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ELSEVIER

22 May 1997

PHYSICS LETTERS B

Physics Letters B 401 (1997) 139-149

Search for excited leptons in e+e annihilation at y/s = 161 GeVL3 Collaboration

M. Acciarri20, O. Adrianir, M. Aguilar-Benitezab, S. Ahlene, B. Alpataj, J. Alcarazab,G. Alemanni11, J. Allabys, A. Aloisioae, G. Alversonm, M.G. Alviggiae, G. Ambrosi“,

H. Anderhubaz, V.P. Andreevs,an, T. Angelescun, F. Anseimo i D. Antreasy an j,A. Arefievad, T. Azemoon0, T. Azizk, P. Bagnaiaam, L. Baksayat, S. Banerjeek,

K. Baniczav, R. Barillères, L. Baroneam, P. Bartaliniaj, A. Baschirottoac, M. Basile^R. B attiston , A. B ayx, F. Becattini1, U. Beckerq, F. Behneraz, J. Berdugoab, P. Bergesq,

B. Bertuccis, B.L. Betevaz, S. Bhattacharyak, M. Biasini\ A. Bilandaz, G.M. B ileiaj,J.J. Blaising8, S.C. Blythak, GJ. Bobbinkb, R. Bocka, A. Böhma, B. Borgiaam,

D. Bourilkovaz, M. Bourquin11, S. Braccini11, J.G. Bransonap, V. Brigljevicaz, I.C. Brockak, A. Buffinir, A. Buijsau, J.D. Burgerq, WJ. Burger“, J. Busenitzat, A. Button0, X.D. Caiq, M. Campanelliaz, M. Capellq, G. Cara Romeo-*, M. Cariaa*, G. Carlinoae, A.M. Cartaceir,

J. Casausab, G. Castellimi F. Cavallariam, N. Cavalloae, C. Cecchiu, M. Cerradaab,F. Cesaroniy, M. Chamizoab, A. Chanbb, Y.H. Changbb, U.K. Chaturvedi

S.V. Chekanovag, M. Chemarinaa, A. Chenbb, G. C hen\ G.M. Chenh, H.F. C h en \H.S. Chenh, X. Chereaud, G. Chiefariae, C.Y. Chiene, M.T. Choias, L. Cifarellia0,

F. CindoloJ, C. Civininir, I. Clareq, R. Clareq, H.O. Cohnab, G. Coignetd, A.P. Colijnb, N. Colinoab, V. Commichau3, S. Costantiniag, F. Cotorobai”, B. de la Cruzab, A. Csilling0, T.S. D aiq, R. D ’Alessandror, R. de Asmundisae, A. Degréd, K. Deitersaw, D. della Volpeae,

P. Denesat>', F. DeNotaristefaniam, D. DiBitontoat, M. Diemozam, D. van Dierendonckb,F. Di Lodovicoaz, C. Dionisiam, M. Dittmaraz, A. Dominguezap, A. Doriaae, M.T. Dova1,4,

E. Dragoae, D. Duchesneaud, P. Duinkerb, I. Duranaq, S. Duttak, S. Easoaj,Yu. Efremenkoah, H. El Mamouniaa, A. Englerak, F.J. Epplingq, F.C. Ernéb,

J.P. Ernenweinaa, P. Extermannu, M. Fabreaw, R. Facciniam, S. Falcianoam, A. Favarar,J. Fayaa, 0 . Fedinan, M. Felciniaz, B. Fenyiat, T. Fergusonak, D. Fernandezab, F. Ferroniam,

H. Fesefeldt3, E. Fiandrini3', J.H. Field“, F. Filthautak, P.H. Fisherq, G. Forconiq,L. Fredj“, K. Freudenreichaz, C. Furettaac, Yu. Galaktionovad,q, S.N. Gangulik,

P. Garcia-Abiaab, S.S. Gaum, S. Gentileam, N. Gheordanescun, S. Giaguam, S. Goldfarbx, J. Goldsteine, Z.F. Gongv, A. Gougas6, G. Grattaai, M.W. Gruenewald1, V.K. Guptaai,A. Gurtuk, LJ. Gutayav, B. Hartmann3, A. Hasanaf, D. Hatzifotiadouj, T. Hebbeker1,

A. Hervés, W.C. van Hoekas, H. Hoferaz, H. Hooraniak, S.R. Houbb, G. Hue,

0370-2693/97/$ 17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PII S 0 3 7 0 - 2 6 9 3 ( 9 7 )0 0 4 0 4 -8

V. Innocente5, K. Jenkesa, B.N. Jinh, L.W. Jones', P. de Jong8,1. Josa-Mutuberriaab,A. Kasserx, R.A. Khan1, D. Kamraday, Yu. Kamyshkovah, J.S. Kapustinskyz,

