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PHYSICS LETTERS B
ELSEVIER Physics Letters B 370 (1996) 211-221
Search for excited leptons in e+e annihilation at y /s = 130-140 GeV
L3 Collaboration
M. Acciarriac, A. Adamav, O. Adrianir, M. Aguilar-Benitezab, S. Ahlen/', B. Alpataj,J. Alcarazab, J. Allaby\ A. Aloisioae, G. Alverson111, M.G. Alviggiae, G. Ambrosi^,
H. Anderhubaz, V.P. Andreevan, T. Angelescu", D. Antreasyan-*, A. Arefievad,T. Azemoonc, T. Azizk, P. Bagnaiaam,s, L. Baksayat, R.C. Ballc, S. Banerjeek,
K. Baniczav, R. Barillère\ L. Baroneam, P. Bartalini aJ, A. Baschirottoac, M. Basilej,R. Battistona-\ A. Bayx, F. Becattinir, U. Beckerq, F. Behneraz, Gy.L. Bencze0,J. Berdugoab, P. Bergesq, B. Bertuccis, B.L. Betevaz, M. Biasinis, A. Bilandaz,G.M. BileiaJ, J.J. Blaising8, S.C. Blyth3*, GJ. Bobbink5, R. Bocka, A. Böhma,
B. Borgiaam, A. Bouchamd, D. Bourilkovaz, M. Bourquin“, D. Boutignyd, E. Brambilla*1, J.G. Bransonap, V. Brigljevicaz, I.C. Brockak, A. Buijsau, A. Bujakav, J.D. Burgerq,
WJ. Burger“, C. Burgosab, J. Busenitzat, A. Buytenhuijsag, X.D. Cai1, M. Campanelliaz, M. Capellq, G. Cara Romeo-i, M. Cariaaj, G. Carlinod, A.M. Cartaccir, J. Casausab,G. Castellinir, R. Castelloac, F. Cavallariam, N. Cavalloae, C. Cecchiu, M. Cerradaa\
F. Cesaroniy, M. Chamizoab, A. Chanbb, Y.H. Changbb, U.K. Chaturvedi1, M. Chemarinaa, A. Chenbb, C. Chenh, G. Chen11, G.M. Chen*1, H.F. Chen\ H.S. Chenh, X. Chereaud,
G. Chiefariae, C.Y. Chien6, M.T. Choias, L. Cifarelliao, F. Cindoloj, C. Civininir, I. ClareR. Clareq, T.E. Coanz, H.O. Cohnah, G. Coignetd, A.P. Colijnb, N. Colinoab,V. Commichau3, S. Costantiniam, F. Cotorobai11, B. de la Cruzab, T.S. Daiq,
R. D’Alessandror, R. de Asmundisae, H. De Boeckag, A. Degréd, K. Deitersaw, E. Dénes0, P. Denesa£, F. DeNotaristefaniam, D. DiBitontoat, M. Diemozam, D. van Dierendonckb,F. Di Lodovico a\ C. Dionisiam, M. Dittmaraz, A. Dominguezap, A. Doriaae, I. Dorned,
M.T. Dova1,4, E. Dragoae, D. Duchesneaud, P. Duinkerb, I. Duranaq, S. Duttak, S. Easo0-*,Yu. Efremenkoah, H. El Mamouniaa, A. Englerak, FJ. Epplingq, F.C. Ernéb,
J.P. Ernenweinaa, P. Extermann“, R. Fabbrettiaw, M. Fabreaw, R. Facciniarn, S. Falcianoam,A. Favarar, J. Fayaa, M. Felciniaz, T. Fergusonak, D. Fernandezab, G. Fernandezab,F. Ferroniam, H. Fesefeldt3, E. Fiandrinia\ J.H. Field“, F. Filthautak, PH. Fisherq,
G. Forconiq, L. Fredj“, K. Freudenreich“ , M. Gailloudx, Yu. Galaktionovad,q,S.N. Gangulik, P. Garcia-Abiaab, S.S. Gaum, S. Gentileam, J. Geralde, N. Gheordanescun,
0370-2693/96/$l 2.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 3 7 0 -2 6 9 3 ( 9 6 ) 0 0 0 4 5 -7
S. Giaguam, S. Goldfarb*, J. Goldstein'', Z.F. Gongv, E. Gonzalezab, A. Gougase,D. Goujon11, G. Gratta3*, M.W. Gruenewald*, V.K. Guptaai, A. Gurtu\ H.R. Gustafson0,
L.J. Gutayav, K. Hangartera, B. Hartmann2, A. Hasanaf, J.T. He*1, T. Hebbeker*,A. Hervés, W.C. van Hoekag, H. Hoferaz, H. Hooraniu, S.R. Houbb, G. Hu1, M.M. Ilyas1,
V. Innocentes, H. Janssend, B.N. Jinh, L.W. Jonesc, P. de Jongq, I. Josa-Mutuberriaab,A. Kasserx, R.A. Khan1, Yu. Kamyshkovah, P. Kapinosay, J.S. Kapustinskyz,
Y. Karyotakisd, M. Kaur‘~\ M.N. Kienzle-Focacciu, D. Kime, J.K. Kima\ S.C. Kimas, Y.G. Kimas, W.W. Kinnisonz, A. Kirkby3*, D. Kirkbyai, J. Kirkby5, W. Kittelag,A. Klimentovq,ad, A.C. König3g, E. Koffeman*5, A. Köngeter3, V. Koutsenko q'ad,
A. Koulbardisan, R.W. Kraemerak, T. Kramerq, W. Krenza, H. Kuijtenag, A. Kuninq,ad,P. Ladrón de Guevaraab, G. Landir, C. Lapointq, K. Lassila-Periniaz, P. Laurikainenw,
