-
Nuclear PhysicsB403 (1993) 3—24 ~ ~North-Holland
Searchfor Z°decaysto two
leptonsandachargedparticle—antiparticlepair
DELPHI Collaboration
P. Abreut, W. A~am~,T. Adye~.E. Agasi~’,R. Aleksanarn G.D.
A1ekseev~,A. Algeritm, P. AllenaY, S. Almehedw. S.J.Alvsvaagd,U.
Ama1di~,
A. Andreazza~,P. Anti1ogus’~,W-D. Ape!°,R.J.Apsimon~,Y.
ArnoudamB. Asrnan~,J-E.Augustinr, A. Augustinus~~,P. B~j11~n~,P.
Bambader,
F. Baraot, R. Barate~,G. Barbie1lini~’,D.Y. Bardin~,G.J.
Barkerah,
A. Baroncelliao,0. Barring~,J.A. Barrio~,W. Bartlaz, M.J.
Batesak,M. Battagliatm,M. Baubillierv, K-H. Becks~’,C.J.
Beeston~’,M. Begalliai,
P. Beilliere’, Yu. Be1okopytov~l~,P. Beltran, D. Bcnedich,A.C.
Benvenuti~,M. Berggrenr,D. Bertrandb,F. Bianchiat, M.S. Bi1enky~,P.
Billoirv,
J. Bjarncw, D. Bloch”, S. B1yth~’,V. Boccia~,P.N. Bogolubov’~,T.
Bologneseam M. Bonesiniaa,W. Boniventoaa,P.S.L. Boothu,G. ~
H. Borner~C. Bosioao, B. Bostjancica~,S. Bosworth~’,0.
Botneraw,E. ~ B. Bouquet’, C. Bourdariosr,T.J.V. Bowcock~’,M.
Bozzok,
S. Braibantb,P. Branchiniao,K.D. Brand~°,R.A. Brenner~,H.
Briand”,C. Bricmanb,R.C.A. Browns,N. Brummer~~d,J-M. Brunet~,L.
Bugge~’~,
T. Burana~,H. Burmeister~,J.A.M.A. Buytacrt~,M. Caccia~,M.
Ca1vi~,
A.J. CarnachoRozasap,R. Campion~’,T. Carnporesi~,V. Canale~,F.
Caob,F. Car~na~,L. Carroll U C. Casok,M.V. Castillo Gimenez~~,A.
Cat~ai~,F.R. CavaIlo~,L. Cerritoaf, V. ~ A. Chana,Ph.
Charpentjer~,
L. Chaussardr,J. Chauveauv,P. Checchia’°,GA. Chelkov~,L.
Chevalier~’m,P. Chliapnikovao,V. Chorowiczv,J.T.M. Chrinay, P.
CollinSat,
J.L. Contreras~,R. Contrik, E. Cortinaay,G. Cosrne’,F.
Couchotr,H.B. Crawleya,D. Crenne1la~(,G. Crosettik, M. Crozon~,J.
Cuevas~
S. Czellarm, E. Dah1-Jensen~,B. Dalmagner,M. Dam~~,G.
Damgaard~,G. Darbok, E. Daubieb, A. Daurn°,P.D. Daunceyat, M.
Davenport~,
P. Davidv, J. Davies~,W. Da Silva”. C. Defoix~,P. Delpierre~,N.
Demariaat,A. De Angelis~’,H. De Boeckb,W. De Boer°,C. De
Clercqb,
M.D.M. De FezLaso”~,N. De Groota~~,C. Dc La Vaissierev,B. De
Lotto~’,A. De Minaa, H. Dijkstra~,L. Di Ciaccio~’,J. Dolbeau~,M.
DonszeI~~nn~,K. Doroba’~,M. Dra~~s~,J. Drees~’,M. Drjsae,Y.
D~four~,F. Dupont~,
D. Edsalla,L-0. Eekaw,P.A.-M. Eeroia~~R. Ehret°,T. Ekelofaw,G.
Ekspongas,A. Elliot Peisertal,J-P. Engelh,N. ErshaidaY,D.
Fassouliotisae,
0550-3213/93/S06.00 © 1 993—ElsevierSciencePublishersB.V. All
rights reserved
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RAPID COMMUNICATION
4 DELI’I!I (ol/aborailon / Z0 decat,sea/c/I
M. F~indt~,M. FernandezAlonsoap.A. Ferreray.T.A. Filippasae,
A. Firestonea H. Foethg E. Fokitis ae F. Fontanellik K.A.J.
ForbesUJ-L. Foussctz,S. Franconx,B. Franekak,P. Frenkiel~,D.C.
Fries0,A.G. Frodesend,R. Fruhwirth~,F. Fulda-Quenzcr’,K.
Furnival”’,
H. Furstenau°,J. Fustcr~D. Garnbaat,C. Garciaay,J. Garciaap,C.
Gaspar~,U. Gasparini”, Ph. ~avi1l~~~,E.N. Gazisae,J-P.Gerberh,P.
GiacomeIli~,
R. Gokieli~,B. Golobar, V.M. Golovatyuk~,J.J.GomezY Cadenas~,G.
Gopal~,L. Gorna,M. Gorski~,V. Graccok,A. Gr~nt~,F. Grardb,
E. Grazianiao G. Grosdidierr E. Grossg P. Grosse-Wiesrnanng
B. Grossetetev,S. ~urneny~ka~, J. Guy’~,U. Hacdinger°,F.
HahnuJM. Hahn°,S. Haider~’,A. Hakansson’~,A. Hallgren~,K.
Hamacher~,
G. HamelDc Monchenaultam,W. Haoad F.J. Harris’~,V. Hedbcrgw,T. ~
J.J.Hernandez”~,P. Herquetb,H. H~rr~,T.L. Hessingu,
I. Hietanentm,C.0. Higginsu,E. HigonaY, H.J. Hjlk~~,S.D.
Hodgson~’,
T. Hofmokl’~,S-0. Holrngren~,D. Holthuizen’~,P.F. Honoree,J.E.
Hoopera~),M. Houldenu,J. Hrubecaz,K. Huetb,P.O. Huithas,
K. Hultqvistas,P. loannouc,P-S. Iversend,J.N. Jacksonu P.
Jalocha~,G. Jar1skog~v,P. Jarry”tm, B. Jean-Marie’,E.K.
Johansson~,D. Johnsonu,
M. Jonker~L. Jonsson’~,P. Juilloth, G. Kalkanis’, G. Kalrnus~,F.
Kapustav,M. Kar1~~~n~,E. Karvelas’, S.
Katsanevase,E.C.Katsoufishie,R. Keranen~J. Kesternanb,B.A.
