search for single top production via fcnc at lep at 189 ... · physics letters b 590 (2004) 21–34...

b

Physics Letters B 590 (2004) 21–34

www.elsevier.com/locate/physlet

Search for single top production via FCNC at LEPat

√s = 189–208 GeV

DELPHI Collaboration

J. Abdallahac, P. Abreuz, W. Adambf, P. Adzico, T. Albrechtu, T. Alderweireldb,c,d,R. Alemany-Fernandezl, T. Allmendingeru, P.P. Allportaa, U. Amaldiag,

N. Amapaneay, S. Amatobc, E. Anashkinan, A. Andreazzaaf, S. Andringaz, N. Anjosz,P. Antilogusac, W.-D. Apelu, Y. Arnoudr, S. Askad, B. Asmanax, J.E. Augustinac,

A. Augustinusl, P. Baillonl, A. Ballestreroaz, P. Bambadex, R. Barbierae, D. Bardint,G. Barkeru, A. Baroncelliaq, M. Battaglial, M. Baubillierac, K.-H. Becksbh,

M. Begalli h,i,j , A. Behrmannbh, E. Ben-Haimx, N. Benekosaj, A. Benvenutig, C. Beratr,M. Berggrenac, L. Berntzonax, D. Bertrandb,c,d, M. Besanconar, N. Bessonar,

D. Blochm, M. Blomai, M. Bluj bg, M. Bonesiniag, M. Boonekampar, P.S.L. Boothaa,G. Borisovy, O. Botnerbd, B. Bouquetx, T.J.V. Bowcockaa, I. Boykot, M. Brackoau,av,aw,R. Brennerbd, E. Brodetam, P. Bruckmanv, J.M. Brunetk, L. Buggeak, P. Buschmannbh,

M. Calvi ag, T. Camporesil, V. Canaleap, F. Carenal, N. Castroz, F. Cavallog,M. Chapkinat, Ph. Charpentierl, P. Checchiaan, R. Chiericil , P. Chliapnikovat,J. Chudobal, S.U. Chungl, K. Cieslikv, P. Collinsl, R. Contriq, G. Cosmex,

F. Cossuttiba,bb, M.J. Costabe, B. Crawleya, D. Crennellao, J. Cuevasal, J. D’Hondtb,c,d,J. Dalmauax, T. da Silvabc, W. Da Silvaac, G. Della Riccaba,bb, A. De Angelisba,bb,W. De Boeru, C. De Clercqb,c,d, B. De Lottoba,bb, N. De Mariaay, A. De Minan,

L. de Paulabc, L. Di Ciaccioap, A. Di Simoneaq, K. Dorobabg, J. Dreesbh,l , M. Drisaj,G. Eigenf, T. Ekelofbd, M. Ellert bd, M. Elsingl, M.C. Espirito Santoz, G. Fanourakiso,D. Fassouliotiso,e, M. Feindtu, J. Fernandezas, A. Ferrerbe, F. Ferroq, U. Flagmeyerbh,H. Foethl, E. Fokitisaj, F. Fulda-Quenzerx, J. Fusterbe, M. Gandelmanbc, C. Garciabe,

Ph. Gavilletl, E. Gazisaj, R. Gokielil,bg, B. Golobau,av,aw, G. Gomez-Ceballosas,P. Goncalvesz, E. Grazianiaq, G. Grosdidierx, K. Grzelakbg, J. Guyao, C. Haagu,

A. Hallgrenbd, K. Hamacherbh, K. Hamiltonam, S. Haugak, F. Hauleru, V. Hedbergad,M. Henneckeu, H. Herrl, J. Hoffmanbg, S.-O. Holmgrenax, P.J. Holtl, M.A. Houldenaa,K. Hultqvistax, J.N. Jacksonaa, G. Jarlskogad, P. Jarryar, D. Jeansam, E.K. Johanssonax,

P.D. Johanssonax, P. Jonssonae, C. Joraml, L. Jungermannu, F. Kapustaac,S. Katsanevasae, E. Katsoufisaj, G. Kernelau,av,aw, B.P. Kersevanl,au,av,aw, U. Kerzelu,

0370-2693/$ – see front matter 2004 Published by Elsevier B.V.doi:10.1016/j.physletb.2004.03.051

http://www.elsevier.com/locate/physletb

22 DELPHI Collaboration / Physics Letters B 590 (2004) 21–34

A. Kiiskinens, B.T. Kingaa, N.J. Kjaerl, P. Kluit ai, P. Kokkiniaso, C. Kourkoumelise,O. Kouznetsovt, Z. Krumsteint, M. Kucharczykv, J. Lamsaa, G. Lederbf, F. Ledroitr,L. Leinonenax, R. Leitnerah, J. Lemonneb,c,d, V. Lepeltierx, T. Lesiakv, W. Liebigbh,