M. Lebeaus, A. Lebedevq, P. Lebrunaa, P. Lecomteaz, P. Lecoqs, P. Le Coultreaz,J.S. Leeas, K.Y. Leeas, C. Leggettc, J.M. Le Goffs, R. Leisteay, M. Lentir, E. Leonardiam, P. Levtchenkoan, C. L i \ E. Liebay, W.T. Linbb, F.L. Linde*3, B. Lindemanna, L. Listaae, Z.A. Liuh, W. Lohmannay, E. Longoam, W. Luai, Y.S. Luh, K. Lübelsmeyer3, C. Lucian\D. Luckeyq, L. Ludovici3111, L. Luminariani, W. Lustermannaw, W.G. Mav, A. Macchiolo',
M. Maityk, G. Majumderk, L. Malgeriam, A. Malininad, C. Mañaab, S. Manglak,M. Maolinbayaz, P. Marchesiniaz, A. Marine, J.P. Martinaa, F. Marzanoam,
G.G.G. Massaro*5, K. Mazumdark, D. McNallys, R.R. McNeil8, S. Meleae, L. Merolaae, M. Meschinir, W.J. Metzgerag, M. von der Meya, Y. M ix, A. Mihuln, A.J.W. van M ilag,
G. Mirabelliam, J. Mnichs, M. Möller3, B. Monteleonir, R. Moore0, S. Morgantiam,R. Mount31, S. Müller3, F. Muheim“, E. Nagy0, S. Nahnq, M. Napolitanoae,
F. Nessi-Tedaldiaz, H. Newman3*, A. Nippe3, H. Nowakay, G. Organtiniam, R. Ostonenw, D. Pandoulas3, S. Paoletti3"1, P. Paolucci3e, H.K. Parkak, G. Pascaleam, G. Passaleva1',S. Patricelliae, T. Paulaj, M. Pauluzzia->, C. Paus3, F. Paussaz, Y.J. Pei3, S. Pensottiac,
D. Perret-Gallixd, S. Petrak1, A. Pevsnere, D. Piccoloae, M. Pierir, J.C. Pintoak,P.A. Pirouéaf, E. Pistolesir, V. Plyaskinad, M. Pohlaz, V. Pojidaevad,r, H. Postemaq,
N. Produit“, R. Raghavank, G. Rahal-Callot32, P.G. Rancoita30, M. Rattaggiac, G. Ravenap, P. Razis3f, K. Read3*1, M. Redaelli3c, D. Renaz, M. Rescigno3111, S. Reucroft111, A. Ricker3,
S. Riemann3y, B.C. Riemersav, K. Riles0, O. Rind0, S. Ro3S, A. Robohm32, J. Rodinq, F.J. Rodriguezab, B.P. Roe0, S. Röhnera, L. Romeroab, S. Rosier-Leesd, Ph. Rosseletx, W. van Rossumau, S. Roth3, J.A. Rubios, H. Rykaczewskiaz, J. Salicios, J.M. Salicioab,
E. Sanchez3b, A. Santocchiaaj, M.E. Sarakinosw, S. Sarkark, M. Sassowsky3, C. Schäfer3,V. Schegelskyan, S. Schmidt-Kaerst3, D. Schmitz3, P. Schmitz3, M. Schneegansd,
B. Schoeneichay, N. Scholzaz, H. Schopperba, DJ. Schotanus3g, R. Schulte3, K. Schultze3, J. Schwenke3, G. Schwering3, C. Sciacca3e, P.G. Seileraw, J.C. Sensbb, L. Servoliaj,
S. Shevchenko31, N. Shivarovar, V. Shoutko3d, J. Shuklaz, E. Shumilovad, T. Siedenburg3,D. Sonas, A. Sopczakay, V. Soulimov36, B. Smithq, P. Sp illantin iM. Steuerq,
D.P. Sticklandaf, F. Sticozziq, H. Stone3Í, B. S toy ano v 3r, A. Straessner3, K. Strauchp,K. Sudhakark, G. Sultanov1, L.Z. Sunv, G.F. Susinno“, H. Suteraz, J.D. Swain',
212 L3 Collaboration / Physics Letters B 370 (1996) 211 -221
L3 Collaboration / Physics Letters B 370 (1996) 211-221 213
X.W. Tangh, L. Tauscherf, L. Taylor01, Samuel C.C. Tingq, S.M. Tingq, O. Tokeraj,F. Tonischay, M. Tonutti3, S.C. Tonwark, J. Tóth0, A. Tsaregorodtsevan, C. Tullya<!,
H. Tuchschererat, K.L. Tungh, J. Ulbrichtaz, L. Urbán0, U. Uwers, E. Valente“1”,R.T. Van de Walle0*, I. Vetlitskyad, G. Viertelaz, M. Vivargentd, R. Volkertay, H. Vogelak,
H. Vogtay, I. Vorobievad, A.A. Vorobyovan, An.A. Vorobyovan, L. Vuilleumierx,M. Wadhwaf, W. Wallraffa, J.C. Wangq, X.L. Wangv, Y.F. Wangq, Z.M. Wangv,
A. Webera, R. Weillx, C. Willmottab, F. Wittgenstein \ S.X. Wu\ S. Wynhoffa, J. Xuf, Z.Z. Xuv, B.Z. Yangv, C.G. Yangh, X.Y. Yaoh, J.B. Yev, S.C. Yehbb, J.M. Youak,
C. Zaccardelliai, An. Zalitean, P. Zempaz, J.Y. Zengh, Y. Zenga, Z. Zhangh, Z.P. Zhangv,B. Zhou*, G.J. Zhouh, Y. Zhouc, G.Y. Zhuh, R.Y. Zhuaì, A. Zichichij-s'1,