Khomenko’’, N.N. Khovanski0,B. King”,
N.J. Kjaer~,H. Klein~,A. Klovning~’,P. Kluit~1,A.
Koch-Mehrin~,J.H. Koehne°,B. Koene~’,P. Kokkinias’, M.
Koratzinos~,A.V. Korytov”,
V. ~ C. Kourkournelis’, 0. Kouznetsov”,P.H. Krarner~,C.
Kreuter°,J. Krolikowskiu~,I. Kronkvist~,U. Kruener-Marquis~,
W. Krupinski~,K. Kulkaaw, K. Kurvinentm, C. Lacastaay,C.
Lambropoulos’,J.W. Lamsaa,L. Lanceri”, V. Lapina~,i-P. Laugicr”tm,
R. Lauhakangastm,
G. Lederaz,F. Ledroit~,R. Leitner~,Y. Lemoigne”tm, J.
Lernonneb,G. Lenzenu~,V. Lepeltier’, T. Lesiak~,J.M. Levy”, E.
Lieb~,D. Likoaz,
J. Lindgrenm,R. Lindnerb~),A. Lipniackau~,I. L~r~ia,B.
Loerstadw,M. Lokajicek~,J.G. Loken~’,A. Lopez~Fernandez~,M.A.
LopezAguerah~P,M. LOSad D. Loukas’, J.J.Lozanoay,P. Lutz~,L.
Lyonsah,G. Maehlumaf,
J. Maillard~,A. Maiot, A. Maltezos’, F. Mandl~,J. Marco”~,M.
Margoni~,J-C.Marjn~A. Markou’, T. Maronb~~,S. Martiay, F.
Matorras”,
C. Matteuzzi~a,G. Matthiae”~,M. Mazzucatoal,M. Mc Cubbinu,R. Mc
Kaya, R. Mc Nulty”, G. Meola’~,C. Meroni~,W.T. Meyer”,
M. Michelottoai ~ Mikulec az L. Mirabito X WA. Mitaroffaz,G.V.
Mitselrnakher0,U. Mjoernrnark’~’,T. Moaas,R. Moellera~),K.
Moenig~,
M.R. Mongek, P. Morettinik, H. Mueller0, W.J. Murray ,Ik, B.
Muryn~,G. Myatt~’,F.L. Navarriae,P. Negri~a,R. Nicolaidouc, B.S.
Nielsen~,
B. Nijjhar’~,V. ~ P.E.S.Nilsend, p~Nissas,A.
Nornerotski”’.v~Obraztsov~,A.G. Olshevski0.R. Oravatm,A. ~ K.
Osterbergm,
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RAPID COMMUNICATION
DELPHI Collaboration / L° decaysearch 5
A. Ouraou”m,M. Paganoniaa,R. Painv H. Palka”, Th.D.
Papadopoulou~~e,L. Papc~,F. Parodik,A. Passeriao,M. Pegoraro”’, J.
Pennanentm,L. Peraltat.
V. ~ H. Pernegger’~,M. Pernicka~~z.A. Perrottac,C. PetridouhiU
4~Petrolinik L. Petrovichaq F. Pierream M. Pimenta
0. Pingoi)~,S. Plaszczynski’.0. Podobrin°,M.E. Pol~,G. Polok”,P.
Poropata~~,V. Pozdniakov”, P. Privitera°.A. Pullia”’. D.
Radojicic~,
S. Ragazzi””, H. Rahmani~e,P.N. Ratoffs,A.L. Read”, p. ~N.G.
Redaelliaa,M. Regler~~z,D. ~ P.B. Rentonall L.K. Resvanisc,
F. Richard’, M. Richardson”.J. Ridky~,G. Rinaudo’°,I Roditi~A.
Romeroat,1. Roncagliolok,P. Ronchese”’,C. Ronnqvistm,El.
Rosenberg”,
S. Rossi~,E. R0550~P. Roudeau’,T. Rovelli’. W. Ruckstuhl~d,
V. Ruhlrnann~Kleiderarn,A. Ruiz~,K. ~ H. Saarikkom,
Y. Sacquin”tm,G. Sajot’, J. Salt~’~,J. Sanchez~,M. Sannino~”,S.
Schael~,H. Schneider°,M.A.E. Schynsb~~,G. Sciollaal, F. Scuri””,
A.M. Segar”,
A. Seitz°,R. Sekulin~,M. Sessaau.G. Sette’~,R. Seufert°,R.C.
Shellardaj,I. Siccama&~,P. Siegrist”tm, S. Simonettik, F.
Simonetto”’, AN. Sisakian’’,
G. Skjevling”, G. Smadjaam~,0. Smirnova’’. G.R. Srnith~k,R.
Sosnowski~,D. Souza~SantosaJ,T.S. Spassoff’,E. Spiritiao,S.
Squarcia’~,H. Staeck~,
C. Stanescu”°,S. Stapnes”’,G. Stavropoulos’,F. Stichelbautb,A.
Stocchi’,J. Straussaz,J. Straver~,R. Strub’’, B. Stugu’’, M. ~M.
Szeptyckaba,i. szyrnanskiba,T. Tabarelliaa,0. Tchjkjlevaq
G.E. Theodosiou’.A. Tilquinz. J. Tirnmerrnans~,V.G.
Timofeev”,L.G. Tkatchev”, T. Todorov1’. D.Z. Toet”~.0. Tokertm, B.
Tome’.
E. Torassa”’, L. Tortora”°,D. Treil1e~,U. Trevisank,W.
Trischuk~,G. Tristran~,C. Troncon”, A. Tsjrou~E.N. Tsyganov”, M.
Turala”,
M-L. Turluer”tm, T. Tuuvam, l.A. Tyapkinv. M. Tyndel””, S.
Tzamarias”,S. Ueberschaerb~),0. ~ v~ ~-~act G. Valenti’, E.
VaIlazza~~t,
J.A. Valls Ferrer”~,C. VanderVeldeb.G.W. Van Apeldoorn~’.P. Van
Dam~’,M. Van Der Heijden~’,W.K. Van Doninck”, P. Vaz~,G.
Vegnia~,
L. Ventura”’, W. Venusa~(,F. Verbeureb,M. Verlato”’, L.S.
Vertogradov’’,D. Vilanovaam,P. Vincent~,L. Vitaletm, E.
Vlasov”~,A.S. Vodopyanov”,
M. VolImerb~),M. Voutilainentm. V. Vrba”°,H. Wahlenu~,C.
Walck”5,F. Waldner”, A. Wehr’~,M. Weierstallb~~,P. Wejlharnrner~,J.
Wernerb~),A.M. We~herell~J.H. Wickensb,G.R.WilkiflSon”~’,W.S.C.