Y. Karyotakisd, M. Kaur1,5, M.N. Kienzle-Focacciu, D. Kime, J.K. Kim“ , S.C. Kimas, Y.G. Kimas, W.W. Kinnisonz, A. Kirkbyai, D. Kirkbyai, J. Kirkby8, D. Kiss0, W. Kittel“«,

A. Klimentovq'ad, A.C. Königag, I. Korolkoad, V. Koutsenko q-ad, R.W. Kraemerak,W. Krenza, A. Kuninq’ad, P. Ladrón de Guevaraab, I. Laktinehaa, G. Landir, C. Lapointq,

K. Lassila-Periniaz, P. Laurikainenw, M. Lebeau8, A. Lebedevq, P. Lebrunaa, P. Lecomteaz,P. Lecoqs, P. Le Coultreaz, J.S. Leeas, K.Y. Leeas, J.M. Le Goffs, R. Leisteay,

E. Leonardiam, P. Levtchenkoan, C. L iv, E. Liebay, W.T. Linbb, F.L. Lindeb,s, L. Listaae, Z.A. Liuh, W. Lohmannay, E. Longoam, W. Luai, Y.S. Luh, K. Liibelsmeyera, C. Luciam,

D. Luckeyq, L. Luminariam, W. Lustermannaw, W.G. M a \ M. Maityk, G. Majumderk,L. Malgeriam, A. Malininad, C. Mañaab, D. Mangeolag, S. Mangla\ P. Marchesiniaz,

A. Marin*, J.P. Martinaa, F. Marzanoam, G.G.G. Massarob, D. McNally8, R.R. M cNeilg, S. M eleae, L. Merolaae, M. Meschinir, WJ. Metzgerag, M. von der Meya, Y. M ix,

A. Mihul", A.J.W. van M ilag, G. Mirabelliam, J. Mnich8, P. Molnar1, B. Monteleonir,R. Moorec, S. Morganti3”1, T. Moulikk, R. Mount01, S. Müller“, F. Muheim11,

AJ.M. Muijsb, E. Nagy °, S. Nahnq, M. Napolitanoae, F. Nessi-Tedaldiaz, H. Newmanai,T. Niessena, A. Nippe3, A. Nisatiam, H. Nowakay, H. Opitza, G. Organtiniam,

R. Ostonenw, D. Pandoulasa, S. Paolettiam, P. Paolucciae, H.K. Parkak, G. Pascaleam,G. Passalevar, S. Patricelliae, T. Paul"1, M. Pauluzzi3-*, C. Pausa, F. Paussaz, D. Peach8,

Y.J. P e i \ S. Pensotti30, D. Perret-Gallixd, B. Petersenag, S. Petrak1, A. Pevsnere,D. Piccoloae, M. Pierir, J.C. Pintoak, P.A. Piroué3Í, E. Pistolesi30, V. Plyaskinad, M. Pohlaz,

V. Pojidaevad,r, H. Postemaq, N. Produit“, D. Prokofievan, G. Rahal-Callotaz,P.G. Rancoitaac, M. Rattaggi30, G. Ravenap, P. Razisaf, K. Readah, D. Renaz,

M. Rescignoam, S. Reucroftm, T. van Rhee3U, S. Riemannay, K. Rilesc, S. R oas,A. Robohm3Z, J. Rodinq, F.J. Rodriguez3b, B.P. Roec, L. Romeroab, S. Rosier-Leesd,

Ph. Rosseletx, W. van Rossumau, S. Roth3, J.A. Rubios, H. Rykaczewskiaz, J. Salicios,E. Sanchezab, M.P. Sanders3g, A. Santocchiaaj, M.E. Sarakinosw, S. Sarkark,

M. Sassowsky3, C. Schäfer3, V. Schegelsky3n, S. Schmidt-Kaerst3, D. Schmitz3,P. Schmitz3, N. Scholzaz, H. Schopperba, D.J. Schotanusag, J. Schwenke3, G. Schwering3,

C. Sciaccaae, D. Sciarrino“, L. Servoliaj, S. Shevchenko3', N. Shivarov3r, V. Shoutko3d,J. Shukla2, E. Shumilov3d, A. Shvorob31, T. Siedenburg3, D. Son3S, A. Sopczakay,B. Smithq, P. Spillantinir, M. Steuerq, D.P. Stic kl andaí, H. Stone 3‘, B. Stoyanov3r,

A. Straessner3, K. Strauchp, K. Sudhakark, G. Sultanov1, L.Z. Sunv, G.F. Susinno“,H. Suteraz, J.D. Swain', X.W. Tangb, L. Tauscherf, L. Taylorm, Samuel C.C. Tingq,