B.C.C. van der Zwaanbu I. Physikalisches Instituí, RWTH, D-52056 Aachen, FRG 1 111. Physikalisches Instituí, RWTH, D-52056 Aachen, FRG 1
h National Institufe for High Energy Physics, NIKHEF, and University o f Amsterdam, NL-1009 DB Amsterdam, The Netherlandsc University o f Michigan, Ann Arbor, AI I 48109, USA
cl Uiboratoire d 'Annecy-le - Vie ux d e Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy ~le-Vie ux CEDEX, Francec Johns Hopkins University, Baltimore, MD 21218, USA
1 Institute o f Physics, University o f Basel, CH-4056 Basel, Switzerland £ Louisiana State University, Baton Rouge, LA 70803, USA
11 Institute o f High Energy Physics, I HEP, 100039 Beijing, China1 Humboldt University; D-10099 Berlin, FRG 1
' INFN-Sezione di Bologna, 1-40126 Bologna, Italy k Tata Institute o f Fundamental Research, Bombay 400 005, India
f‘ Boston University, Boston, MA 02215, USA m Northeastern University, Boston, MA 02115, USA
11 institute o f Atomic Physics and University o f Bucharest, R-76900 Bucharest, Romania n Central Research Institute fo r Physics o f the Hungarian Academy o f Sciences, H-1525 Budapest 114, Hungary2
p Harvard University, Cambridge, MA 02139, USA *1 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, FBUA Project, CH-1211 Geneva 23, Switzerland u Uni versi ty o f Geneva, CH -1211 Geneva 4, Switze rland
Chinese University o f Science and Technology, USTC, Hefei, Anhui 230 029, China 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, lialy
' Los Alamos National Laboratory, Los Alamos, NM 87544, USA Instituí de Physique Nucléaire de Lyon, IN2P3-CNRS, Universi té Claude Bernard, F-69622 Villeurbanne, France
ab Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, E-28040 Madrid, Spainac 1NFN-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 al Department o f Natural Sciences, University o f Cyprus, Nicosia, Cyprus
ag University o f Nymegen and NIKHEF, NL-6525 ED Nymegen, The Netherlands0,1 Oak Ridge National Laboratory; Oak Ridge, TN 37831, USA ai California Institute o f Technology, Pasadena, CA 91125, USA
a-i INFN-Sezione di Perugia and Università Degli Studi di Perugia, 1-06100 Perugia, Italyak Carnegie Mellon University, Pittsburgh, PA 15213, USA
df Princeton University, Princeton, NJ 08544, USA am INFN-Sezione di Roma and University o f Rome, "La Sapienza ” , 1-00185 Rome, Italy
an Nuclear Physics Institute, St. Petersburg, Russia
V
w
214 L3 Collaboration /Physics Letters B 370 (1996) 211-221
ao University und 1NFN, Salerno, I-84100 Salerno, Italy11 p University o f California, San Diego, CA 92093, USA
114 Dept, de Fisica de Particulas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain1 Bulgarian Academy o f Sciences, Central Laboratory o f Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria as Center for High Energy Physics, Korea Advanced Inst. o f Sciences and Technology; 305-701 Taejon, South Korea
aI University o f Alabama, Tuscaloosa, AL 35486, USA au Utrecht University and NIKHEF, NL-3584 CB Utrecht, The Netherlands
Purdue University, West Lafayette, IN 47907, USA Paul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
iiy DESY-Institut fü r Hochenergiephysik, D-15738 Zeuthen, FRG az Eidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zurich, Switzerland
11:1 University o f Hamburg, D-22761 Hamburg, FRG hh High Energy Physics Group, Taiwan, ROC
Received 20 December 1995 Editor; K, Winter
av
aw
Abstract