Williams’”’,
M. Winter”, M. Witek~,G. Wormser’, K. Woschnagg~~v,N.
Yamdagni~~s,P. Yepes~,A. Zaitsevaq,A. Zalewska~,P. Zalewskir, D.
Zavrtanik”’,E. Zevgolatakos’,0. Zhangb~~.N.I. Zimin~,M. Zilo R.
Zuberi’’~’,
R. ZukanovichFunchal~,G. Zumerle”’, J. Zuniga”~
“ Ames Laboratory and Departntentof J’/itsics, 10/ta ,Stcilr’
LJ,i,e/sitY,Ames Il 50011, US.1
b P/tistcsDepartment, (nit. Imistelhng -imtttrerpen,
Umtirersiteitsp/ein I, B-26 /0 14//ni/k, Belgium
and huE, LLB- 1 LB Plein/aan 2, B- /050 Brussels, Belgium, and
l’actilte desSciences,(jim.dc If/tat Moos,Ar. Maist,iau 19,
8-70001lons, Belgium
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RAPID COMMUNICATION
6 DELPhI Collaboration / Z0 decaysea/c/I
Physics Laboratory, Lm,iversit~’ofAtliens,SolonosSt,. 104,
GR-10680 Athens,Greeced DepartmentofJ’hysics, University of
Bergen,Allégaten55, N-5007 Bergen, Norway
Dipartimento di Fisica, LTnirersith di Bolognaand INFN, Via
Irnerio 46, 1-40126 Bologna, Italy
CollegedeFrance, Lab.
dePhysiqueCorpusculaire,1N2P3-CNRS,F-75231Paris Cedex05, France
g C’L’Rtvi ChI-1211 Geneva23, Switzerlandh
CentredeRechercheNuclcaire, 1N2P3 - CNRS/ULP- BP2O,
F-67037Strasboto’gCedex,France
/ Institute ofNuclear Physics, N.C.S.R.De,nokritos, P.O. Box
60228, GR-15310At/tens, GreeceJ FZU, Inst. ofPhysicsof the (‘AS.
Il/gb EnergyPhysicsDivision, Na Slovance2, CS-18040
Prague 8, Czechoslovakiak Dipartimnento di Fisica, Universith di
Genovaand JNFN, Via Dodecaneso33, 1-16146
Genova,ItalyInstitut desSciencesNuclCaires,
1N2P3-CNRS,UniversitC de Grenoble 1, F-38026
Grenoble, FianceReseam’chInstitute for High Energy Physics,SEFT,
Siltavuorenpenger20 C, SF00170
Helsinki, Finland“ Joint Institute for Nuclear
Research,Dubna,HeadPost Office, P.O. Box 79, 10/ 000
Moscow, RussianFederationInstitut für
ExperimmientelleKernphysik, Universität Karlsruhe, Postfach6980,
D-7500
Karlsruhe 1, Germany~ High EnergyPhysics Laboratory, Institute
ofNuclear Physics, UI. Kaivior,’ 26 a, PL-30055
Krakow 30, Poland“ Centro Brasileiro dc PesquisasFisicas, ma
Xarier Sigaud /50, RJ-22290Rio deJaneiro, Brazil
(~,u,’e,’sitCde Panis-Sud,Lab. de l’Acchlhrateur LinCaim’e,
1N2P3-CNRS,Bat 200, F-91405Orsa,’, France
S Schoolof Physicsand Materials, Universityof Lancaster,
LancasterLA] 4YB, UKLIP, 1ST, FCUL - .4v. Elias Game/a, 14 - 10,
P-1000Lisboa (‘odex, Portugal
Departmentof Physics,University of Liverpool, P.O. Box /47,
Liverpool L69 3BX, UKV LPNHE, 1N2P3-CNRS,UniversitésParis 1’I et
VII, Tour 33 (RdC), 4 placeJussieut,F- 75252
Paris Cedex05, FranceDepartmentof Physics,University of Lund,
So”l~’egatamt14, S-22363Lund, Sit’eden
UniversitC ClaudeBerna,’d deLyon, IPNL, 1N2P3-CNRS,F-69622 I
‘illeurbanneC’edex, FranceY UniversidadComplutense,Avda.
Coinplutenses/n, E-28040 Madrid, Spain
Unit’. dl4ix - Marseille II - CPP, 1N2P3-CNRS,F-13288 Marseille
Cedex09, Franceaa Dipartimmiento di Fisica, Università di Milano
and INFN, I ‘ia Celoria /6, 1-20/33Milan, Italy
ab Niels Bohr Institute, Blegdamnsrej17, DK-2100 C’openhagen0,
Den,miarkac NC, Nuclear Centreof ‘tIFF, Charles University, Am’eal
MFF, I’ Holesorickach2, CS-180 00,
Prague 8, Czechoslovakiaad NIKHEF-H, Postbus41882, NL-1009 DB
Amsterdam,The Nethem’lands
ae National TechnicalUniversity, Physics Department,Zogm’afou
Campus, GR-15773
Athens,Greecead PhysicsDepartment,University of Oslo, Blinder,,,
N- /000 Oslo 3, Aoru’ai’ag Dpto. Fisica, Univ. Oviedo,
C’/P.JimenezCasas,S/N-33006Ot’iedo, Spain
ah Departmentof Physics,University of Oxford, Keble Road,
OxfordOX/ 3R11, UKai Dipartimnento di Fisica, Universith di Padova
and INFN, ‘ia Mar:olo 8, 1-35131Padua, Italy
“~Depto. deFisica, Pont/f/cia Univ. Cathlica, C.P. 38071
RJ-22453Rio de Janeiro, Brazilak RutherfordAppletonLaboratory,
Chilton, Didcot OXJ I OQX, UK
af Dipartimento di Fisica, Uni,’ersith di RomnaII and INFN, Tor
I ‘ergata, 1-00173Rome,Italyam Centred’Etude de Saclay,DSM/D.IPNIA,
F-9119] G/fLsum’~YvetteC’ede.v, France
~ Dipa,’timnentodi Fisica-Universith di Salem’no,
1-84100Salerno, ItalyIstituto Superioredi Sanith, 1st. Naz. di
Fisica NucI. (INFN,), Via/c ReginaElena 299, 1-00161
Rome, Italy“P C.E.A.F.M., CS/C. - Unit’. (‘antabria, Arda. los
C’astros, S/N-39006Santander,Spain
aq Inst. for Ihigh Enem’gy Physics, Serpukov
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RAPID COMMUNICATION
DELPHI Collaboration / Z°decayscare/i 7
P.O. Box 35, Protvino (Mosco/vRegion). RussianI”ederation
J. StefanInstitute and Departimientof Physics, University of
Ljumb/jana,Jamova39, SI-6 1000
Ljubljana, Slo,’enia~ Fysikum, StockholimiUniversity, Box 6730,
5’- 113 85 Stockholmmi,Siveden
a~Dipartimnento di I”isica Sperimmientale,Universith di Torino
and INFN, I ‘ia P. Giuria 1, 1-10125Turin, Italy
Dipartimmiento di Fisica, Università di Triesteand IIVFV, I ia
A. I ‘a/eGo 2, 1-34127Trieste, Italy
and Istituto di Fisica, Universith di (‘dine, 1-33100(dine,
Italy~W Departimientof RadiationSciences,University qf L~’psalaP.O.
