated hadronic jets with energy greater than 2 GeV and one to three associated tracks. The tau en- ergy must be contained in a cone of 10^◦half-opening angle around the jet direction. No additional tracksand no more than two additional calorimetric clusters are required in a further cone of 30^◦ half-opening angle. The ratio of the energiesin the two coneshasto be lessthan 2.0. Remain- ing clusters and tracks are classified as hadrons. Jets are reconstructed with the DURHAM algorithm [22]. The jet resolution parametery_mn isdefined asthe y_cut value at which the event configuration changesfrom n to m jets.

Njets8 isthe number of jetsclustered withycutfixed and equal to 0.008. The acollinearity (θ_acol) and acoplanarity (θ_acop) anglesare calculated by forcing hadronic and lep- tonic objects in every event into exactly two jets. At least one time of flight measurement has to be consistent with the beam crossing to reject cosmic rays.

4 λ

analysis

Table 2 shows the possible topologies arising when the λijk couplingsdominate. The different selectionscan be classified into four categories as follows: 2+E/, 4+E/,

Table 2. Processes considered in the λijk coupling analysis and corresponding selections. ˜χ⁰mχ˜⁰nindicates neutralino pair- production withm= 1,2 andn= 1, ..,4. “Cascades” refers to all possible final state combinations of Table 1

Direct decays Selections

e⁺e⁻→χ˜⁰mχ˜⁰n→ νν 4+E/ e⁺e⁻→χ˜⁺₁χ˜⁻₁ → 6

νν 4+E/

νννν 2+E/

e⁺e⁻→˜⁺_R˜⁻_R → νν 2+E/

e⁺e⁻→˜ν˜ν → 4+E/

Indirect decays

e⁺e⁻→χ˜⁰mχ˜⁰n(n≥2) → cascades (≥4)+ (jets) +E/ e⁺e⁻→χ˜⁺₁χ˜⁻₁ → χ˜⁰₁₍₂₎χ˜⁰₁₍₂₎W^∗W^∗ (≥4)+ (jets) +E/ e⁺e⁻→˜⁺_R˜⁻_R → νν (≥4)+ (jets) +E/ e⁺e⁻→˜ν˜ν → νννν 4+E/

6, (≥ 4) plus possible jets and E/. E/ (missing energy) indicatesfinal state neutrinosescaping detection. After a common preselection, a dedicated selection is developed for each group, taking into account lepton flavours, parti- cle boosts and virtual W and Z decay products.

Events are preselected by requiring at least two charged leptons(N_e,µ,τ), to reject q¯q eventsand hadronic W⁺W⁻ and ZZ decays. Events have to contain at least 3 charged tracks(Ntracks) and 4 calorimetric clusters (N_clusters) in order to remove e⁺e⁻ → e⁺e⁻, µ⁺µ⁻ and purely leptonicτ⁺τ⁻and W⁺W⁻ decays. The visible en- ergy (Evis) isrequired to be lessthan 90% of√

s, in order to reject background from lepton pair-production. If the number of tracksisat least 5, the cut on the visible energy is not applied, otherwise it would suppress signal events with six charged leptons. Back-to-back events, in particu- larτ⁺τ⁻, are reduced by requiringy34 to be greater than 0.0002. For low multiplicity signal events belonging to the

Data qq, ll γγ interactions WW, Weν, Zee, ZZ

χ⁰1 χ⁰1

M = 94 GeV χ⁺1 χ^-1

M = 94 GeV, ∆M = 40 GeV

0 250 500 750 1000

2 3 4 5

N_leptons

Events

0 50 100 150 200

0 1 2 3

Acollinearity (rad)

Events/0.1 rad

0 50 100 150

0.2 0.4 0.6 0.8 1 1.2 E_visible/√s^

Events/0.025

0 50 100 150

-10 -7.5 -5 -2.5 0 ln(y₃₄)

Events/0.25

a) b)

c) d)

L3 L3

L3 L3

Fig. 1a–d.Data and Monte Carlo distributions ofathe num- ber of leptons, b acollinearity, c the normalised visible en- ergy andd ln(y34) after the λijk preselection. The solid his- tograms show the expectations for Standard Model processes at √

s = 189 GeV. The dotted and dashed histograms show two examples of signal, withdominant coupling λ133. Th e dotted histograms represent the process e⁺e⁻ → χ˜⁰₁χ˜⁰₁, for M_χ˜⁰₁= 94 GeV, corresponding to five hundred times the lumi- nosity of the data. The dashed ones represent e⁺e⁻→χ˜⁺₁χ˜⁻₁, withM_χ˜±