We report on a search for the excited leptons e*, ¡x*, r* and v * in e+e*~co!lisions at yfs = 130-140 GeV using the L3 detector at LEP. No evidence has been found for their existence. From an analysis of the expected pair produced t t * in the channels eeyy, f.ifiyy, r r y y , eeWW, and v v y y , we determine the lower mass limits at 95% CL, of 64.7 GeV for e*, 64.9 GeV for 64.2 GeV for r \ 57.3 GeV (eW decay mode) and 61.4 GeV ( v y decay mode) for v * . From an analysis of the expected singly produced t t* in the channels eey, ¡x¡xy, r r y , ^eW and v v y , we determine upper limits on the couplings A/m/* up to w * = 130 GeV.
1 . Introduction
The Standard Model [ 1 ] has been successful in describing present experimental data, including all LEP results. However, it has a large number of arbitrary parameters and leaves many fundamental questions unexplained such as the lepton-quark spectrum, mass generation and the origin of the Higgs mechanism. One way to explain the number of families and to make the fermion masses and weak mixing angles calculable, would be to assume that quarks, leptons and gauge bosons are composite [2]. As a consequence of such a model, one would expect the existence of excited lepton state, £*, for each known lepton, L
1 Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.
2 Supported by the Hungarian OTKA fund under contract numbers 2970 and T14459.
* Supported also by the Comisión Interministerial de Ciencia yéTechnologia,
4 Also supported by CONICET and Universidad Nacional de La Piala, CC 67, 1900 La Plata, Argentina.
5 Also supported by Panjab University, Chandigarh-160014, India.
At e+e” colliders, excited leptons would be produced either in pairs (e+e“ ™> £*£*) or singly (e+e ~ —> ££*). In the first case the maximum t mass is limited to the beam energy, whereas in the second case it can reach mass regions close to the center of mass energy. An excited lepton t is assumed to have spin It could have both the left- and the right-handed component [3] or only a left- handed component as in the Standard Model. Since the production cross section of excited lepton pairs is smaller if only the left-handed component is present, we use this assumption in order 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.
The excited leptons e*, /¿*, r* and v* have been extensively searched for at the LEP e+e” collider with < s / s = 91 GeV and at the HERA ep collider in the last five years [4]. In this paper we describe a new search for excited leptons with L3 at center of mass energies of 130-140 GeV.
L3 Collaboration / Physics Letters B 370 (1996) 211-221 215
The Z and y are assumed to have the same coupling to an excited lepton pair £*£* as to the standard lepton pair £L The r-channel contribution for e* and v* is neglected since the couplings V£*£ are expected to be much smaller than normal couplings V££ and V £.*£,*> where V = y, Z, W. The lowest order pair production cross section can be found in Ref. [2 ].
For single t production, the effective La- grangian [3] can be written as:
£ eff = - Dv£*0/5)^KV^i'/„■ J A ^V=y,Z,W
h.c.
where A is the composite mass scale and Cy/?*£ and Dw*/ are unknown coupling constants. The precise g — 2 measurements impose \Cvt*f] = |Dv£**|. The absence of the electric dipole moment of electrons suggests that both Cym and D y a r e real. Therefore we use in the following Cyf+t = D y ^ . The coupling constants can be written as;
Cyi.f = i ( t 3/ + i Y / ) ,
Cz i-t = { ( t3 ƒ cot 0w ~ ^Y/'tan^w).
