Search for excited leptons at LEP

Volume 244, number 1

PHYSICS LETTERS B

12 July 1990

Search for excited leptons at LEP OPAL Collaboration M.Z. Akrawy a, G. Alexander b, j. Allison c, p.p. Allport d, K.J. Anderson ~, J.C. Armitage f, G.T.J. Arnison g, P. Ashton c, G. Azuelos h,~, J.T.M. Baines c, A.H. Ball i, j. Banks c, G.J. Barker a, R.J. Barlow c, J.R. Batley d, j. Becker J, T. Behnke k, K.W. Bell g, G. Bella b, S. Bethke ~, O. Biebel ,1, U. Binder J, I.J. Bloodworth n, p. Bock ~, H. Breuker k, R.M. Brown g, R. Brun k, A. Buijs k, H.J. Burckhart k, p. Capiluppi o, R.K. Carnegie f, A.A. Carter a, J.R. Carter d, C.Y. Chang i, D.G. Charlton k, J.T.M. Chrin c, I. Cohen b, W.J. Collins o, J.E. Conboy ~, M. Couch ", M. Coupland q, M. Cuffiani o, S. Dado r, G.M. Dallavalle o, P. Debu s, M.M. Deninno o, A. Dieckmann ~, M. Dittmar t, M.S. Dixit u, E. Duchovni v, I.P. Duerdoth k.2, D. Dumas f, H. El Mamouni h, P.A. Elcombe d, P.G. Estabrooks f, E. Etzion b, F. Fabbri o, p. Farthouat s, H.M. Fischer m, D.G. Fong i, M.T. French g, C. Fukunaga w, A. Gaidot s, O. Ganel v, J.W. Gary ~, J. Gascon h, N.I. Geddes g, C.N.P. Gee g, C. Geich-Gimbel m, S.W. Gensler ¢, F.X. Gentit s, G. Giacomelli o, V. Gibson o, W.R. Gibson a, J.D. Gillies g, J. Goldberg r, M.J. Goodrick d, W. Gorn t, D. Granite r, E. Gross v, P. Grosse-Wiesmann k, j. Grunhaus b, H. Hagedorn J, J. Hagemann k, M. Hansroul k, C.K. Hargrove °, J. Hart d, P.M. Hattersley n, M. Hauschild k, C.M. Hawkes k, E. Heflin t, R.J. Hemingway f, R.D. Heuer k, J.C. Hill o, S.J. Hillier n, C. Ho t, J.D. Hobbs e, P.R. Hobson x, D. Hochman v, B. Holl k, R.J. Homer ~, S.R. Hou i, C.P. Howarth P, R.E. Hughes-Jones c, P. Igo-Kemenes ~, H. Ihssen ~, D.C. Imrie x, A. Jawahery i, P.W. Jeffreys g, H. Jeremie h, M. Jimack k, M. Jobes ~, R.W.L. Jones a, p. Jovanovic ", D. Karlen f, K. Kawagoe w, T. Kawamoto w, R.G. Kellogg i B.W. Kennedy P, C. Kleinwort k, D.E. Klem u, G. Knop m, T. Kobayashi w, T.P. Kokott m, L. KSpke k, R. Kowalewski f, H. Kreutzmann m, j. von Krogh ~, J. Kroll e, M. Kuwano w, p. Kyberd a, G.D. Lafferty c, F. Lamarche h, W.J. Larson t, J.G. Layter ', P. Le Du ~, P. Leblanc h, A.M. Lee ~, D. Lellouch k, p. Lennert ~, L. Lessard h, L. Levinson v, S.L. Lloyd a, F.K. Loebinger c, J.M. Lorah i, B. Lorazo h, M.J. Losty u, J. Ludwig J, N. Lupu r, j. Ma t,3, A.A. Macbeth ~, M. Mannelli k, S. Marcellini o, G. Maringer m, A.J. Martin a, j.p. Martin h, T. Mashimo w, p. M~ittig k, U. Maur m, T.J. McMahon ", A.C. McPherson f,4, F. Meijers k, D. Menszner ~, F.S. Merritt ~, H. Mes ", A. Michelini k, R.P. Middleton g, G. Mikenberg v, D.J. Miller P, C. Milstene b, M. Minowa w, W. Mohr J, A. Montanari o, T. Mori w, M.W. Moss c, P.G. Murphy c, W.J. Murray d, B. Nellen m H.H. Nguyen e, M. Nozaki w, A.J.P. O'Dowd ~, S.W. O'Neale k.5, B.P. O'Neill t, F.G. Oakham ", F. Odorici o, M. Ogg f, H. Oh t, M.J. Oreglia e, S. Orito w, j.p. Pansart s, G.N. Patrick g, S.J. Pawley c, p. Pfister J, J.E. Pilcher ~, J.L. Pinfold v, D.E. Plane k, B. Poli o, A. Pouladdej f, T.W. Pritchard a, G. Quast k, j. Raab k, M.W. Redmond ~, D.L. Rees n, M. Regimbald h, K. Riles ', C.M. Roach d, S.A. Robins a, A. Rollnik m, J.M. Roney e, S. Rossberg J, A.M. Rossi o,6, p. Routenburg f, K. Runge J, O. Runolfsson k, S. Sanghera f, R.A. Sansum g, M. Sasaki w, B.J. Saunders g, A.D. Schaile J, O. Schaile J, W. Schappert Y, P. Scharff-Hansen k, H. v o n d e r Schmitt e, S. Schreiber m, j. Schwarz J, A. Shapira ", B.C. Shen t, p. Sherwood P, A. Simon m, p. Singh a, G.P. Siroli o, A. Skuja i, A.M. Smith k, T.J. Smith ", G.A. Snow i, E.J. Spreadbury p,7, R.W. Springer ~, M. Sproston g, K. Stephens ~, H.E. Stier J, R. Str6hmer ~, D. Strom ~, H. Takeda w, T. Takeshita w, T. Tsukamoto w, M.F. Turner o, 0370-2693/90/$ 03.50 © 1990 - Elsevier Science Publishers B.V. ( North-Holland )