S.M. Tingq, M. Tonutti3, S.C. Tonwark, J. Tóth0, C. Tully3 H. Tuchscherer31,K.L. Tungh, Y. Uchidaq, J. Ulbricht32, U. Uwers, E. Valenteatn, R.T. Van de Walleag,

G. Vesztergombi0, 1. Vetlitskyad, G. Viertelaz, M. Vivargentd, R. Völkert3y, H. Vogelak,H. Vogtay, I. Vorobievad, A.A. Vorobyovan, A. Vorvolakosaf, M. Wadhwaf, W. Wallraff3,

J.C. Wangq, X.L. Wangv, Z.M. Wangv, A. Weber3, F. Wittgensteins, S.X. W u‘,

140 L3 Collaboration / Physics Letters B 401 {¡991) 13 9 - 149

L3 Collaboration / Physics Letters B 401 (1997) 139-149 141

S. Wynhofp, J. X u£, Z.Z. X u \ B.Z. Y ang\ C.G. Yangh, X.Y. Yaoh, J.B. Yev, S.C. Yehbb, J.M. Youak, An. Zalitean, Yu. Zalitean, P. Zempaz, Y. Zenga, Z. Zhang \ Z.P. Zhangv,

B. Zhou e, G.Y. Zhuh, R.Y. Zhuai, A. Z i c h i c h i F. Ziegleraya A Physikalisches Institut, RWTH, D-52056 Aachen, F R G 1

III. Physikalisches Institut, RWTH, D-52056 Aachen, FRG 1 h National Institute fo r High Energy Physics, NIKHEF, and University o f Amsterdam, NL-1009 DB Amsterdam, The Netherlands

c University o f Michigan, Ann Arbor, MI 48109, USA d Laboratoiré d ’Annecy-le-Vieux de Physique des Par tic ules, L A PP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France

c Johns Hopkins University, Baltimore, MD 21218, USA 1 Institute o f Physics, University o f Basel, CH-4056 Basel, Switzerland

g Louisiana State University, Baton Rouge, LA 70803, USA h Institute o f High Energy Physics, I HEP, 100039 Beijing, China6

1 Humboldt University, D -10099 Berlin, FRG 1 J University o f Bologna and INFN-Sezione di Bologna, 1-40126 Bologna, Italy

k Tata Institute o f Fundamental Research, Bombay 400 005, India£ Boston University, Boston, MA 02215, USA

m Northeastern University, Boston, MA 02115, USA n Institute o f Atomic Physics and University o f Bucharest, R-76900 Bucharest, Romania

0 Central Research Institute fo r Physics o f the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2

p Harvard University, Cambridge, MA 02139, USA Massachusetts Institute o f Technology, Cambridge, MA 02139, USA

r INFN Sezione di Firenze and University o f Florence. 1-50125 Florence, Italy s European Laboratory fo r Particle Physics, CERN, CH-1211 Geneva 23, Switzerland

1 World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland u University o f Geneva, CH-1211 Geneva 4, Switzerland

v Chinese University o f Science and Technology, USTC, Hefei, Anhui 230 029, China 6 w SEFT, Research Institute fo r High Energy Physics, P.O. Box 9, SF-00014 Helsinki, Finland

x University o f Lausanne, CH-1015 Lausanne, Switzerland y INFN-Sezione di Lecce and Università Degli Studi di Lecce, 1-73100 Lecce, Italy

z Los Alamos National Laboratory, Los Alamos, NM 87544, USA aa Institut de Physique Nucléaire de Lyon, IN2P3-CNRS, Università Claude Bernard, F-69622 Villeurbanne, France

ab Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, E-28040 Madrid, Spain3

ac INFN-Sezione di Milano, 1-20133 Milan, Italy ad Institute o f Theoretical and Experimental Physics, 1TEP, Moscow, Russia ac INFN-Sezione di Napoli and University o f Naples, 1-80125 Naples, Italy af Department o f Natural Sciences, University o f Cyprus, Nicosia, Cyprus

aß University o f Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlandsah Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ai California Institute o f Technology, Pasadena, CA 91125, USA