C / + ( y--- 1
2 \ / 2 sin0w
where ƒ and ƒ' are respectively the free parameters for SU (2) and U ( l ) , t3 is the third component of weak isospin of £*, Y is the hypercharge of £* and the Weinberg angle. Throughout this paper we assume that t3 and Y for the £* are the same as for the standard L For excited charged leptons, we assume that ƒ = f so that ƒ / A (= is the only free parameterin the Lagrangian [5]. The differential cross section formulae can be found in Ref. [3] and the total cross section is obtained by integration.
An excited lepton is expected to have a narrow width and its mean decay length is less than 1 jam at LEP [6 ]. An excited charged lepton is expected to decay into a standard lepton and a photon with a 100% branching ratio, if its mass is smaller than that of the W and the Z. At large mass, the decays £* —> Z£ and £* become important and the branching ratioof £* ~ > £y is then a function of mf* [6 ]. An excited neutrino, v *, can decay by emitting a y, Z or W, which
2. Production and decays of i* are either virtual or real depending on the mass of v*. We have studied the following two cases:(1) ƒ = f \ In this case the yvv* coupling vanishes
and v * —> vlu and v* —* eW are the only decay modes allowed (the searches are limited to the v*, which is expected to be the lightest excited neutrino). Since the eW branching ratio is more than 65% [6 ] for mv* = 40-140 GeV, and its experimental signatures are much cleaner than those of the Z channel decays, we investigate only the W channel decays. The visible final state is an electron plus two jets if the W decays hadronically or an electron plus another lepton if the W decays leptonically. The coupling parameter in the Lagrangian is the same as for the charged excited leptons: ƒ / A( = ).
(2) ƒ f : In this case the yvv* coupling exists. The neutrino has a magnetic moment [7] and the decay v* vy would have a large branching ratio. Hence the W and Z channel decays are neglected in this analysis. The signature of a v * is a single energetic photon. There are two coupling parameters ƒ and f in the Lagrangian. For simplicity we study the two extreme cases: ƒ = 0 or ƒ = 0 .
All the above processes have been generated by a Monte Carlo program according to the differential cross section of Ref. [3] with an angular distribution of 1 + cos# assigned to the £* decay. The relevant branching ratios of £* decays are taken from Ref. [6 ]. The subsequent r decays are simulated by the KO- RALZ Monte Carlo program [ 8 ] and the hadronic fragmentation and decays are simulated by the JET- SET Monte Carlo program [9]. The effect of initial state radiation is not included in the Monte Carlo generator but is taken into account in our cross section calculations. All generated events have been passed through the L3 detector simulation [10] which includes the effects of energy loss, multiple scattering, interactions and decays in the detector and the beam pipe.
3. The L3 detector and the data sample
The L3 detector is described in detail in Ref. [11]. It consists of a silicon microstrip vertex detector, a central tracking chamber (TEC), a high resolution elec-
216 L3 Collaboration /Physics Letters B 370 (1996) 211-221
tromagnetic 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 11° 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 (air core; 36° < 6 < 144°), forward (iron core; 22° < 0 < 36°) and backward (iron core; 144° < 9 < 158°) regions.
The data used in these searches were taken with the L3 detector at LEP between October and November 1995. The integrated luminosity is 2.75 pb~I at yfs = 130 GeV, 2.27 pb" 1 at y/s = 136 GeV and 0.05 pb" 1 at V 5 = 140 GeV
4. Search for excited electrons
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 ref) projection, it is identified as a photon.
To remove the background from two-photon collisions, hadrons and taus, we apply the following selection criteria:
i) there is at least one electron in the event;ii) the number of tracks is at least 1 and at most 4;
iii) the total energy in the electromagnetic calorimeter is more than half the beam energy.
In the following we describe the selection of excited electron candidates in the channels eeyy and eey.
4.1. Pair production of e*
Event selection for the reaction e+e'"--* e*e* —> eeyy requires two electrons and at least two photons, all with energy greater than 5 GeV Two events pass the selection. The invariant mass is reconstructed for all possible combinations of ey, but no structure is evident in the spectrum. All masses are less than 50 GeV The main background is due to radiative Bhabha events e+e ~ —> eeyy. Owing to uncertainties of Monte Carlo predictions for hard radiative Bhabha processes, we conservatively make no background subtraction in calculating the upper limit,
The detection efficiency for the signal is estimated from Monte Carlo to be 55%, independent of the mass of the e*. The decay branching ratio, e* —> ey, is about 100% for the mass region concerned. Taking into account the luminosity, the efficiency and the production cross section of e*, 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 64.7 GeV
4.2. Single production of e*
Event selection for the reaction e+e - —> ee* —■> eey requires at least one electron with energy greater than 5 GeV and exactly one photon with energy greater than 10 GeV To reduce the background from Bhabha scattering, the photons are required to be in the barrel region. Since the t-channel contribution is large, one electron could be missed in the beam pipe. For events with only one observed electron, we therefore require that the thrust axis of the event should be within the barrel region.