Box 535, S-751 21
Uppsala, Sweden“~ IFIC’, I’alencia-CSIC, amid D.F..4.M.N., U. dc
Valencia,Arda. Dr. .lloliner 50, E-46100
Burjassot(Valencia,), Spainaz Institut fir
Hochenergiephysik,Osterm’. ,4kad. d. U
‘issensch.,Nikolsdorfem’gasse18. .4-1050
Vienna,Austriaba Inst. Nuclear Studiesand Uni,’e,’siti’ of U
‘arsair, (‘1. Ilo:a 69, PL-00681 Warsaw, Poland
bb Fac/ibereichPhysik, Universityof 14’ttppei’tal, Post/a~’li/00
127, D-5600 U’uppertal 1, Germany
Received26 May 1993(Revised10 June 1993)
Acceptedfor publication 15 June 1993
Basedon a sampleequivalentto 365000 hadronicZ0 decays,the
searchin DELPHIdatafor pairs of leptonsaccompaniedby a pair of
chargedparticles is described.A totalof 11 eventswere found in the
electronchannel,9 in the muon channeland 7 in the
tauchannel.Resultson leptonpairs with a radiatedphotonare also
presented.The datafromall channelsarecompatible with the
expectationsfrom standardprocesses.However, oneeventwas found in
the tau channelwith an unusually high massof the
chargedparticlepair.
1. Introduction
This article describesa study of eventsconsistingof a lepton
pair and onepair of chargedparticles,recordedin the DELPHI
detectorat LEP. Theleptonpair, f+Fy, may be e+e, ~ or i~r. The
otherpair of particles,hereafter
called V, canbe eithera lepton pair or a hadronpair. Eventsof
this type canbeproducedthrough second-orderQEDprocesses,whenan
off-mass-shellphotonfrom initial or
final-statebremsstrahlungmaterializesinto a pair of
fermions.Calculationsof ratesanddifferential distributions
havebeenmadein ref. [1].Thesesimple final statesare of interest for
several reasons.They allow one
to extendthe precisiontestsof QED from the final stateswith a
real photonradiatedto the case where a virtual photon is
produced.Also, although theyare rather rare, they can be a
significant backgroundin searchesfor the Higgsbosons[2j. Finally,
an anomalyin the productionrateor distributionof eventscould be a
signalof new physicsbeyond the standardmodel. A
possiblesmallexcessin the ~ channelwas suggestedin 1990 by the AMY
collabo-ration [3j, working in the TRISTAN/KEK accelerator,and
later the ALEPH
collaboration [4] reportedan excessof t~rV events.In July 1991
the Markil
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RAPID COMMUNICATION
8 DELPhI Col/ahom’ation/ L° deca,’search
[5] collaborationusing data taken at PEP/SLACand the OPAL [61
and theDELPHI [71collaborationsusingdatafrom LEP/CERN all quotedno
discrep-ancieswith the standardmodel. More recently,the AMY
collaborationreportedthat with the increasein statistics,their
dataare not significantly different fromexpectations[8].
The dataanalyzedhere wererecordedby DELPHI during 1990 and 1991
in
a samplecorrespondingto 365000 hadronicZ°decays.The
eventsweretakenat centreof massenergiesbetween88.2and 94.2GeV with
74% at the peakofthe Z°crosssection~the integratedluminosity was
15.7pb’.
The organizationof the paperis the following: in sect. 2 a
descriptionof the
essentialfeaturesof the DELPHI detectoris given. In sect. 3 the
Monte-Carlosimulationis presented.The eventselectionis explainedin
detail in sect.4 andthe eventidentification is describedin sect. 5.
In sect. 6 theresultsaregiven, andin sect. 7 a checkwith H’y
eventsis presented.Finally conclusionsare drawnin
sect. 8.
2. Apparatus
An exhaustivedescription of the DELPHI detector can be found
elsewhere[9]. The specific propertiesrelevant to this
analysisarenow described.
Chargedparticle tracksare measuredin the 1.2 T magneticfield by
threecylindrical trackingchambers:the Inner Detector(ID)
coverspolaranglesfrom30°to 150°at radii 12 to 28 cm, theTime
ProjectionChamber(TPC), the maintrackingdevice,coverspolar
anglesfrom 20°to 1600 and radii between35 and111 cm andthe Outer
Detector (OD) coverspolar anglesfrom 43°to 137°atradii between198
and 206 cm.
The Microvertex Detector(VD) consistsof threecylindersmadeof two
inde-pendenthalf-shellsinsertedbetweenthe beampipe andthe ID.
Eachhalf-shell
containscoaxiallayersof microstrip detectorslocateda radii 6.3
(only for 1991data), 9 and 11 cm. They measurethe
transversecoordinatesandcover polaranglesfrom 43°to 137°.The VD
wasusedin the analysisafter the eventselec-tion to checkon
possiblephotonconversionsoccurringbetweenthe VD and theTPC
sensitiveregions.
The energy of the photons and electrons is measured by the High
DensityProjection Chamber (HPC) and by the
ForwardElectromagneticCalorimeter(FEMC). The HPC has nine layersof
lead andgas coveringpolar anglesfrom43°to 137°andradii between208
and260 cm.The FEMC hasleadglassblockscoveringpolar anglesfrom 10°to
36°andfrom 144°to 170°.
Hadronshowerenergiesaremeasuredby combiningthe
measurementsfromthe Hadron Calorimeter (the instrumentediron return
yoke for the magnet),
coveringpolar anglesfrom 10°to 170°,andthe
electromagneticcalorimeter.
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RAPID COMMUNICATION
DELPHI (‘ollabom’atiomi / /0 decai’search 9
Muonsareidentifiedby their penetrationthroughtheyoke to theMuon
Barrel
(MUB) and Forward (MUF) chambers,which havelayersembeddedin
andoutsidethe iron yoke,andcoverpolaranglesfrom 9°to43°,from 52°to
128°andfrom 137°to 17 1°.The calorimetersalso distinguishhadronor
electromagneticshowersfrom minimum ionizing particles.