INFN-Sezione di Perugia and Università Degli Studi di Perugia, 1-06100 Perugia, Italy^ Carnegie Mellon University, Pittsburgh, PA 15213, USA

a£ Princeton University, Princeton, NJ 08544, USA am INFN-Sezione di Roma and University o f Rome, “La Sapienza ”, LOO 185 Rome, Italy

an Nuclear Physics Institute, St. Petersburg, Russia ao University and INFN, Salerno, 1-84100 Salerno, Italy ap University o f California, San Diego, CA 92093, USA

aq Dept, de Fisica de Partículas Elementales, Univ, de Santiago, E-15706 Santiago de Compostela, Spain ur Bulgarian Academy o f Sciences, Central Lab. o f Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria as Center fo r High Energy Physics, Korea Adv. Inst, o f Sciences and Technology; 305-701 Taejon, South Korea

al University o f Alabama, Tuscaloosa, AL 35486, USA au Utrecht University and NIKHEF, NL-3584 CB Utrecht, The Netherlands

av Purdue University, West Lafayette, IN 47907, USA aw Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland

ay DESY-Institut für Hochenergiephysik, D-15738 Zeuthen, FRG az Eidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland

142 LJ Collaboration/Physics Letters B 401 (1997) 139-149

ba University o f Hamburg, D-22761 Hamburg, FRG High Energy Physics Group, Taiwan, ROC

Received 29 January 1997 Editor: K. Winter

Abstract

A search for excited leptons e*, /jl*, r* and in e+e~collisions at yjs =161 GeV is performed using the 10.8 pb 1 of data collected by the L3 detector at LEP. No evidence has been found for their existence. From an analysis of the expected £*i* pair production in the channels eeyy, [ifxyy, rryy, eeWW, and vvyy, the lower mass limits at 95% C.L. are 79.7 GeV for e \ 79.9 GeV for f x \ 79.3 GeV for r* and 71.2 GeV for p* assuming the same couplings as for standard leptons. From an analysis of the expected i t * single production in channels eey, fifiy, rry, ^eW and vvy, the upper limits on the couplings A/nt£* up to mi* = 161 GeV are determined. © 1997 Published by Elsevier Science B.V.

1. Introduction

The existence of excited leptons would be a clear signal for composite models, which can explain the number of families and make the fermion masses and weak mixing angles calculable [1]. The excited lep­tons e*, , r* and v* have been extensively searched for at the LEP e+e~collider with yfs = 91 GeV [2,3] and y/s = 130-140 GeV [4,5] and at the HERA ep collider [6]. In this paper we report a search based on the 10.8 pb” 1 of data collected by the L3 experiment at the centre of mass energy of 161 GeV.

An excited lepton £* is assumed to have spin It could have both a left and a right-handed compo­nent [7] or only a left-handed component as in the Standard Model. Since in the latter case the produc­tion cross section of excited lepton pairs is smaller, we use this assumption to make conservative estimates. An excited lepton, £*, is expected to decay immedi­ately into its ground state, £, by radiating a photon or a massive vector boson, Z or W.

1 Supported by the German Bundesministerium für Bildung, Wis­senschaft, Forschung und Technologie.

2 Supported by the Hungarian OTKA fund under contract number T 14459.

•* Supported also by the Comisión Interministerial de Ciencia y Technología.

4 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.

5 Also supported by Panjab University, Chandigarh-160014, India.6 Supported by the National Natural Science Foundation of China.

At e+e” colliders, excited leptons can be produced either in pairs (e+e~—> £*£*) or singly (e+e~—> ££*). For £*£* pair production, the Z and y are assumed to have the same coupling to an excited lepton pair £ * t as to the standard lepton pair £1. The f-channel contribution for e* and v* is neglected since the cou­plings W£*£ are expected to be much smaller than nor­mal couplings Y££ and V£*£*t where V = y, Z, W. The lowest order pair production cross section can be found in Ref. [1].

For single £* production, the effective Lagrangian [7] can be written as

Aff = - ^w £ * e y 5 )^^ fiV uv=y. z,w

h.c.,

where A is the composite mass scale and Cyp*£ and D y ^ are unknown coupling constants. The precise muon g-2 measurements impose |C y ^ | = |Dw+i| [7,8]. The absence of the electric dipole mo­ment of electrons suggests that both CVm and Dv are real. We assume that the excited lepton and its corresponding excited neutrino are a weak doublet,therefore the coupling constants satisfy Cy£*/? = D y ^ ,and we have

YCzi*t = cot6w - - f t e n f f w) ,

c _ /Cwf.f - r - , ,

2v2sin By

L3 Collaboration /P hysics Letters B 401 (1997) 139-149 143

where ƒ and ƒ7 are the free parameters for SU(2 ) and U (l) respectively, ti is the third component of the weak isospin of £*, Y is the hypercharge of £*, and #w is the weak mixing angle. Throughout this paper we assume that ¿3 and Y are the same for £ and £*.