135

Volume 244, number l

PHYSICS LETTERS B

12 July 1990

G. Tysarczyk-Niemeyer ~, D. Van den plas h, G.J. VanDalen ~, G. Vasseur ~, C.J. Virtue u, A. Wagner ~, C. Wahl J, C.P. Ward a, D.R. Ward d, j. Waterhouse r, P.M. Watkins ", A.T. Watson ", N.K. Watson n, M. Weber ~, S. Weisz k, N. Wermes ~, M. Weymann k, G.W. Wilson ~, J.A. Wilson n, I. Wingerter k, V.-H. Winterer J, N.C. Wood P, S. Wotton k, B. Wuensch m, T.R. Wyatt c, R. Yaari v, y . Yang t,3, G. Yekutieli v, T. Yoshida w, W. Zeuner k and G.T. Zorn a b c a e f g h i J k

Queen Mary and Westfield College, University of London, London E1 4NS, UK Department of Physics andAstronomy, TelAviv University, TelAviv 69978, Israel Department of Physics, Schuster Laboratory, The University, Manchester M I 3 9PL, UK Cavendish Laboratory, Cambridge CB3 OHE, UK Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, 1L 60637, USA Department of Physics, Carleton University, Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 RutherfordAppleton Laboratory, Chilton, Didcot, Oxfordshire OXI10QX, UK Laboratoire de Physique NuclOaire, UniversitO de MontrOal, Montreal, Quebec, Canada H3C 3J7 Department of Physics andAstronomy, University of Maryland, College Park, MD 20742, USA Fakultiitfur Physik, Albert Ludwigs Universitdt, D-7800 Freiburg, FRG CERN, European Organisationfor Particle Physics, CH-1211 Geneva 23, Switzerland Physikalisches Institut, Universitiit Heidelberg, D-6900 Heidelberg, FRG m Physikalisches Institut, Universitdt Bonn, D-5300 Bonn 1, FRG n School of Physics andSpace Research, University of Birmingham, Birmingham B15 2TT, UK o Dipartimento di Fisica dell" Universitdt di Bologna and INFN, 1-40126 Bologna, Italy P University College London, London WC1E 6BT, UK q Birkbeck College, London WCIE 7HV, UK r Department of Physics, Technion- Israel Institute of Technology, Haifa 32000, Israel DPhPE, CEN Saclay, F-91191 Gif-sur- Yvette, France t Department o f Physics, University of California, Riverside, CA 92521, USA u NationalResearch Council of Canada, Herzberg Institute of Astrophysics, Ottawa, Ontario, Canada KIA OR6 Nuclear Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel w International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo 113, Japan and Kobe University, Kobe 657, Japan Brunel University, Uxbridge, Middlesex UB8 3PH, UK Received 17 April 1990

Excited leptons have been searched for using data recorded by the OPAL detector at LEP. No evidence for such particles has been found. From the study of e+e - ~ + ~ - Y 7 events, lower limits on the masses of spin-~ excited leptons are found to be 44.9 GeV at 95% confidence level. From the study of e + e - ~ ~+~-7 events, upper limits on their couplings are set up to ~* masses close to the mass of the Z ° boson.