TheSmallAngleTagger(SAT) wasusedto measuretheluminosity. It
consistsof an electromagneticcalorimetercoveringpolar anglesfrom 43
to 1 35 mrad.
3. The simulationof known physicsprocesses
At the Z°pole the main processescontributingto thef~fV
signalarethe Z°decaysinto leptonsandthei-channel
photonexchange,bothwith virtual photonproductionproviding the other
pair of chargedparticles.There is a small con-tribution to the
signal from multiperipheraland conversion-typediagrams[1].
The generationof the l+8V eventsusesa Monte-Carloprogramthat is
de-
rived from the onedescribedin ref. [1], but with the
Z°diagramsaddedto thephotonones.The electromagneticcoupling
constantis assumedequalto 1/128
for the Z°propagatorand 1 / 137 for
thephotonpropagator.Thisgeneratorpro-gramdoesnot include initial
radiation, hencethe crosssectionsobtainedweremultiplied by a
correctionfactor dependingon the energy in thecentreof mass
(0.74 at the Z°peak).The programwasmodified to accountfor final
statescontaininga pion pair.
This wasdone by generatingC +[/L+/1 events,changingthe muonpair
in apion pair, andthenweighting eachof them accordingto thetimelike
pion formfactorasmeasuredin the ratio of~~rvm_productionto /1+u~
production [10] inlow-energye+e annihilations.This factor,which
wasassumedto be zeroabovea V massof 1.6 GeV, takes into account
both the resonantmassdependencearoundthe p regionandthe different
angulardecaydistributionsfor pion andmuonpairs.Final
statescontainingotherhadronpairshaveacontribution one
orderof magnitudesmaller [11] andwere not simulated.A total of
500 events(about 250 pb~)weregeneratedfor eachfinal state,
requestingpair invariant massgreaterthan 50 MeV/c2 andpolar
anglegreaterthan 15°(except for the i~tV channel).They
weresubmittedto the full de-
tectorsimulation [12] allowing for all decaysand
interactions,andthe resultswere processedby the
samereconstructionand analysis programsas the rawdatafrom LEP.
In addition to the simulatedC +1_V events,other sets of
generatedeventswere also passedthrough the detectorsimulation,
reconstructionand analysisprogramsin orderto
estimatethebackground.Thedatasamplesusedcomprised20 pb’ of
simulated Bhabhaevents (etc -~ e~e(;’)) from the programBABAMC
[13], 35 pb’ of simulatedMuon events (eke jr~u(y)) from
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10 DELPHI Collaboration / Z°decaysearch
DYMU3 [14], 50 pb~of simulatedTau events (eke r~t~)’))
fromKORALZ [15] and 25 pb~of simulatedhadronicevents (e~e~
q~j(~’))from JETSET7.3[16].
4. Event selection
Thechargedparticlemultiplicity distinguishesC ~CV eventsfrom
mostdilep-tonandhadronicfinal states.Candidateeventsarerequiredto
havefourcharged
particlesor, to accepty+z’V eventsin which one of the r
decaysinto threechargedpions,six
chargedparticles.Backgroundeventscomefrom t~t eventswith
3-prongdecay of oneor both of the t’s, C + C ~‘ eventswherethe
photon
converts,and hadronicjets at small anglesto the beam,where the
efficiencyfor reconstructingparticlesis low. In order to
reducethesebackgrounds,thecharacteristictopology of C’CV eventsis
exploited, favoring a quasi 3-bodyconfiguration, andwith a wide
range in the invariant mass of the CV system(constrainedin the i~r
events)or of the V pair (peakedat very low massesin the
contributionfrom realphotonconversions).
In the following subsectionsthe successiveselectioncriteria will
bedescribedin detail.
4.1. SELECTION OF CHARGED PARTICLES
Charged particles must come from the interaction region within 6
cm along thebeamdirectionand within 3 cm in the transverseplane (4
cm for polar angleslower than 40°or greaterthan 140°).They
shouldhavea polar anglebetween
25°and 155°,andmomentumgreaterthan0.2 GeV/c. Eventsmust
havefouror six chargedparticleswith zero total charge.If therewere
only four charged
particles,up to two additionalchargedparticlesnotcomingfrom
theinteractionregionwereallowed;thesemay be theresult of an
associatedphotonconversion.To removephoton-photoninteractionsthe
sumof the absolutemomentaof the
chargedparticlesmust beabove 15 GeV (seefig. 1) andtheremustbe
at leasttwo jets. Thejet topologywasdeterminedusingthejet
clusteralgorithm JADE[16] with default parameterYcut = 0.05.
At this first levelabout 4100 eventswere kept.
4.2. SUPPRESSIONOF r~v BACKGROUND
The backgroundfrom t~i eventscan be much reducedby applying a
cuton the lowest invariant massof all possibletriplets of
particles:in a perfectlyreconstructedtau decay, this mass should
not exceed 1.7 GeV, while for thesignalno suchconstraintis
present.For the samplewith four chargedparticlesa
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0
DELPHI500
LI) IH-z[0
4QQ
300-
200-
10: ilO 20 310 40 510 60 70 810 910 100
Sum of Momenta (GeV/c)
Fig. 1. Sum of the momentaof the chargedparticles.
minimal massof 2.5 GeV/c2wasimposedon all combinationsof
threechargedparticles.For the samplewith six chargedparticles,all
possiblecombinationsof threechargedparticleswith total charge+ 1
and invariant masslower than1.7 GeV/c2werecounted.Eventswith
oneandonly onesuchcombination(atdecaycandidate)andan
invariantmassof the otherthreeparticlesgreaterthan2.5 0eV/c2
wereretained.The acceptedparticlecombinationwasreplacedbyits total
momentum,reducingthis caseto four chargedparticles.
Fig. 2 shows that the triplet massspectrumis dominatedup to 1.6
GeV/c2by the x into 3m contribution.The reliability of the Monte
Carlo simulationofthe 3m massdistribution wasverified
usingselecteddecaysof the Z°into t~r[17]. It canbe seenthat the 2.5
GeV/c2cut removesmostof this background;
the contaminationremainingin the final samplewasestimatedto be
1.6 ±0.5events.This contributioncomesfrom eventsin whichoneof the
chargedpionshassuffereda nuclearinteraction in the
materialbetweenthe decayvertexandtheTPC, resultingin
abadlyreconstructedr mass.Above 1.6 GeV/c2,theexcess
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12 DELPIII Collahoratiomi / /0 decayseai’cli
~000 DELPHI
U
>Solid — DATA
~ 800 -d Dashed — r Background MC
- Dotted —I1V Monte—Carlo*1 00if)H-z[0
600 -
400 -
200-- CL.T
0-0 2 4 6 8 10
MASS (GeV/c2)
Fig. 2. Invariant massof the threenearestchargedparticles, for
eventswith only four tracksandtotal chargezero.The solid line
representsthe data, the dashedline representsthe Monte-Carlofor the
simulation of ~ background,and the dotted line representsthe
Monte-Carlo for the
signal for a luminosity correspondingto hundredtimes the
luminosity of the datasample.
abovetheexpectedC ~CV signalis mainlydueto the backgroundfrom
hadronic
eventsand leptonic eventswith convertedphotons.After the
massselection3 16 eventsremained.