For excited charged leptons, we assume ƒ = ƒ' in the above formula, so that ƒ /A(= y/2A/me*) is the only free parameter in the Lagrangian [9]. An excited charged lepton is expected to decay immediately into its corresponding standard lepton and a photon with a 100% branching ratio if its mass is smaller than that of the W. For higher mass, the decays £* —► Z£ and £* —> become important and the branching ratio of £* —* £y is then a function of [10]. For the excited neutrino v*9 both ƒ = ƒ / and ƒ ¥= f cases, described in detail in [4], are studied. In the former case the yvv* coupling vanishes and v* —> vL and v* —► eW are the only decay modes allowed, whereas in the latter case the yvv* coupling exists and v* vy decay would have a large branching ratio. In this analysis the searches are limited to the v*, which is assumed to be the lightest excited neutrino.

All the above processes have been generated by a Monte Carlo program according to the differential cross section of Ref. [7] with an angular distribu­tion of 1 + cos 6 assigned to the £* decay. The rel­evant branching ratios of £* decays are taken from Ref. [10]. The subsequent r decays are simulated by the TAUOLA and KORALZ Monte Carlo pro­grams [ 11] and the hadronic fragmentation and de­cays are simulated by the JETSET Monte Carlo pro­gram [ 12]. The effect of initial state radiation is not included in the Monte Carlo generators but is taken into account in our cross section calculations. The gen­erated events have been passed through the L3 detec­tor simulation [13] which includes the effects of en­ergy loss, multiple scattering, interactions and decays in the detector and the beam pipe.

2. The L3 detector

The L3 detector is described in detail in Ref. [ 14]. It consists of a silicon microstrip vertex detector, a cen­tral tracking chamber (TEC), a high resolution elec­tromagnetic calorimeter composed of bismuth germa­nium oxide (BGO) crystals, plastic scintillation coun­ters, a uranium hadron calorimeter with proportional

wire chamber readout and a precise muon spectrom­eter. These detectors are installed in a 12 m diame­ter solenoid magnet which provides a uniform field of 0.5 T along the beam direction.

The BGO electromagnetic calorimeter covers the polar angle from 110 to 169°. It is divided into a barrel (42° < 6 < 138°) and endcaps (11° < 6 < 38°, 142° < 6 < 169°). The energy resolution for photons and electrons is less than 2% for energies above 1 GeV, The angular resolution of electromagnetic clusters is better than 0.5° for energies above 1 GeV. The hadron calorimeter covers the polar angle from 5.5° to 174.5°.It measures the event energy, with the help of TEC and BGO, with a resolution of about 10% at 91 GeV. The muon chambers cover the polar angle from 22° to 158°. They are divided into barrel (36° < 6 < 144°), forward (22° < 6 < 36°) and backward (144° < 6 < 158°) regions.

3. Pair production of excited leptons

In the L3 detector an electron is identified as an electromagnetic shower with a matched track within 5° in the r<p projection. If the shower is isolated from all tracks by more than 15° in the r4> projection, it is identified as a photon. Muons are identified from tracks in the muon chambers with measurements in both the r4> and rz projections. The transverse and the longitudinal distances of closest approach to the inter­action vertex are both required to be less than 200 mm. For tau identification, a jet clustering algorithm [ 15] is used which groups neighbouring calorimeter energy depositions. The algorithm normally reconstructs one jet for a single isolated electron, photon, muon, high energy tau or hadronic jet.

Event selection for the reaction e+e_—> £ * t —» t£yy is based on the fact that these processes contain two leptons and two photons. The Standard Model background from lepton pair production with initial and final state radiation can be efficiently reduced by requiring two energetic photons.

To remove the background from two-photon colli­sions, in all channels except in the single photon final state, the visible energy of the event is required to be more than half of the beam energy.

144 L3 Collaboration/Physics Letters B 401 (1997) 139-149

3.1. Pair production of e

* * e eEvent selection for the reaction e+e eeyy requires the following criteria:

(i) there must be two electrons and two photons, each with an energy greater than 02E heam, where £beam is the beam energy;

( ii) for at least one of the two possible e, y permuta­tions for m\(G\, y i ) , m2(e2, 72). the mass dif­ference mj — m2 must be less than 10 GeV and the average mass must be greater than 50 GeV.

No events pass the selection. The main background is due to radiative Bhabha events e+e ~ —> eeyy. Atotal of 0.4 events is predicted from the BHAGENE3 Monte Carlo [16].

The detection efficiency for the signal is estimated from Monte Carlo to be 51%, which is slightly depen­dent on the mass of the e*. The decay branching ratio for e* —> ey is close to 100% with its mass below 80 GeV

Taking into account the luminosity, the efficiency and the production cross section of e* [7,10], we ob­tain the number of expected e* as a function of me* ■ From Poisson statistics, we determine the lower mass limit for excited electrons at 95% Confidence Level (C.L.) to be 79.7 GeV

3.2, Pair production of fx

Event selection for the reaction e+e (JbfJLjy requires the following criteria:

(i) there are exactly two tracks in the central track­ing chamber, with at least one being an identi­fied muon with an energy greater than 20 GeV;

(ii) there are at least two photons with energiesgreater than 0.2J5beam>

(iii) if the two muons are measured by the muon chambers, cut (ii) in Section 3.1 is applied for muons; if only one muon is measured, at least one (xy combination must have an invariant mass between 50 GeV and 100 GeV.