1 Also at TRIUMF, Vancouver, Canada. 2 On leave from Manchester University, Manchester M 13 9PL, UK. 3 On leave from Harbin Institute of Technology, Harbin, P.R. China. 4 Now at Applied Silicon Inc. s On leave from Birmingham University, Birmingham B 15 2TT, UK. 6 Present address: Dipartimento di Fisica, Universith della Calabria, 1-87036 Rende, Italy. 7 Deceased 6 February 1990.

136

The existence of excited states ofleptons and quarks is one of the natural consequences of composite models [ 1 ]. These models offer explanations for the existence of generations and the relations between leptons and quarks (i.e. similar SU (2) properties and the simple ratio of electric charges), which are not explained within the framework of the standard model. This paper presents a search for spin-½ charged excited leptons (2*; l = e , ~t and z) produced in e+e collisions at LEP at centre of mass energies around the mass of the Z° boson and extends the previous

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limits [2,3] ~ on the mass and the strength of the couplings. In this analysis an excited lepton is assumed to have spin-½ and to decay into an ordinary lepton and a photon with a 100% branching ratio. If the mass of an excited lepton is lighter than half of the available centre of mass energy, it can be pairproduced. The single-production of an excited lepton is also possible through the coupling £'2 V ( V= 7, Z°). In this case, a search can reach an 2" mass region close to the available centre of mass energy. To this purpose the processes e + e - - - , e + e - 7 7 , e + e - - + e + e - 7 , e + e - ~ t + ~ t - 7 7 , e+e---,p.+~t-7, e+e---,T+~-~, 7 and e + e - - , z + x - 7 have been studied. The process e + e - - , ( e + ) e T 7, in which one of the electrons is scattered at small angles and escapes detection, has also been studied, because the single-production of e* is dominated by t-channel photon exchange. In this paper the Z ° is assumed to couple to a spin½ excited lepton pair in the same way as to ordinary lepton pairs. The lowest order pair-production cross section of excited leptons is given by ~re+e-~*+~*--

+

X

e - , ,,v 5 v +h Left= v=~zo A 2 a" (cv-dvy)2F~,, .c.,

4//'a 2 ( 3 - - , 8 2 ) 3s fl 2

(1 _ 4 sin20~)2 s(s-MZz) 1+ 8 sin2Owcos20w ( s - M z )2 2 + M z2F z 2

×

( 1 - 4 sin20w)2 + 1 256 sin40w cos40w

(

( 1 - 4 sin20w)2+

s2

( s - M z )2 ~ - + M z2F z 2

]

12 July 1990

'

where s is the square of the centre of mass energy, Mz and Fz are the mass and width of the Z ° boson, 0w is the weak mixing angle, and 13 is the velocity of the excited leptons in the final state:

fl-x/1-4M~./s. The production cross section depends sensitively on the mass of the excited lepton, M~., through ,8, and higher order radiative corrections substantially reduce this cross section. The effective lagrangian for the magnetic transition o f spin-½ excited leptons to ordinary leptons is generally expressed as [ 5 ] ~ The limits obtained at PETRA and PEP are summarized in ref. [4].