A total of 34 events pass the selection. Fig. la shows the invariant mass, mcy, of all combinations. The mass resolution for e* is about 1 GeV, estimated from Monte Carlo events. No significant structure can be seen. Owing to uncertainties of Monte Carlo predictions for hard radiative Bhabha processes, we conservatively make no background subtraction in calculating the upper limit. For this reason, no background distribution is shown in Fig. la.
The detection efficiency is estimated from Monte Carlo to be 44% at me* = 7 0 GeV and 66% at me* =139 GeV The decay branching ratio of e* —> ey changes from 100% at ?ne* < 80 GeV to 40% at me* =
L3 Collaboration / Physics Letters B 370 (1996) 211-221 217
L3
> 5<u °ar 4
.2 3 k*c oIII *
1
oo 50 100
6> 5aS" 40)
ft 4 M 3k*c 2UJ
10
0 50 100mey [GeVj "V, [GeV]
>0)O
$€ 1 cII!
2 -
0
8>a)a 6
§ . £ 4 a>5 2
d)• DATA
-□MC vvym MC yyy
0 50 100[GeV]
40 60 80 100 120 m,.ooii IGeV]
Fig. I . Selected events in single production searches; a) invariant mass of all ey combinations; b) invariant mass of all jxy combinations; c) invariant mass of all t y combinations (after applying kinematic constraints); d) recoil mass of single photons. For each plot the bin size is comparable to the expected mass resolution.
140 GeV. We obtain an upper limit for the number of e* events in each mass bin at 95% C.L. 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. 2a. The more stringent limit in the region between 65-90 GeV, as determined from a previous analysis of Z decays [ 12], is combined with the present measurement.
5. Search for excited muons
Muons are identified from tracks in the muon chambers with measurements in both the rep and rz projections. The transverse and the longitudinal distances of closest approach to the interaction vertex are required to be less than 200 mm.
To remove the background from cosmic rays, two- photon collisions, hadrons and taus, we apply the following selection criteria:
i) there is at least one identified muon with momentum greater than 10 GeV;
ii) there is at least one scintillator signal in time with the bunch crossing ( < 4 ns);
■>o0
1
1m.. [GeV]
b)f = 0
10-1
........ f 0
Ea
-2 10 V
10-3
10-4 1 _L_
60 70 80 90 100 110"V [GeV]
120 130 140
Fig. 2. The upper limit of the coupling constant A/nif* at 95%C.L. as a function of ni{*: a) ƒ = ƒ' for all excited leptons; b) ƒ 4* f for excited neutrinos with A = ƒ if ƒ = 0, and A = f if ƒ = 0. The excluded region is above and to the left of the curves.
iii) there are at least two tracks in the central tracking chamber;
iv) the number of clusters in the electromagnetic calorimeter is less than 10;
v) the energy deposited in the hadron calorimeter is less than 10 GeV;
vi) the total visible energy is greater than half the beam energy.
In the following we describe the selection of excited muon candidates in the channels ¡i/xyy and fxfxy.
5.1. Pair production of /x
Event selection for the reaction e+ef i /xyy requires at least two photons with energy greater than 5 GeV Two events pass the selection. The invariant mass is reconstructed for all possible combinations of ¡xy, but no structure is evident in the spectrum. AH masses are less than 52 GeV except one combination which has a mass of 70 GeV. The
218 L3 Collaboration /Physics Letters B 370 (1996) 211-221
main background is due to radiative dimuon events/¿/xyy. A total of 0.8 events is predicted by+
the Monte Carlo program, KORALZ.The detection efficiency for is estimated from
Monte Carlo to be 62%, independent of the mass ofthe /A*. The decay branching ratio, fX* —> ¡my, is about 100% for the mass region concerned. We determine the lower mass limit for excited muons at 95% C.L. to be 64.9 GeV.
5.2. Single production o f t
Event selection for the reaction e+e ¡ip.¡¿fly requires exactly one photon with energy greater than 5 GeV. A total of 15 events pass the selection. The main background is due to radiative dimuon events
following selection criteria:i) the number of tracks is at least two and at most
7;ii) the number of clusters in the electromagnetic
calorimeter is less than 16;iii) the visible energy is greater than half the beam
energy;iv) there is at least one tau whose energy deposition
in the calorimeters is not consistent with that ofa muon;
v) the total energy deposition in the electromagnetic calorimeter is less than 85% of the center- of-mass energy;
vi) the polar angle of the missing momentum shouldsatisfy | cos#| < 0.9.