No events pass the selection. The main background is due to radiative dimuon events e+e~—► ¡JL/xyy. A total of 0.1 events is predicted from the KORALZ Monte Carlo program.

The detection efficiency for jul* is estimated from a Monte Carlo study to be 60%, independent of the mass of the ¿¿*. The decay branching ratio for /¿*

( iy is close to 100% with its mass below 80 GeV We determine the lower mass limit for excited muons at95% C.L. to be 79.9 GeV

3.3. Pair production of r

+ a ------- T*T*Event selection for the reaction e+e r ry y requires the following criteria:

(i) the number of tracks is at least 2 and at most 7; there is at least one tau with energy deposition in the calorimeters not consistent with that of a muon; at least one tau has an energy greater than 0,2Ztbearm

(ii) in the electromagnetic calorimeter, there are at least two photons with energies greater than 0 .2£’beam; total visible energy must be less than 85% of the centre of mass energy;

(iii) if two tau jets are identified, cut (ii) in Section 3.1 is applied for tau leptons; if only one tau jet is identified, at least one r y combination must have an invariant mass between 50 GeV and 100 GeV

No events pass the selection. The background from radiative r r events, e+e~ —> rry y , is estimated to be 0.1 events using the KORALZ program.

The detection efficiency for t* is estimated to be 40%, independent of the mass of r*. The decay branching ratio of r* —> r y is close to 100% with its mass below 80 GeV We determine the lower mass limit for excited taus at 95% C.L. to be 79.4 GeV

3.4. Pair production of v*e

For the event selection of excited electron neutrinos in the eW decay mode, the jet cluster algorithm used in the tau analysis is used. An isolated electron is defined as being more than 15° away from any other calorimetric cluster. The branching ratio for the eW decay mode ( ƒ = f ) rises from 72% at m v+ = 60 GeV to 80% at mv* = 8 0 GeV For the vy decay mode (ƒ ^ ƒ )> the branching ratio is close to 100% for the mass region concerned.

Event selection for the reaction e+e - eeWW requires the following criteria:

(i) the number of tracks must be at least 4 and the number of jets must be at least 3; the polar angle of the missing momentum must satisfy | cos 6\ <0.95;

* *

L3 Collaboration/Physics Letters B 401 (1997) 139-149 145

(ii) there must be at least one isolated electron with an energy between 5 GeV and 50 GeV to remove qq(y) background with a converted photon; the number of jets must be at least 5 if there is no identified second electron with an energy greater than 1 GeV.

No events pass the selection. The background from the two-photon process is predicted to be negligible, 0.4 events are predicted from hadronic events and 0.3 events are predicted from W pair production [17] as estimated from Monte Carlo studies.

The detection efficiency is estimated from Monte Carlo to be 46% between 60 and 70 GeV The effi­ciency decreases to 18% at mv* = 80 GeV where the production and decay of a W at its energy threshold reduces the production of isolated electrons of suffi­cient energy. The decay branching ratio for v * —> eWis about 77% in the mass region 60 to 80 GeV. The lower mass limit for excited neutrinos at 95% C.L., combined with our previous data [4], is determined to be 64.5 GeV

Event selection for the reaction e+e” vt vQyy requires the following criteria:

(i) there are two photons in the barrel region each with an energy greater than 20 GeV; there are no tracks in the central tracking chamber;

(ii) the energy deposited in the electromagnetic calorimeter must be less than 75% of the centre of mass energy and the energy deposited in the hadron calorimeter caused by the leakage of the electromagnetic showers must be less than 15 GeV

No events pass the selection. The main background is due to the radiative vv events, e+e“—> vvyy . A total of 0.2 events are predicted from KORALZ.

The detection efficiency is estimated to be 50%. It is slightly dependent on the mass of the v*. The decay branching ratio of v * —► vey is 100% with its mass below 80 GeV We determine the lower mass limit for excited neutrinos to be 71.2 GeV at 95% C.L.

more than 25 GeV and no other photons present.