where A is the composite mass scale and Cv and dv are the coupling constants. The precise g - 2 measurements imply Jcv[ = Idv], and the absence of electric dipole moments for electrons and muons require that Cvand dv have large real components. The coupling constants can be written as cr = - ~ ( f + f ' ) and Czo = - ~ (fcot Ow-f' tan 0w) in terms of the weak mixing angle and free p a r a m e t e r s f a n d f ' [ 5 ]. In the following analysis we assume f = f ' . In this scheme f/A is the only free parameter in the lagrangian. According to the full formula of the differential cross section given in ref. [ 5 ], a Monte Carlo program was made to generate single-production of spin-½ excited leptons. We assume that the branching ratio Br(2"--,27) is 100%. Using the effective lagrangian given above, the angular distribution of the decay 2"-~27 has a ( 1 + cos 0) dependence with respect to the spin direction in the 2" rest frame [5]. This angular dependence was taken into account in the Monte Carlo event generation. The decay of z leptons was simulated using the JETSET Monte Carlo program [ 6 ]. The effect of initial state radiation was included in the event generator for the excited lepton pairs, whereas it was not included in that of the single-production. The overall reduction factor for the cross section of the single production (s-channel) was estimated to be 77%. The single-production cross section used in the following analysis includes this reduction factor. The detailed description of the OPAL detector can be found elsewhere [7]. The components of the OPAL detector relevant to this analysis are described below. The trajectories and momenta of charged particles are measured by a central tracking detector in a uniform magnetic field of 0.435 T. It includes a precision vertex chamber, a large volume jet chamber which gives precise tracking information in the plane perpendicular to the beam direction and z-chambers for tracking in the plane parallel to the beam direction. The main tracking is done with the jet chamber, a drift chamber approximately four metres long and two metres in radius. It provides up to 159 measured space points along a track and covers the polar angular range [cos 01 < 0.97. The time-of-flight system 137

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(TOF) covers the region Icos 0[ <0.82 and consists of 160 scintillator bars. The barrel part of the electromagnetic calorimeter consists of 9440 lead-glass blocks of 24.6 radiation lengths thickness pointing towards the interaction region. The barrel part covers the region Icos01 <0.82. The blocks are slightly tilted from a perfect pointing geometry to prevent photons from escaping through inter-block gaps. With the material in front (about 2 radiation lengths) and the current uncertainty in the gain calibration, the effective energy resolution is about 5% for 10 GeV and 3% for 45 GeV electrons. The position resolution is better than 5 mrad in the polar and azimuthal angles for electromagnetic showers of more than 10 GeV. The two endcaps of the electromagnetic calorimeter consists of 2264 lead-glass blocks of 20 radiation lengths thickness, covering the polar angular range of 0.81< Icos01 <0.98. The iron return yoke of the magnet is instrumented with 9 layers of streamer tubes which provide hadron calorimetry and muon identification over nearly the whole solid angle. Muons are also identified in four layers of drift chambers surrounding the hadron calorimeter. The forward detector, used for the luminosity measurement, is composed of two identical elements placed around the beam pipe at either end of the central detector, each consisting of a lead-scintillator calorimeter and proportional tube chambers. They cover the polar angles between 40 and 150 mrad and 2~ in azimuthal angle. The systematic error in the determination of the integrated luminosity is estimated to be 2.2% [8]. The data used in this paper were collected during an energy scan of the Z ° resonance at centre of mass energies between 88.3 and 95.0 GeV. After applying quality cuts on the status of the electromagnetic calorimeter, jet chamber and forward detector, the integrated luminosity used in this analysis was 1.15 pb-i. The trigger of the OPAL experiment has a high degree of redundancy and high efficiency for the events under study in this paper. The trigger conditions relevant to this search were as follows. (a) More than 6 GeV in the barrel or more than 10 GeV in one endcap was deposited in the electromagnetic calorimeter. (b) At least two tracks with Icos0[ <0.7 were found by the central detector trigger processor. 138

12 July 1990

(c) Hits were found in at least 3 of the 4 layers of muon chambers and were associated in azimuthal angle with either a track or a TOF counter hit. (d) More than 4 GeV was deposited in the barrel electromagnetic calorimeter and at least one track was found or one TOF counter was hit. If any of the above conditions were satisfied, the event was recorded. The overall trigger efficiency is estimated to be more than 99% for each type of event discussed below. The selection criteria for e + e - - - , e + e - ? ( e + e - ~ e + e - y y ) events were: (i) The number of electromagnetic clusters with more than 100 MeV of energy had to be less than 10. (ii) At least three (four) energetic electromagnetic clusters were required, each with an energy greater than 10% of the beam energy and Icos 01 < 0.70. (iii) The energy sum of the three (four) most energetic clusters had to exceed 80% of the centre of mass energy. (iv) The opening angle between any two energetic clusters had to be greater than 10 °. (v) For e+e - ~ e + e - 7 events, the sum of the opening angles between the three most energetic clusters had to be greater than 357 °. This cut ensured that the events were planar. Using these criteria, we selected a total of 29 events as e+e--~e+e ~/and zero events as e + e - - . e + e - ~ q ,. All the events selected as e+e - --+e+e-~, were visually scanned and confirmed as clear e+e ---+e+e-~/events. No e + e - - , ' / y ? candidate was found in the angular region considered. The expected number of e+e---,e+e-7 events is 31.3_+0.6, calculated by the BABAMC Monte Carlo program [9]. In the same data sample, we observed 746 collinear e+e ---+e+e events in the angular region Icos 01 < 0.7 selected by the cuts described in ref. [ 8 ]. The invariant mass of e±3, pairs was reconstructed using the energies and angles of the electromagnetic clusters, without requiring any further constraints on the kinematics. The mass resolution is calculated to be typically 1.0 GeV by Monte Carlo simulation. The reconstructed invariant mass distribution ofe±~, pairs is shown in fig. la together with the expectation from the BABAMC Monte Carlo program. The structure around Me+v = 30 GeV, both in the real data and the Monte Carlo, is due to the kinematics of final state radiation and