4.3. REJECTIONOF CONVERTED PHOTONS
Photonconversionsare recognizedsincethey give two
chargedparticletrackswhich havealmostthe samepolar angleandvery
small invariant mass. In thetransverseplane the circular
trajectoriesof the two tracksare almosttangent.Therefore,for every
pair of particles,the transversetrajectorieswereextrapo-
latedto
thepointwherethetangentswereparallelandthedistancebetweenthemwassmallest.The
pair wasconsidereda convertedphoton if at that point the
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140- DELPHI
U
~ 120(2
0-
DATA
80 - i/il/I//i IIV Monte—Carlo *10
60 -
40 -
20- CUT
0 - r~—~r~-1‘9—n-i r r-r~0 0.~ 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
MASS (GeV/c2)
Fig. 3. Smallestinvariantmass (assumingthe sameazimuthal
angle)of all possiblecombinationsof two chargedparticles.Thcsolid
line representsthe data,and the shadedregion
representstheMonte-Carlo for the signal for a luminosity
correspondingto ten timesthe luminosity of the data
sample.
invariant masswasbelow 85 MeV/c’2 and the distancebetweenthe
trajectorieswasless than4.5 cm. For particleswith momentumlower
than0.6 6eV/c, the
cut wasincreasedto takeinto accountmultiple
scatteringeffects.Forthe samplewith six chargedparticles,the
threeparticlesmaking the t candidatewere not
used.In fig. 3 the invariant massof thepair is
shown,comparingdataandsignalMonte-Carlosimulation (multiplied by
10). No eventsfrom the BhabhaandMuon
Monte-Carlosimulatedsamplessurvive this selection.
Thesecuts removed246 eventsandthus 70
candidateeventsremained.
4.4. REMOVAL OF HADRONIC BACKGROUND
ThepreferredtopologiesofP’~1Veventsare 1-3 (i.e. two jetswith
theV andonelepton in the samejet) and 1-1-2 (i.e. threejetswith the
V in a separatejet),
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14 DL’LPlII Collaboration / ~0 decaysearch
while hadronicbackgrounddueto lost tracksoften have2-2 and 1-3
configu-
rations.Hadronic eventsin 1-3 topologyarehighly suppressedby the
previous
triplet masscut. Theeventswith 2-2 topologywere rejected.With
this cut 39 ofthe 2-2 type eventswereremovedfrom the 70
candidateevents.This criterionis stablefor variationsof up to +0.03
in the .T’cut parameter.
The 39 eventsremovedhadmoreextra-tracks,that is tracksnot
extrapolating
to the interactionregion, and very few identified
leptons;indeed, only 6 outof the 31 remainingcandidateshad extra
tracks, while 23 out of the 39 2-2eventshadat leastone extratrack,
andonly 4 out of the 39 2-2 eventshadanidentifiedlepton (3
candidatemuonswith momentumgreaterthan 5 6eV/c,and1
candidateelectronwith energygreaterthan 5 GeV/c).
Theseresultsconfirm
that the rejectedsamplemostly containedq~events.The quasi 3-body
configuration of the 1+1—V eventswas usedas a further
selection,to eliminate hadroniceventswith
undetectedparticles.After mergingthetwo particlesthat maketheV into
a singleparticle,theresultingconfigurationshould be planar,evenfor
t+rV events,sincein this casethe chargeddecayproductsfollow closely
the direction of the tau. Thereforethe sumof its three
internal anglesshould be approximately360°.Rejectingthe
eventsfor whichthereis no combination of two
oppositelychargedparticlesfor the V giving asumof anglesgreaterthan
357°,2 out of the 31 candidateswereremoved.Theefficiencyof this cut
is high for thesignal: 99%for thee~e V andjt ~jiV eventsand 85%for
the t4tV events.
Finally, in hadronicevents,largeamountsof
hadronicenergynotassociatedto
chargedparticlescanbe expected,dueto
neutralparticlesandchargedparticleslost in the forward region,
while for the ~ eventsthis is not the case:99%of the
simulatede~eVandp~j~Veventsand95% of the simulatedz~yVeventsdeposit
less than 10 6eV in the hadronic
calorimeterunassociatedtochargedparticles.Thereforetwo
candidateswhichhad depositsof unassociatedhadronicenergygreaterthan
10 GeV wereremoved.
After all the selections,27 candidateeventswereretained,which
constituted
the final sample.Applying this analysisto
thesimulatedq~backgroundsample,1.8+ 1.0events
survive the cuts.However, as the q~Monte-Carlo programsare not
well tested in the low
chargedmultiplicity domain,an estimateofthe
hadronicbackgroundwasmadefrom the data.In this estimatethe
eventswith non zero total chargewere used,sincethe
incompletehadroniceventsdo not necessarilyhavezero total charge.In
fact,before thetopologycut, wherethe
othermainbackgroundswerealreadyhighly suppressed,39
zerototalchargeeventsand32 nonzerototal chargeeventswerefound with
topology2-2. Removingthe chargeconservationcut, two non-zero
totalchargeeventssurviving all the othercuts
werefound.However,afterinspection,oneof thesetwo
eventswasidentified as aC+1V eventwhere there
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DELPHI Collaboration / ~0 decayscald/i 15
wasa wrong chargeidentification in oneof the tracks(0.7
eventsareexpectedfrom the CCV Monte-Carlo).Therefore,accordingto
the ratio of the hadronicsamplesin the zero and non zero total
chargeevents,the remaininghadronicbackgroundin the
candidatesampleis estimatedto be 1 .2 + 1.2 events.
This is in good agreementwith the aboveMonte Carlo estimate;
taking theweightedaverageonegets 1.6 ±0.8 events.
5. Identification of events
The 27 candidateeventswereclassifiedas t+TV, e+eV orp~uV,
accord-
ing to the multiplicity, the total measuredenergy,andthe lepton
identification.To definethe V, eachpair of
oppositelychargedparticleswasmergedinto a
singleone.If theresulting3-particleconfigurationhadthe sumof its
threeinter-nal angleshigher than357°,the pair waschosenas the V
candidate.Whenmorethan one such pairexisted,and if the two
particlescomposingthe V werenotidentifiedas leptonsof different
flavour, theonewith thelowest masswastaken.