In the following we describe the selection of excitede +e /ifjuy, which is predicted by KORALZ to be tau candidates in the channels r r y y and rry.12.3 events. Fig. lb shows the invariant mass, of all combinations together with the Monte Carlo prediction for the background. The mass resolution for /¿* 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 singly produced ¡jl* is estimated to be 67%. The efficiency drops to 58% at /z* mass close to the center-of-mass energy. The decay branching ratio of fi* —► ¡uy changes from 100% at 50%. The efficiency is slightly dependent on the mass mM* < 80 GeV to 40% at = 140 GeV We obtain of the r* and is taken into account. The decay branch- an upper limit of the coupling constant A/mM* at 95% ing ratio of r* r y is about 100% tor the mass re- C.L. as a function of , which is combined with the gion concerned. We determine the lower mass limit previous analysis [12], as shown in Fig, 2a. for excited taus at 95% C.L. to be 64.2 GeV.
6.1. Pair production of r*
Event selection for the reaction e+e ~ —> r*r* r ry y requires at least one tau with energy greater than 2 GeV and two photons each with energy greater than 5 GeV No events pass the selection. The background from radiative rr events, e+e ~ —> r r y y , is estimated to be 0.3 events by KORALZ.
The detection efficiency for r* is estimated to be
6.2. Single production o f t6* Search for excited taus
This analysis uses a jet clustering algorithm [13] 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. A photon should fulfill the requirements as mentioned in Section 4. In addition, it must be isolated from any other calorimetric cluster by at least 15°. A jet is identified as a tau if its energy is greater than 2 GeV and it is not identified as a photon.
To remove background from hadrons, two-photon collisions, muons and Bhabha events, we apply the
Event selection for the reaction e+e TT rryrequires at least one tau with energy greater than 2 GeV and exactly one photon with energy greater than5 GeV A total of 4 events pass the selection. The main background is due to radiative rr events, e+e ~ —► r r y , and is estimated to be 4.9 events by KORALZ. Due to the large Lorentz boost, most of the tau neutrinos have the same direction as taus. Using this assumption and energy and momentum conservation, we can estimate the total momentum of the taus and reconstruct the invariant mass of r y . From Monte Carlo events we estimate the mass resolution of r*, after applying these kinematic constraints, to be about 2 GeV Fig. tc
L3 Collaboration / Physics Letters B 370 (1996) 211-221 219
shows the invariant mass of the 4 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 singly produced r* is estimated to be 66%, The efficiency drops to 48% for a t* with mass close to the center-of-mass energy. The decay branching ratio of r* —> r y changes from 100% at mT* < 80 GeV to 40% at mT* = 140 GeV. We obtain an upper limit of the coupling constant A/mr* at 95% C.L, as a function of mT*, which is combined with the previous analysis [32], as shown in Fig. 2a.
7. Search for excited neutrinos in the channelv eW
The jet cluster algorithm used in the tau analysis is also used here. A reconstructed jet corresponds to an isolated high energy lepton, photon or hadronic jet. An electron is identified as an electromagnetic shower with an associated track within 5°. It must be isolated from any other calorimetric cluster by at least 15°.
In the following we describe the selection of excited neutrino candidates in the channels eeWW and v tW.
7.Ì. Pair production of v*
p * p *Event selection for the reaction e+e eeWW requires the following:
i) there must be at least one isolated electron with energy greater than 2 GeV. It should be inconsistent with a converted photon;
ii) the number of tracks must be at least 4;iii) the energy deposited in the electromagnetic
calorimeter must be less than 90% of the center- of-mass energy;
iv) the total visible energy should be greater than 20 GeV
v) the number of jets should be at least 3;v i) the number of jets should be at least 5 if there is
no identified second electron with energy greater than 1 GeV
No events pass the selection. The backgrounds from two-photon collisions and hadronic events are estimated from Monte Carlo to be negligible.
The detection efficiency is estimated from Monte Carlo to be 59%. The efficiency is slightly dependent on the mass of the v* and is taken into account. The decay branching ratio of v* —► eW is about 71% in the mass region concerned. The lower mass limit for excited neutrinos at 95% C.L. is determined to be 57.3 GeV.
7.2. Single production of v*
vvEvent selection for the reaction e+e ^eW requires the following:
i) there must be exactly one isolated electron with energy greater than 5 GeV It should be inconsistent with a converted photon and its polar angle should satisfy | cos#| < 0.9;
ii) the total visible energy should be greater than 40% of the beam energy;
iii) the polar angle of the missing momentum direction should be greater than 18°;
iv) 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 should satisfy cos (f> < 0.9;
v) if the W decays hadronically, i.e. the number of tracks is greater than or equal to 5, there must be three jets in the event, each with energy greater than 5 GeV
No events pass the selection. Backgrounds from tt, two-photon collision and hadrons are estimated from Monte Carlo to be negligible.