D. Liko bf, A. Lipniackaax, J.H. Lopesbc, J.M. Lopezal, D. Loukaso, P. Lutzar,L. Lyonsam, J. MacNaughtonbf, A. Malekbh, S. Maltezosaj, F. Mandlbf, J. Marcoas,R. Marcoas, B. Marechalbc, M. Margonian, J.-C. Marinl, C. Mariottil , A. Markouo,

C. Martinez-Riveroas, J. Masikp, N. Mastroyiannopouloso, F. Matorrasas,C. Matteuzziag, F. Mazzucatoan, M. Mazzucatoan, R. McNultyaa, C. Meroniaf,

W.T. Meyera, E. Miglioreay, W. Mitaroff bf, U. Mjoernmarkad, T. Moaax, M. Mochu,K. Moenigl,n, R. Mongeq, J. Montenegroai, D. Moraesbc, S. Morenoz, P. Morettiniq,

U. Muellerbh, K. Muenichbh, M. Muldersai, L. Mundimh,i,j , W. Murrayao, B. Murynw,G. Myattam, T. Myklebustak, M. Nassiakouo, F. Navarriag, K. Nawrockibg,

R. Nicolaidouar, M. Nikolenkot,m, A. Oblakowska-Muchaw, V. Obraztsovat,A. Olshevskit, A. Onofrez, R. Oravas, K. Osterbergs, A. Ouraouar, A. Oyangurenbe,

M. Paganoniag, S. Paianog, J.P. Palaciosaa, H. Palkav, Th.D. Papadopoulouaj, L. Papel,C. Parkesab, F. Parodiq, U. Parzefalll, A. Passeriaq, O. Passonbh, L. Peraltaz,

V. Perepelitsabe, A. Perrottag, A. Petroliniq, J. Piedraas, L. Pieriaq, F. Pierrear,M. Pimentaz, E. Piottol, T. Podobnikau,av,aw, V. Poireaul, M.E. Polh,i,j , G. Polokv,V. Pozdniakovt, N. Pukhaevab,c,d,t, A. Pulliaag, J. Ramesp, L. Ramleru, A. Readak,

P. Rebecchil, J. Rehnu, D. Reidai, R. Reinhardtbh, P. Rentonam, F. Richardx, J. Ridkyp,M. Riveroas, D. Rodriguezas, A. Romeroay, P. Ronchesean, E. Rosenberga,

P. Roudeaux, T. Rovellig, V. Ruhlmann-Kleiderar, D. Ryabtchikovat, A. Sadovskyt,L. Salmis, J. Saltbe, A. Savoy-Navarroac, U. Schwickerathl, A. Segaram, R. Sekulinao,M. Siebelbh, A. Sisakiant, G. Smadjaae, O. Smirnovaad, A. Sokolovat, A. Sopczaky,

R. Sosnowskibg, T. Spassovl, M. Stanitzkiu, A. Stocchix, J. Straussbf, B. Stuguf,M. Szczekowskibg, M. Szeptyckabg, T. Szumlakw, T. Tabarelliag, A.C. Taffardaa,

F. Tegenfeldtbd, J. Timmermansai, L. Tkatchevt, M. Tobinaa, S. Todorovovap,B. Tomez, A. Tonazzoag, P. Tortosabe, P. Travnicekp, D. Treillel, G. Tristramk,M. Trochimczukbg, C. Tronconaf, M.-L. Turluerar, I.A. Tyapkint, P. Tyapkint,

S. Tzamariaso, V. Uvarovat, G. Valentig, P. Van Damai, J. Van Eldikl,A. Van Lysebettenb,c,d, N. van Remortelb,c,d, I. Van Vulpenl, G. Vegniaf, F. Velosoz,

W. Venusao, P. Verdierae, V. Verzi ap, D. Vilanovaar, L. Vitaleba,bb, V. Vrbap,H. Wahlenbh, A.J. Washbrookaa, C. Weiseru, D. Wickel, J. Wickensb,c,d,

G. Wilkinsonam, M. Winterm, M. Witekv, O. Yushchenkoat, A. Zalewskav,P. Zalewskibg, D. Zavrtanikau,av,aw, V. Zhuravlovt, N.I. Zimin t, A. Zintchenkot,