1 = 94 GeV and ∆M =M_χ˜± 1 −M_χ˜0

1 = 40 GeV, cor- responding to twenty times the luminosity

2+E/ topologies, the following cuts are applied: events must have at least 2 and less than 6 tracks, between 2 and 15 calorimetric clusters, the visible energy has to be less than 60% of √

s and Njets8 hasto be at least 2. In this case, the preselection requirement ony34isnot applied. In addition, for the 2+E/topologies, the acollinearity and acoplanarity anglesare required to be below 176^◦in order to reject⁺⁻ background.

Untagged two-photon interactionsare removed by re- quiring the visible energy to be greater than 20% of√ The polar angle (θmiss) of the missing momentum vec-s.

tor hasto be between 15^◦ and 165^◦. Background from two-photon interactionsisfurther reduced by requiring the missing transverse momentum (p^T_miss) to be greater than 7 GeV. Tagged two-photon interactionsare rejected by requiring the sum of the energies measured in the small angle calorimetersbetween 1.5^◦ and 9.0^◦ to be less than 10 GeV.

After the preselection isapplied, 995 eventsare se- lected in the data sample and 984±6 eventsare expected from Standard Model processes, of which 398 are from W⁺W⁻, 136 from W^±e^∓νdecaysand 258 from q¯q events.

Figure 1 shows the number of leptons, acollinearity, nor- malised visible energy and ln(y34) distributions after the preselection. The data are in good agreement with the Monte Carlo expectations.

The four groupsof final selectionsare shown in Ta- bles10 to 13 of Appendix A. The efficienciesare summa- rized in Tables3 and 4. Here and in the following sections

Data qqγγ interactions WW, Zee, ZZ

χ⁰1 χ⁰1

M = 15 GeV χ⁺1χ^-1

M = 94 GeV, ∆M = 55 GeV

0 1000 2000

-12 -10 -8 -6 -4 -2 ln(y₃₄)

Events/0.5

0 500 1000 1500

0.5 0.6 0.7 0.8 0.9 1 Thrust

Events/0.02

0 250 500 750 1000

0.4 0.6 0.8 1 1.2 E_visible/√s^

Events/0.02

0 250 500 750 1000

0 1 2 3

θ_missing

Events/0.1

a) b)

c) d)

L3 L3

L3 L3

Fig. 2a–d.Data and Monte Carlo distributions ofaln(y34), bthrust,cnormalised visible energy anddpolar angle of the missing momentum after theλ_ijk preselection. The solid his- tograms show the expectations for Standard Model processes at √

s = 189 GeV. The dashed and dotted histograms show two examples of signal, withcoupling λ311. The dashed his- tograms represent the process e⁺e⁻→χ˜⁰₁χ˜⁰₁→4 jetsνν, with¯ M_χ˜⁰₁ = 15 GeV, corresponding to thirty times the luminosity of the data. The dotted ones represent e⁺e⁻ → χ˜⁺₁χ˜⁻₁, with M_χ˜±

1 = 94 GeV and∆M =M_χ˜± 1 −M_χ˜0

1= 55 GeV, withsubse- quent ˜χ⁰₁χ˜⁰₁decays into 4 jetsνν, corresponding to one hundred¯ times the luminosity

Data qqγγ interactions WW

χ⁰1 χ⁰1

M = 94 GeV χ⁰1 χ⁰1

M = 30 GeV

0 200 400 600 800

-12 -10 -8 -6 -4 -2 ln(y₃₄)

Events/0.04

0 500 1000

0.5 0.6 0.7 0.8 0.9 1 Thrust

Events/0.02

0 200 400 600 800

-12 -10 -8 -6 -4 ln(y₄₅)

Events/0.4

0 250 500 750 1000

0 0.2 0.4 0.6 0.8 1 Width of most energetic jet

Events/0.01

a) b)

c) d)

L3 L3

L3 L3

Fig. 3a–d.Data and Monte Carlo distributions ofaln(y34), bthrust,cln(y45) anddwidthof the most energetic jet after the λ_ijk preselection. The solid histograms show the expec- tations for Standard Model processes at√

s = 189 GeV. The dashed and dotted histograms show two examples of signal, withdominant couplingλ212, corresponding to decays into c, d and s quarks. The dashed histograms represent the process e⁺e⁻→χ˜⁰₁χ˜⁰₁, withM_χ˜0