4. Single production of excited leptons

Event selection for the reaction of e+e i tt t y is based on the fact that these processes contain one energetic photon plus leptons. The photon is re­quired to be in the barrel region and to have an energy

4.1. Single production of e*

ee eeyEvent selection for the reaction e+e~ — requires the following criteria:

(i) there is at least one electron with an energy greater than 25 GeV and the number of tracks is at most 2;

(ii) the total energy deposited in the hadron calorimeter caused by leakage of the electro­magnetic shower must be less than 10 GeV;

(iii) for events with only one observed electron, the thrust axis of the event must be within the barrel region.

A total of 29 events pass the selection; 23 of them have only one visible electron. The main background is due to radiative Bhabha events. A Monte Carlo cal­culation utilizing the TEE program [18], which de­scribes the configuration with one visible electron, predicts 15.6 background events passing the selection cuts. Fig. la shows the invariant mass, mer, of all com­binations together with only the TEE prediction for the background. No significant structure is observed. The mass resolution for e* is about 1 GeV, estimated from Monte Carlo events. We use the mass resolution and conservatively make only the background subtraction from the TEE prediction in calculating the upper limit.

me* = 90 GeV and 67% at me* = 160 GeV The decay branching ratio of e* —> ey changes from 88% at me* = 90 GeV to 38% at = 160 GeV. Com­bining our present data with that for y/s = M z and y/s = 130-140 GeV [2,4] and taking into account the luminosity, the efficiency and the branching ratio, the upper limit of the coupling constant A/me* at 95% C.L. as a function of me+ is shown in Fig. 2.

4.2, Single production of f i

jXfXEvent selection for the reaction e+e ¡jLfiy requires the following criteria:

(i) there is at least one identified muon with mo­mentum greater than 15 GeV, and the number of tracks is at most 2;

(ii) the total energy deposited in the hadron calorimeter must be less than 25 GeV;

(iii) the polar angle of the missing momentum must satisfy I cos 9\ < 0.95.

146 U Collabora!ion/Physics Letters B 401 (1997) !39-149

>oO 4cn H

1 /3<L>- 2 c *•W

0 0

1o

20 40

i—i—i—i—|—i—i—i—;—i—i—i—|—i—i—1—i—'—1—1—i—1 ' 1 i 1 1 1 I 1 1

: a) . DATAMC

n n . Î60 80

mey (GeV)120 140

%a 24)

• ^h î a 1wo

l T " 11! I I 1 I

b)

0 20 40

• DATA MC

60 80 100 120 140 160m^Y (GeV)

>oO 2CO

VP F < (

c 1 W0

J j I I I I j I j I J I I ■ ■

F c)

. n.naJj-LrLrL

• DATA MC

VI wH-ur io 20 40 60 80 100 120 140 160

(GeV)

Fig. 1. Selected events in single production searches: a) invariant mass of all ey combinations; b) invariant mass of all /¿y combinations; c) invariant mass of all ry combinations.

ny(GeV)

Fig. 2. The upper limit of the coupling constant A/w;* at 95% C.L. as a function of mg* for excited leptons with ƒ = / . The excluded region is above and to the left of the curves.

A total of 4 events pass the selection. The main background is due to radiative dimuon events e+e“ —> fijxy, which is predicted from KORALZ to be 5.9 events. Fig. lb shows the invariant mass, mMr, of all combinations together with the Monte Carlo predic­tion for the background. The mass resolution for /a *

is about 3 GeV for muons in the barrel (90% of ob­served events). No significant structure can be seen from the plot. We conclude that the observed events are compatible with the expected background.

The detection efficiency for a singly produced /¿* is estimated to be 62%. The efficiency decreases to 57% at a ¿¿* mass close to the centre of mass energy. The decay branching ratio of /jl* —» jjlj changes from 88% at = 90 GeV to 38% at m^* = 160 GeV. Combined with our previous data [2,4], we obtain an upper limit at 95% C.L. for the coupling constant A/m^* as a function of as shown in Fig. 2.

L3 Collaboration / Physics Letters B 401 (1997) 139-149 147

4.3. Single production o f r

Event selection for the reaction e+e~ requires the following criteria:

(i) at least one tau jet energy must be greater than 2 GeV;

(ii) the number of tracks must be at least 2 and at most 7, the number of clusters in the electro­magnetic calorimeter must be less than 15;

( iii) the total energy deposited in the electromagnetic calorimeter must be less than 85% of the centre of mass energy; the thrust axis of the event must be in the barrel region.

A total of 8 events pass the selection. The main background is due to radiative r r events, e+e - —» Try, and is estimated to be 10.4 events by KORALZ. After applying kinematic constraints, we estimate the mass resolution of r* to be about 2 GeV from Monte Carlo events. Fig. lc shows the invariant mass of the 8 se­lected events for all r y combinations, together with the Monte Carlo prediction for background. No struc­ture is seen in the plot. We conclude that all events are compatible with the expected background.