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Invariant Mass 2 !::

L-

_~ al b)e+e----->,u,+,u,-7 E z 6 4 2 o

20

40

mass

60

80

1 O0

(CeV)

Fig. 1. The reconstructed invariant mass distributions (points with error bars, two entries per event) for (a) M~÷v in e+e---,e+e-7 events and (b) M~_+r in e+e-~ ~t+~t-T events compared with the Monte Carlo simulations (solid histograms). The dotted histograms show expected peaks corresponding to f/A = 1/ ( 300 GeV ) and M~. = 70 GeV. the angular and energy cuts imposed. The agreement between the measurement and the expectation is good. To search for e* single-production through t-channel photon exchange, e + e - - - , ( e + )e:~), events were selected using the following criteria. (i) The number of electromagnetic clusters with more than 100 MeV of energy had to be less than 10. (ii) At least two energetic electromagnetic clusters are required, each with an energy greater than 10% of the beam energy and leos 01 <0.70. (iii) The energy sum of the clusters other than the two most energetic clusters had to be less than 10% of the beam energy. (iv) The energy sum of the two most energetic clusters and the electron escaping into the beam pipe (i.e. E, + E2 + IEl cos 01 + E2 cos 02 I, where Ei and 0i were the energy and the polar angle of the energetic cluster i) had to exceed 80% of the centre o f mass energy. (v) The acollinearity angle between the two energetic clusters had to be greater than 10 °.

12 July 1990

(vi) The acoplanarity angle (acollinearity angle in the plane perpendicular to the beam direction) between the two energetic clusters had to be less than 10 °. (vii) One of two energetic clusters had to be isolated from any charged tracks by at least 45 ° in azimuth, while the other had to associated with at least one charged track. Two events satisfied these criteria. The reconstructed invariant masses of the observed e-+y pairs were 40.8 GeV and 73.9 GeV. The expected number of events is 2.6 _+0.2 estimated by the T E E G G Monte Carlo program [10] and is consistent with the observation. The selection criteria for e + e - ~ t + l a - T ( e + e - - ~ ~t+~t-),y) events were: (i) The number of high-Pt tracks was required to be between 2 and 6, where a high-Pt track was defined as a track with at least 36 of a possible 159 hits, satisfying: P, > 4 GeV, Idol < 2 cm and I Zo] < 70 cm, where P, is the m o m e n t u m of the track in the plane transverse to the beam, do is the distance of closest approach of the track to the beam axis and Zo is the longitudinal displacement from the nominal interaction point at the point of closest approach to the beam. (ii) The number of electromagnetic clusters with more than 100 MeV of energy had to be less than 10. (iii) The energy sum of electromagnetic clusters had to be less than 75% of the centre of mass energy. (iv) At least two high-Pt tracks, each with associated electromagnetic energy less than 4 GeV, were required in the polar angular region I cos 01 <0.70. (v) At least one (two) neutral electromagnetic cluster (s) with an energy larger than 10% of the beam energy and Icos 01 < 0.70 was (were) required. (vi) Any opening angle between the high-Pt tracks and the neutral cluster (s) had to be greater than 10 °. (vii) The sum of the momenta of the two high-P, tracks and the energy of the neutral cluster (s) had to exceed 50 GeV. (viii) For e+e--,~t+~t-T events, the sum of the opening angles had to be greater than 357 ° Using these criteria, we selected a total of 19 events as e+e--*la+la-y and one event as e+e-~p.+la-TT. Through visual inspection, it was found that all the selected events had at least one clear track consistent with a muon in the hadron calorimeter or in the muon chambers. The expected numbers for the standard 139