5.1. IDENTIFICATION OF r~rV EVENTS
Events with 6 chargedparticles, or eventswith four
chargedparticlesand
sumof their absolutemomentalowerthan 65
GeV/candtotalelectromagneticenergybelow 65 GeV, wereconsideredT+rV
events.This cut retains 86% ofthe simulatedr+1V events,while the
contaminationfrom simulatede+eVor p4 jrV eventsis 1%. Therewere7
eventsclassifiedas
All eventsbut one containedat leastoneidentified electronor
muon, a con-
dition which was not imposedon the selectionand which confirms
that thecandidateeventsarenot significantly dominatedby
hadronicbackground.Thespreadin mass of the lowest masstriplet of
chargedparticle tracksand thefact that at least one particle of
this triplet was identified as a lepton in 5out of the 7
r+z~Vevents, also excludesany significant contaminationdueto poorly
reconstructedt~r decayinginto 3m. The total backgroundto
thischannel is estimated,as describedin the previoussections, to be
1.6 ±0.5eventsfrom t~t backgroundand 1.6+ 0.8 eventsfrom
hadronicbackground.
5.2. IDENTIFICATION OF e~eVAND1G1iV EVENTS
The remaining20 eventswere classifiedase+e_V or ~14jrV
events.
Out of these20 events, 11 events were identified as e+e—V by
demandingthat one of the leptons (not making the V) had
Eem/pbetween0.6 and 1.7,
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16
U
V ‘5’....LD0005.,) .-INOC —‘,
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00 C”— ‘f-i CC- ‘a- 0 00 C’) 0. 0. — 0. 0’ C’) C- — 0’ C”.U — U 0.
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CC”)
Lu u~~
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DELPHI Collaboration / /~ decam’scare/i 17
TABLE 2
Observednumbersof eventscomparedto the expectednumbersof
events:/ + I — V is the expectedsignal, /‘).f~ representsthe added
contribution from Bhabhaand Muon backgroundsimulated
samples.~r is the a-~r backgroundsampleand q0 is the
hadronicbackgroundsample.
Channel Data Monte-CarloExpectations
Signal Backgroundl~I’ 1990 1991 Total I/V II:’ q~e~e 0 11 11
9.5±1.1 0 0 0
3 6 9 9.3±0.9 0 0 02 5 7 3.9 ±0.4 0 1.6 0.5 1.6 ±0.8
Total 5 22 27 22.7± 1.4 0 1.6±0.5 1.6 ±0.8
TABLE 3
Distribution of the candidatesaccordingto the V
classification,as describedin the text.
l~l’ V = cc V = ~ V = mo V = ~u,m7r V =? Total
e~e 2 5 0 3 1 115 I 1 2 — 93 3 I — — 7
Total 10 9 2 5 1 27
where Eem was the energy measuredin the
electromagneticcalorimeterandpthe measuredmomentum.
Of the remaining9 events,7 hadhits in the
muonchambersassociatedto oneof the chargedparttclesnot making the
V, andwere classifiedas JS1LV events.Of the other two events,onehad
both leptonswith Eern/p below 0.1 andEhad/Pbelow 0.1 (Ehad is the
energymeasuredin the hadroniccalorimeterassociatedto the
particle)and sowasalso classifiedas ~i1iV event.The
othereventhadoneleptonwith Ehad/Pbelow 0.1 andE~~/p=
5.9andtheotherlepton with Ehad/PandEem/p below 0.1. It wasassumedto
be a ,L/C’V eventwhere the radiatedphotonwascollinearwith oneof the
muons.
5.3. CLASSIFICATION OF THE V
If a particle making the V wasenergetic,it was classifiedusing
the electro-magneticcalorimeterandmuonchambersinformation. Lower
energyparticleswereidentified usingtheenergyperunit
track-lengthdepositedin the gasof theTPC (dE/dx).
In all candidateeventsexceptone,at leastoneof theparticlesof
theV wassuc-cessfullyidentified (with
someambiguitybetweenmuonsandpionsfor low mo-mentumparticles)andthe
V wascompatiblewith being a particle—antiparticlepair.
The information from the Microvertex Detectorwas used to confirm
that
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15 DELPHI Collaboration / ~0 decaysearch
86 ~ ~:“ ~5’~ ‘96ECM (GeV)
3-
b)TTV DELPHI
if)I-zUs
1-
0 86 ‘ 87 ‘‘‘88’ ‘ ‘ 89 ‘ 909192 ‘ ‘ 93 ‘ ‘ 96ECu (CeV)
Fig. 4. Distribution of the centreof massenergyfor the (a) e’)eV
and /i+/1V, and (b) trVcandidateevents,
photonconversionswereproperlyeliminatedby thecutsdescribedin
section4.3.Of the 8 V’s with massbelow 0.4 6eV/c2 falling within
the acceptanceof theVD, 7 hadassociatedhits in it, whereasmost of
the conversionsshouldtakeplaceafterthis detector.
6. Results
The propertiesof the 27 candidatesare given in table 1. The
i’esults of this
analysisare summarizedin tables2 and 3. Fig. 4 shows the
distribution of thecentreof massenergyof the events,ECM. Figs. 5—7
show the comparisonsofinvariant massesandV energybetweenthe
observeddataandthe expectationsfor the signal. The invariant mass
of the pair C~C (the leptonsnot makingthe V), Inil, wascalculatedas
the recoil massfrom the V, for all eventsexceptthe one with the V
massof 17.5 6eV/c2. For this last case,a x2 4-constraint
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DELPHI Collaboration/ ~0 decayseai’c/t 19
9—
8- DELPHI
r a)eeV+~zV
5 ±DATA
d ~ ~_ t~IIV Monte—Carla
3 - /1111/, ‘r-r+qq Bacliground MC
0 24 68 ‘ 10 ‘ 12 14 16 ~ 20
V MASS (GeV/c2)
3,5-
DELPHI~2.5 -
b)TTV2-
2,1.5-
(2H- -
~0.5~ jI L ___
0 -f’~ , I , ‘r’r’fl,1,,,~ , ‘ T’’’P’fl’
0 2 4 6 8 10 12 14 16 18 20V MASS (GeV/c
2)
Fig. 5. Distribution of the V invariantmassfor the (a) c’) eV
and ~i’~ ~i V, and (b) nV candidateevents,comparedto
Monte-Carlosimulation for the signal (line) and
background(shaded).
kinematicalfit of the four particlesmomenta(onebeingthe tau
candidate,i.e.
the vector sum of threeparticles) wasperformedto improve the
errors on thecalculatedmasses(thevaluesof the massesfor the V or
1+C havenot changedsignificantly from the previousmethods).