The detection efficiency for a singly produced r* is estimated to be 52%, independent of the mass of the

r r r r y

r*. The decay branching ratio of r r y changesfrom 88% at m T* = 90 GeV to 38% at mT+ = 159 GeV Combined with our previous data [2,4], we obtain an upper limit at 95% C.L. for the coupling constant A/mr* as a function of m T+, as shown in Fig. 2.

4.4 . Single production o f v *e

+ o - VzVlEvent selection for the reaction e+e VqQW requires the following criteria:

(i) there must be exactly one isolated electron with an energy greater than 5 GeV The polar angle of the missing momentum must satisfy | cos#| <0.95.

(ii) if the W decays leptonically, i.e. the number of tracks is less than 5, there must be only two jets in the event and their acoplanarity angle must satisfy cos <f> < 0.9;

(iii) if the W decays hadronically, i.e. the number of tracks is greater than or equal to 5 and there must be three jets in the event, each with an energy greater than 5 GeV.

A total of 6 events pass the selection. Backgrounds are predicted to be 4.1 events from W pair produc­tion [17] and 1.5 events from hadronic events. The backgrounds from r r and two-photon collisions are negligible.

The detection efficiency is estimated to be 33% at m v+ = 90 GeV and 44% at > 110 GeV Thec cdecay branching ratio of v* —► eW is more than 95% at m v* = 90 GeV and decreases to 64% at m v+ =e160 GeV Combined with our previous data [2,4], we obtain an upper limit at 95% C.L. for the coupling constant A/mp* as a function of as shown inFig. 2 .

Event selection for the reaction e+e —>VqVqY requires the following criteria:

(i) there must be only one photon in the barrel re­gion with an energy greater than 0 .2£beam;

(ii) the total energy deposited in the hadron calorimeter caused by leakage of electromag­netic showers must be less than 10 GeV.

A total of 32 events pass the selection. The back­ground is mainly due to radiative neutrino events

-+ vvy , and e+e“ —► y y with one y missing in the beam pipe. For radiative neutrino events there are 25.9 predicted from NNGSTR [19] consistent with the 26.1 predicted from KORALZ [11]. The number of events predicted for e + e '- * y y background is 1,4 using the GGG generator [20]. We conclude that all events are compatible with the expected background.

The detection efficiency is estimated to be 73%. It is slightly dependent on the mass of the v* and is the same for both ƒ = 0 and f = 0. For ƒ = 0,

e+e

the decay branching ratio of v *e vey is 100% at

m p* < 91 GeV and decreases to 86% at m v* = 160 GeV. For f = 0, the decay branching ratio is 100% at m v* < 80 GeV and decreases to 12% at = 160 GeV Since it is not possible to reconstruct the v y invariant mass, we derive the upper limit on the basis of 32 observed events with 25.9 expected background events. An upper limit is obtained for the coupling constant X /m v* at 95% C.L. as a function o f m v* for both ƒ = 0 and f = 0. The result is shown in Fig. 3, in which A = ƒ if f = 0, and A = f if ƒ = 0.

It has to be noted that for all the analysis presented in this letter, the 95% C,L. limits include the small effects due to the systematic errors on the luminosity and selection efficiency. The luminosity error is taken as 0 .6% and the uncertainty in selection efficiency

148 L3 Collaboration /P hysics Letters B 401 (1997) 139-149

mv* (GeV)e

Fig. 3. The upper limit of the coupling constant A/mg* at 95% C.L. as a function of mt* for excited electron neutrinos with A = ƒ ( f = 0) and A = f ( ƒ = 0), The excluded region is above and to the left of the curves.

depends on the channel; the largest value is about 3% for the * channel.

5. Conclusion

We see no evidence for excited electrons, muons, taus or electron neutrinos; the observed events are con­sistent with Standard Model expectations. From pair production searches the lower mass limits at 95% C.L. are found to be 79.7 GeV for e*, 79.9 GeV for /ll*, 79.3 GeV for r*, 64.5 GeV (eW decay mode) and 71.2 GeV O ey decay mode) for v*. From single produc­tion searches, we derived upper limits on the couplings A /me* in the range of ( 10~4-1 ) GeV-1 for I* masses up to 161 GeV. Our results are in agreement with sim­ilar searches performed at the same energy [21].

A ckno wl edgem ents

We wish to congratulate the CERN accelerator di­visions for the successful upgrade of LEP at the en­ergy of y/s = 161 GeV and to express our gratitude for the excellent performance of the machine. We ac­knowledge with appreciation the effort of all engi­

neers, technicians and support staff who have partici­pated in the construction and maintenance of this ex­periment. Those of us who are not from member states thanks CERN for its hospitality and help.

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