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processes e + e - ~ p + p - 7 and e+e---.z+z-7 are 20.0 + 1.4 and 1.8 ± 0.7, respectively,calculated by the K O R A L Z Monte Carlo program [ I l ]. The number of e+e-~p+~t - events in the same data sample selected by the criteriagiven in ref. [8] was 676. The invariant masses ofp±7 pairs in e+e ---.p+la-7 events were reconstructed from their opening angles only, assuming the kinematics of a three-particlefinal state. For p-candidate tracks, the angles of associated electromagnetic clusters were used in the reconstruction. The typical mass resolution is estimated to be 0.8 GeV. The invariant mass distribution of p ±7 pairs is shown in fig. Ib together with the expectation from the K O R A L Z Monte Carlo program. There isa structure around M,+v = 30 GeV, again due to the imposed angular and energy cuts. The agreement between the measurement and the expectation is good. Possible combinations of the invariant masses ofp+7 pairs in the e + e - ~ P + 7 - 7 7 event are (26.0 GeV, 17.0 G e V ) and (38.5 GeV, 32.6 GeV). The event sample for the e+e---.z+z-7(7) analysis was obtained by applying the following cuts. (i) The number of good tracks was required to be between 2 and 7, where a good track was defined as a track satisfying: P~>0.05 GeV, I d o l < 2 cm and IZol < 70 cm. (ii) The n u m b e r o f electromagnetic clusters b a d to be less than 10, where the energy thresholds for the clusters were chosen to be 100 MeV and 200 MeV for the barrel and the endcaps, respectively. (iii) The total observed energy in the electromagnetic calorimeter had to be between 3 and 80% o f the centre o f mass energy, and the visible energy, defined as the sum of the electromagnetic energy and mom e n t a o f charged tracks, had to be between 18 and 120% o f the centre o f mass energy. ( i v ) The vertex position along the b e a m direction was required to be within 30 cm o f the nominal b e a m crossing point, and at least one track should satisfy the condition I do I < 0.5 cm. (v) Icos 0misI had to be less than 0.95, where 0m~ was the direction o f the missing m o m e n t u m with respect to the b e a m axis, calculated from the energy vectors o f the electromagnetic clusters. ( v i ) The thrust axis, calculated with the electromagnetic clusters and the charged tracks, had to satisfy I cos Oxl < 0.85, where 0V was the angle between the thrust axis and the b e a m axis. 140

12 July 1990

( v i i ) Events identified as e + e - ~ p + p - by the criteria described in ref. [ 8 ] were rejected. A total o f 763 events r e m a i n e d after applying these cuts. F r o m this z-pair sample, we selected events which had at least three "jets". In this analysis, a "jet" could be either a single c h a r g e d / n e u t r a l particle or a few c h a r g e d / n e u t r a l particles, as expected from the z decays. At first a cone o f 15 ° half opening particle in the cone was searched for. I f a particle was found, the sum o f the m o m e n t a of the two particles was used to define the centre axis of a new cone. This procedure was repeated until no more particle could be found in the cone. If the visible energy in the cone was greater than 10% of the beam energy, the cone was regarded as a "jet". If a jet was found, the same procedure was repeated using the remaining particles. Using this " j e t " finding method, we selected 31 three-jet events and no four-jet events. Out o f the 31 events, four events without any isolated hard photon candidates were rejected through visual inspection. These events were classified as z-pair events with no hard photons. We used the K O R A L Z Monte Carlo program to estimate the n u m b e r of events from the radiative z-pair production. The events generated by K O R A L Z were processed with the O P A L detector simulation program [12] and reconstructed in the same way as the real events. After applying the selection criteria, including the visual inspection, the expected n u m b e r o f events was found to be 23.5 _+2.6. The additional contribution from m u l t i h a d r o n i c events was estimated to be 2.4_+ 1.4 events by the JETSET Monte Carlo program [6]. Hence the observed 27 events are consistent with the expectation. Applying the selection criteria to the events generated by the previously discussed Monte Carlo programs, we calculated numbers of events expected given our integrated luminosity and detection efficiency for single- and p a i r - p r o d u c t i o n o f excited leptons o f various masses. In the Monte Carlo event generation the luminosity distribution at the different centre o f mass energies at which data had been recorded was taken into account. We used our previously measured values of the Z ° mass and width [ 8 ] in the Monte Carlo programs. Fig. 2 shows the expected n u m b e r o f events for each channel, where f/A = 1 / ( 300 GeV ) is assumed for the single-production o f excited leptons. C o m p a r i n g the numbers o f observed events and

Volume 244, number 1

PHYSICS LETTERS B

Number of Expected Events . . . .