1 = 94 GeV, corresponding to one thou- sand times the luminosity of the data. The dotted ones repre- sent the same process, withM_χ˜0

1 = 30 GeV, corresponding to fifteen times the luminosity

Table 3. Efficiency ranges for neutralino and chargino production. ˜χ⁰mχ˜⁰nindicates neu- tralino pair-production with m = 1,2 and n = 1, ..,4. The efficiencies correspond to M_χ˜0

m+M_χ˜0

n= 188 GeV. For direct decays the lowest mass values considered areM_χ˜0 1 = 5 GeV andM_χ˜±

1 = 15 GeV forλ133,M = 15 GeV forλ₃₁₁andM_χ˜0

1= 20 GeV andM_χ˜± 1 = 45 GeV forλ₂₁₂. For direct neutralino decays we quote the ˜χ⁰₁χ˜⁰₁ efficiencies. In the case of indirect decays the chargino selection efficiencies correspond toM_χ˜±

1 = 94 GeV

Direct decays Mass values

Coupling Process M = 5–20 GeV M = 25–50 GeV M=55–94 GeV

λ133 χ˜⁰mχ˜⁰n 4%–13% 21%–35% 41%–51%

λ133 χ˜⁺₁χ˜⁻₁ 8%–10% 15%–36% 43%–45%

λ₃₁₁ χ˜⁰_mχ˜⁰_n, ˜χ⁺₁χ˜⁻₁ 6%–17% 19%–28% 21%–26%

λ212 χ˜⁰_mχ˜⁰_n, ˜χ⁺₁χ˜⁻₁ – 29%–38% 35%–54%

Indirect decays ∆M values

Coupling Process ∆M = 5–20 GeV ∆M= 25–50 GeV ∆M= 55–80 GeV

λ133 χ˜⁰_mχ˜⁰_n(n≥2) 50%–51% 48%–50% 42%–46%

λ133 χ˜⁺₁χ˜⁻₁ 47%–59% 33%–43% 14%–27%

λ₃₁₁ χ˜⁺₁χ˜⁻₁ 25%–28% 29%–30% 17%–23%

λ₂₁₂ χ˜⁰_mχ˜⁰_n(n≥2) 55%–59% 59%–60% 47%–53%

λ₂₁₂ χ˜⁺₁χ˜⁻₁ 54%–62% 60%–66% 43%–56%

Table 4.Efficiency ranges for scalar lepton production. In the case of direct decays the lowest mass value considered isM˜R= 54 GeV forλ12k,M˜ν= 54 GeV forλ121, M˜e_R= 45 GeV forλ311andM˜R= 30 GeV forλ212. For the processes marked with * we refer to four-body decays, as described in Sects. 5 and 6. For indirect decays the scalar lepton selection efficiencies correspond toM˜R(Mν˜) = 94 GeV

Direct decays Mass values

Coupling Process M = 5–20 GeV M = 25–50 GeV M=55–94 GeV

λ12k ˜⁺_R˜⁻_R – – 8%–18%

λ121 ν˜ν˜ – – 6%–8%

λ₃₁₁ ˜e⁺_R˜e⁻_R * – 14%–16% 18%–22%

λ₂₁₂ ˜e⁺_R˜e⁻_R * – 32%–38% 35%–54%

λ212 µ˜⁺_Rµ˜⁻_R * – 32%–38% 35%–54%

λ212 τ˜_R⁺τ˜_R⁻ * – 32%–38% 15%–44%

Indirect decays ∆M values

Coupling Process ∆M = 5–20 GeV ∆M= 25–50 GeV ∆M= 55–80 GeV

λ133 ˜e⁺_R˜e⁻_R 74%–74% 50%–63% 26%–38%

λ133 µ˜⁺_Rµ˜⁻_R 82%–84% 76%–84% 65%–72%

λ133 τ˜_R⁺τ˜_R⁻ 66%–72% 67%–73% 57%–65%

λ133 ν˜ν˜ 52%–52% 39%–49% 28%–37%

λ₃₁₁ ˜e⁺_R˜e⁻_R 30%–60% 50%–60% 60%–76%

λ212 ˜e⁺_R˜e⁻_R 37%–63% 66%–69% 45%–62%

λ212 µ˜⁺_Rµ˜⁻_R 29%–51% 54%–57% 34%–50%

λ₂₁₂ τ˜_R⁺τ˜_R⁻ 56%–66% 39%–62% 15%–16%