Within the statisticsthereis a
reasonableagreementbetweendataandMonte-
Carlo predictions.Therearehoweversmalldiscrepancieswith the
expectedbe-haviour.On fig. 4, thereis an indication of aspreadin
centreof massenergyofthe 1+1_V candidates.There is also a small
peakof 3 low V masseventsandan eventwith an unusuallyhigh V
mass(17.47 + 0.17 6eV/c2) in the r~tVchannel (fig. 5-b)). The
threeeventswith a low V masshad a centreof massenergyout of the
Z°peak,andwerecharacterizedby a very energeticV.
The high V massevent is shown in fig. 8. In this eventthe V is
composedoftwo identified muonsrecoilingagainsta 25.8+ 0.8
6eV/c2invariantmassT~Tsystem.The massresolutionfor this
eventwascalculatedfrom a ,~2 4-constraintkinematicalfit of the four
particlesmomenta.The numberof eventswith a V
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20 DELPhI Collaboration / Z0 decayscare/i
“U 6- DELPHI>
~ a)eeV~~V
~ + DATA
U IIV Monte—Carloliii. TT+qq Background MC
~ - ‘ ~3IQ ‘~~‘‘‘~1’’’’1’ 80
Ii MASS (CeV/c2)
2.5 -
__________________________________________________________
DELPHI‘-‘.~. 2->
b)orV1.5
1- - - - — -
‘2~0’ 310 ~ 6’070 8’O9~
U MASS (GeV/c2)Fig. 6. Distribution of the invariant mass of the
I ‘)/ — pair for the (a) e~eV and
1r~1i V.and (b) na-V candidateevents, comparedto Monte-Carlo
simulation for the signal (line) and
background(shaded).
massgreaterthan 10 6eV/c2andC+1 masslower than40 6eV/c2
predicted
by the C~lV Monte Carlo is 0.013 + 0.013.
7. Checks on £ty events
Anothertestof universalityin
thethreeleptonicchannelswasdonebystudying
the production of Cl;’ events.This is directly related to the
searchfor heavyexcited chargedleptonsdecayinginto two normal
leptonsplus a photon, ananalysisalreadypublishedby DELPHI [181.
This analysisalso correspondstoa total of 365000 hadronicevents.The
eventselectionrequestedtwo chargedparticleswith polar
anglesbetween25°and 155°plus a photonwith energyabove 2 GeV and
with an isolation anglewith respectto the nearestchargedparticle
larger than 30°.Table 4 gives the numberof events selectedfor
each
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DELPIII C’ollalCoraiion / /0 dd’d’dii’ scald/i 21
—
~ 4~44- -f-DATA DELPHI11V Monte—Carlo a) eeV+~V
2 — — Ill/ill/li T’r+qq Background MC
V ENERGY (GeV)2.5-
DELPHI2- -- b)TTV
--
0 ~ ‘ ‘ ‘ 20 25’ 31Q ~ ~ ‘41Q’ 45 50
V ENERGY (CeV)
Fig. 7. Distribution of theenergyof the V for the (a)e’)eV
and1’~jiV, and (b) na-V candidate
events,comparedto Monte-Carlo simulationfor the signal (line)
and background(shaded).
channeltogetherwith the Monte Carlo expectationsfrom
standardZ°decays;for this analysisthe selectionon the centreof
massenergythat wasused in ref.
11181 wasremoved.Table 5 givesthe result of a modifiedstudy in
which the minimum isolation
angle for the photonwasreducedIn 15°,thereforecoveringan
angularregionsimilar to that of the/tV analysis.
As canbe seenin the tables,the numberof eventsfound is in
agreementwiththe standardMonte Carlo predictions.
8. Summary and conclusion
A searchfor C~CVeventswith the DELPHI detector was performed on
asamplecorrespondingto 365 000 hadronicZ°decays.The results
indicate noanomalousbehaviourin any of the threechannels.e+eV,
/1+/rV andy+y~~
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22 DELPHI Collaboration / Z0 decayscare/i
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En’ 3370 C3~:CO 0 3 0 0 0 0Pro.. 28-00C-C99C • ~ 24-Jo,-C992
(CC) (8) (0) (CC) (‘I) (0) 10
Fig. 8. Display of the a-’~a-Veventwith highermassfor the V
(17.5±0.2GeV/c2). The particlescomposingthe V appearon the
left-handside,giving hits in the outerlayersof the
muonchambers,classifyingthe particlesasmuons.On the right-handside,
a clusterof threechargedparticle trackswith invariant masslower
than 1.7 GeV/c2, and a well-separatedtrack with
associatedhadronicenergyare seen, identifying the event as a-+a-V.
The energyin the centreof mass is 89.5 GeV.the massof the V is
17.47±0.17 GeV/d’2, and the massof the a-~a-— system is 25.8±0.8
GeV/c2(the massof the systemrecoiling from the V).
Othercharacteristicscan be found in table 1 (1991
event 28271/3370).
The final sampleshowsa small excessin the low V massregionfor
they+1Vchannel,althoughof no strongstatisticalsignificance.An
eventwasfound withan unusuallyhigh V massof 17.47±0.17 6eV/c2
composedof two identifiedmuons,recoiling from a r systemwith
invariant massof 25.8+ 0.8 6eV/c2.
An independentteston universality of the threelepton channelson
l~C~
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DELPHI Collaboration/ Z°decayscare/i 23
TABLE 4
Number of II’;’ eventsfound for each channel,and numberof
eventsexpectedfrom standardprocesses(photon isolation angle
greaterthan 30°).
Channel Observed Expected
eey 166 169 ± 13123 126 ± 10
a-ny 75 72+6
TABLE 5
Numberof fly eventsfound when the minimum photon isolation
angleO~is reducedto 15°,andnumberof eventsexpectedfrom
standardprocesses.
15° 15°
Channel Observed Expected Observed Expectedeey 23 24+3 189
193+15
29 18±2 152 144+11a-ny 10 Il + 1.5 85 83+7
events,showsa good agreementbetweendataandexpectations.
We would like to thank R. Kleiss for helpful discussionsand
J.Hilgart forproviding an improvedversionof the codefor the1+1—V
model.Wearegreatlyindebtedto our technical collaboratorsand to the
funding agenciesfor theirsupportin building andoperatingthe DELPHI
detector,andto the membersofthe CERN-SLDivision for the
excellentperformanceof the LEP collider.
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