I

. . . .

I

. . . .

I

. . . .

I

io 2 >

e*e

"-110

/~... _

.~

~--<\

/

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-1

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t~lllllllllth

', I ', : ', ', I ','I :

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-1

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lo

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,,

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,

'~-:-i

,,

,

I , ~ , , I , ,

'J

....

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12 July 1990

95% c o n f i d e n c e level limits are s h o w n in fig. 3. As seen in fig. 3a, the l i m i t o b t a i n e d f r o m e + e - ~ (e + ) e T ) , is b e t t e r t h a n that o b t a i n e d f r o m e + e - - ~ e + e - y in general. T h i s is because o f the large contrib u t i o n o f t-channel p h o t o n e x c h a n g e to the single p r o d u c t i o n o f e* a n d o f the relatively low background. W i t h the use o f nearly the full d a t a s a m p l e taken in 1989, the use o f the process e + e - - ~ (e + )eTy, and high d e t e c t i o n efficiency for each process (especially for ~* ), o u r limits are s o m e w h a t m o r e stringent t h a n the recently r e p o r t e d limits [ 3 ] . An i n d i r e c t search for e* is also possible by s t u d y i n g the Q E D reaction e + e - --*yy. At present, h o w e v e r , o u r l i m i t f r o m this r e a c t i o n [ 13 ] is not strong e n o u g h to c o n s t r a i n Me, a b o v e the centre o f mass energy due to small statistics, and at masses b e l o w the centre o f mass energy

-1 >

-1 O P A L 10

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/ J

=

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.

.

.

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-3

lO

( CeV )

-1

Fig. 2. The expected numbers of events from excited lepton productions after applying the selection criteria, shown as functions of the mass of excited leptons. (a) number of events to be observed as e + e - ~ e + e - y y (solid curve), e+e--~e+e-y (dashed curve) and e+e - - , (e + )e:~7 (dash-dotted curve). (b) number of events to be observed as e + e - - ~ + ~ - y y (solid curve) and e + e - - , p + p - y (dashed curve). (c) number of events to be observed as e+e-~x+~-Ty (solid curve) and e+e--~t+~-y (dashed curve). In the single-productions of excited leptons, f/A = 1/ ( 300 GeV ) is assumed.

lO

< ~- lO -I

lO

-2 e x p e c t e d e v e n t s b o t h for signal a n d b a c k g r o u n d , we o b t a i n e d limits on the masses a n d the c o u p l i n g constants o f spin-½ excited leptons. F r o m the pair-productions, the l o w e r limits on Me., M r , a n d M r . are each 44.9 G e V at 95% c o n f i d e n c e level, where the b a c k g r o u n d yields are a s s u m e d to be zero. F r o m the s i n g l e - p r o d u c t i o n s , u p p e r limits on f/A are o b t a i n e d as f u n c t i o n s o f the masses o f the excited leptons. F o r e* a n d ~t*, e v e n t s w h i c h had an £±y pair in the invariant mass region o f M ~ . - 2 G e V < M ~ _ + ~ < M ~ . + 2 GeV, were t r e a t e d as c a n d i d a t e s . N o such i n v a r i a n t mass cut was used for ~+Y pairs a n d all the selected e v e n t s (27 e v e n t s ) w e r e treated as c a n d i d a t e s . T h e

i

_2 b) lO

C)

1o -3

lO -4

lO

0

20

40 l°mass

60

80

O0

( GeV

Fig. 3. The upper limits on the couplingf/A as functions of the mass of the excited leptons. (a) limits for e* obtained from e + e - ~ e + e - 7 (solid curve) and from e+e---,(e-+)eT7 (dashdotted curve). (b) limit for la* obtained from e+e --,la+la-y (solid curve). (c) limit for x* obtained from e+e - ~x+T y (solid curve). The mass limits obtained from the pair-productions are shown by vertical lines. 141