The detection efficiency is estimated to be 25% at m„* = 70 GeV and 45% at mv* > 100 GeV The decay branching ratio of v* eW is more than 95% at mv* = 91 GeV and drops to 67% at m„* = 140 GeV We obtain an upper limit of the coupling constant A/i7iu* at 95% C.L. as a function of m*,*, which is combined with the previous analysis [ 12], as shown in Fig. 2a.
8. Search for excited neutrinos in the channelv vy
Photons are identified as electromagnetic showers. To remove background from charged final states and cosmic rays, we apply the following selection criteria:
220 L3 Collaboration/ Physics Letters B 370 (1996) 211-221
i) there are no tracks in the central tracking chamber;
ii) the energy deposited in the electromagnetic calorimeter is larger than 10 GeV;
iii) the energy deposited in the hadron calorimeter is less than 5 GeV;
iv) the thrust axis has a polar angle | cos0| < 0.91;v) there is at least one scintillator signal in time
with the beam crossing ( < 4 ns) caused by leakage of the electromagnetic showers.
In the following we describe the selection of excited neutrino candidates in the channels vvyy and vvy.
8.1. Pair production of v
Event selection for the reaction e+e+ ~ - v*v*vvyy requires at least two photons, each with energy greater than 10 GeV. The directions of the missing mo-
both NNGSTR [16] and KORALZ [ 8 ] for radiative neutrino events and 1.2 events are predicted by GGG [14] for e+e” —► yy. Fig. Id shows the recoil mass of the 20 observed events compared with Monte Carlo predictions for the background. The peak at 91 GeV shows that most of the events are due to e+e“ —> Zy, where the y is from initial state radiation, and the Z decays to vv. We conclude that all events are compatible with the expected background.
The detection efficiency is estimated to be about 56%. It is slightly dependent on the mass of the v* and is the same for both ƒ = 0 and f = 0. For ƒ =0 , the decay branching ratio of v* —+ vy is 100% at mv* < 91 GeV and drops to about 90% at mv* = 140 GeV. For f = 0, the decay branching ratio is 100% at mv* < 80 GeV and drops to 15% at mv+ = 140 GeV. Since it is not possible to reconstruct the vy invariant mass, we derive the upper limit on the basis
mentum and of both photon candidates are required to of 20 observed events with 16.1 expected backgroundevents. An upper limit is obtained for the coupling constant À/m v* at 95% C.L. as a function of mv* for both ƒ = 0 and f = 0. The result, combined with the
be greater than 20° with respect to the beam direction. This cut removes most of the Bhabha and e+e events. To further remove e+e“
77yy events, we re
quire that the energy sum of two photons is less than 75% of the center-of-mass energy.
No events pass the selection. The main background is due to events of the type e+e" —» yy. A total of 0.6 events are predicted by GGG [ 14].
The detection efficiency is estimated to be 52%. It is slightly dependent on the mass of the v* and is taken into account. The decay branching ratio of v* —»■ vy is 100% in the mass region concerned. We determine the lower mass limit for excited neutrinos at 95% C.L. to be 61 .4 GeV
previous analysis, is shown in Fig. 2b, in which A = ƒ if ƒ' = 0, and A = ƒ' if ƒ = 0.
9. Conclusion
8.2. Single production of v *
Event selection for the reaction e+e vv * vvyrequires exactly one photon with energy greater than 10 GeV This photon is required to be in the barrel part of BGO in order to reject background from radiative Bhabha and e+e~ -* Zy —* vvy events. A total of 20 events pass the selection. The background is mainly due to a) radiative Bhabha events e+e" —► (e ) (e )y with two electrons missing in the beam pipe, b) radiative neutrino events e+e~ —> vvy , and c) e+e ~ —> yy with one y missing in the beam pipe. For the present luminosity, no events are predicted by TEE [15] for radiative Bhabha events, 14.9 events are predicted by
We see no evidence for excited electrons, muons, taus or neutrinos; the observed events are consistent with Standard Model expectations. From pair production searches the lower mass limits are found to be 64.7 GeV for e*, 64.9 GeV for 64.2 GeV for r*, 57.3 GeV (eW decay mode) and 61.4 GeV (v y decay mode) for v*. From single production searches, we derive upper limits on the couplings k/m?* in the range of (10_4-1) GeV-1 for I* masses up to 130GeV
Acknowledgments
We would like to thank D. Zeppenfeld and F. Boud- jema for valuable discussions. We wish to congratulate the CERN accelerator divisions for the successful upgrade of the LEP energy and to express our gratitude for the excellent performance of the machine. We acknowledge with appreciation the effort of all engi-
L3 Collaboration /Physics Letters B 370 (1996) 211-221 221
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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