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PHYSICS LETTERS B

the direct search yields m o r e s t r i n g e n t constraints. In s u m m a r y , excited leptons have been searched for by s t u d y i n g the processes e + e - - , e + e - T T , e+e ---, e+e-T, e+e--,(e-+)e~T, e+e--,g+g-TT , e+e-~ g+~-T, e+e---'~+x-77 and e+e-~+x-T. T h e observed e v e n t s are c o n s i s t e n t with the s t a n d a r d m o d e l e x p e c t a t i o n s a n d n o e v i d e n c e for spin-½ excited lept o n s was f o u n d . F r o m the study o f e+e---,l~+!~-TT events, lower limits on the masses are f o u n d to be Me, > 44.9 GeV, M r , > 44.9 GeV, a n d M~, > 44.9 G e V at 95% c o n f i d e n c e level. F r o m the study o f e + e - - , ~ + ~ - T events, u p p e r l i m i t s o n the ~*~V ( V = T, Z ° ) c o u p l i n g s are set up to ~* masses close to the Z ° mass. It is a pleasure to t h a n k the LEP D i v i s i o n for the efficient o p e r a t i o n o f the m a c h i n e , the precise inform a t i o n on the absolute energy, a n d their c o n t i n u i n g close c o o p e r a t i o n with o u r e x p e r i m e n t a l group. In a d d i t i o n to the s u p p o r t staff at o u r o w n i n s t i t u t i o n s we are pleased to a c k n o w l e d g e the following: T h e B u n d e s m i n i s t e r i u m f'tir F o r s c h u n g u n d Technologie, F R G , T h e D e p a r t m e n t o f Energy, USA, T h e I n s t i t u t de Recherche F o n d a m e n t a l e du C o m m i s s a r i a t a l'Energie A t o m i q u e , T h e Israeli M i n i s t r y o f Science, T h e M i n e r v a Gesellschaft, T h e N a t i o n a l Science F o u n d a t i o n , USA, T h e N a t i o n a l Sciences a n d Engin e e r i n g Research C o u n c i l , C a n a d a , T h e J a p a n e s e M i n i s t r y of E d u c a t i o n , Science a n d C u l t u r e (the M o n b u s h o ) a n d a grant u n d e r the M o n b u s h o Intern a t i o n a l Science Research P r o g r a m , T h e A m e r i c a n Israeli B i - n a t i o n a l Science F o u n d a t i o n , T h e Science

142

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a n d E n g i n e e r i n g Research C o u n c i l , U K a n d T h e A.P. Sloan F o u n d a t i o n .

References [1]For a review, see for example Compositeness Working Group, F. Boudjema and F.M. Renard, Compositeness, CERN Report 89-08, eds. G. Altarelli el al., Vol. 2 (1989). [2] AMY Collab., S.K. Kim et al., Phys. Lett. B 223 (1989) 476; TOPAZ Collab., 1. Adachi et al., Phys. Lett. B 228 (1989) 553; VENUS Collab., K. Abe et al., Phys. Lett. B 213 (1988) 4O0. [ 3] ALEPH Collab., D. Decamp et al., Phys. Lett. B 236 (1990) 501. [4] Review of particle properties, Particle Data Group, G.P. Yost et al., Phys. Len. B 204 (1988) 265. [ 5 ] K. Hagiwara et al., Z. Phys. C 29 ( 1985 ) 115. [6] T. Sjgstrand, Comput. Phys. Commun. 39 (1986) 347. [ 7 ] OPAL Technical Proposal ( 1983 ) CERN/LEPC/83-4; OPAL Collab., K. Ahmet et al., The OPAL Detector at LEP, in preparation. [ 8 ] OPAL Collab., M.Z. Akrawy et al., Phys. Len. B 240 ( 1990 ) 496. [9] M. B~Shm,A. Denner and W. Hollik, Nucl. Phys. B 304 (1988) 687; F.A. Berends, R. Kleiss and W. Hollik, Nucl. Phys. B 304 (1988) 712. [ 10] D. Karlen, Nucl. Phys. B 289 (1987) 23. [ 11 ] S. Jadach et al., Z Physics at LEP 1, CERN Report 89-08, eds. G. Altarelli et al., Vol. 1 ( 1989 ); KORALZ, version 37. [ 12] J. Allison et al., Comput. Phys. Commun. 47 (1987) 55; R. Brunet al., GEANT3, Report DD/EE/84-1, CERN (1989). [ 13 ] OPAL Collab., M.Z. Akrawy el al., Phys. Lett. B 241 (1990) 133.