M. Zupano

a Department of Physics and Astronomy, Iowa State University, Ames, IA 50011-3160, USAb Physics Department, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium

c IIHE, ULB-VUB, Pleinlaan 2, B-1050 Brussels, Belgium

DELPHI Collaboration / Physics Letters B 590 (2004) 21–34 23

d Faculté des Sciences, Université de l’Etat Mons, Av. Maistriau 19, B-7000 Mons, Belgiume Physics Laboratory, University of Athens, Solonos Str. 104, GR-10680 Athens, Greecef Department of Physics, University of Bergen, Allégaten 55, NO-5007 Bergen, Norway

g Dipartimento di Fisica, Università di Bologna and INFN, Via Irnerio 46, IT-40126 Bologna, Italyh Centro Brasileiro de Pesquisas Físicas, rua Xavier Sigaud 150, BR-22290 Rio de Janeiro, Brazil

i Departamento de Física, Pont. Universidad Católica, C.P. 38071, BR-22453 Rio de Janeiro, Brazilj Institute de Física, Universidad Estadual do Rio de Janeiro, rua São Francisco Xavier 524, Rio de Janeiro, Brazil

k Collège de France, Laboratoire de Physique Corpusculaire, IN2P3-CNRS, FR-75231 Paris cedex 05, Francel CERN, CH-1211 Geneva 23, Switzerland

m Institut de Recherches Subatomiques, IN2P3-CNRS/ULP-BP20, FR-67037 Strasbourg cedex, Francen Now at DESY-Zeuthen, Platanenallee 6, D-15735 Zeuthen, Germany

o Institute of Nuclear Physics, N.C.S.R. Demokritos, P.O. Box 60228, GR-15310 Athens, Greecep FZU, Institute of Physics of the C.A.S. High Energy Physics Division, Na Slovance 2, CZ-180 40, Praha 8, Czech Republic

q Dipartimento di Fisica, Università di Genova and INFN, Via Dodecaneso 33, IT-16146 Genova, Italyr Institut des Sciences Nucléaires, IN2P3-CNRS, Université de Grenoble 1, FR-38026 Grenoble cedex, France

s Helsinki Institute of Physics, P.O. Box 64, FIN-00014 University of Helsinki, Finlandt Joint Institute for Nuclear Research, Dubna, Head Post Office, P.O. Box 79, RU-101 000 Moscow, Russian Federation

u Institut für Experimentelle Kernphysik, Universität Karlsruhe, Postfach 6980, DE-76128 Karlsruhe, Germanyv Institute of Nuclear Physics, Ul. Kawiory 26a, PL-30055 Krakow, Poland

w Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, PL-30055 Krakow, Polandx Université de Paris-Sud, Laboratoire de l’Accélérateur Linéaire, IN2P3-CNRS, Bât. 200, FR-91405 Orsay cedex, France

y School of Physics and Chemistry, University of Lancaster, Lancaster LA1 4YB, UKz LIP, IST, FCUL-Av. Elias Garcia, 14-1◦ , PT-1000 Lisboa codex, Portugal

aaDepartment of Physics, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UKab Department of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK

ac LPNHE, IN2P3-CNRS, Université de Paris VI et VII, Tour 33 (RdC), 4 place Jussieu, FR-75252 Paris cedex 05, Francead Department of Physics, University of Lund, Sölvegatan 14, SE-223 63 Lund, Sweden

aeUniversité Claude Bernard de Lyon, IPNL, IN2P3-CNRS, FR-69622 Villeurbanne cedex, Franceaf Dipartimento di Fisica, Università di Milano and INFN-MILANO, Via Celoria 16, IT-20133 Milan, Italy

ag Dipartimento di Fisica, Università di Milano-Bicocca and INFN-MILANO, Piazza della Scienza 2, IT-20126 Milan, Italyah IPNP of MFF, Charles University, Areal MFF, V Holesovickach 2, CZ-180 00, Praha 8, Czech Republic

ai NIKHEF, Postbus 41882, NL-1009 DB Amsterdam, The Netherlandsaj National Technical University, Physics Department, Zografou Campus, GR-15773 Athens, Greece

ak Physics Department, University of Oslo, Blindern, NO-0316 Oslo, Norwayal Departamento de la Fisica, Universidad Oviedo, Avda. Calvo Sotelo s/n, ES-33007 Oviedo, Spain

am Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UKan Dipartimento di Fisica, Università di Padova and INFN, Via Marzolo 8, IT-35131 Padua, Italy

ao Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, UKap Dipartimento di Fisica, Università di Roma II and INFN, Tor Vergata, IT-00173 Rome, Italy

aq Dipartimento di Fisica, Università di Roma III and INFN, Via della Vasca Navale 84, IT-00146 Rome, Italyar DAPNIA/Service de Physique des Particules, CEA-Saclay, FR-91191 Gif-sur-Yvette cedex, France

as Instituto de Fisica de Cantabria (CSIC-UC), Avda. los Castros s/n, ES-39006 Santander, Spainat Institute for High Energy Physics, Serpukov, P.O. Box 35, Protvino (Moscow Region), Russian Federation

au J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Sloveniaav Laboratory for Astroparticle Physics, Nova Gorica Polytechnic, Kostanjeviska 16a, SI-5000 Nova Gorica, Slovenia

aw Department of Physics, University ofLjubljana, SI-1000 Ljubljana, Sloveniaax Fysikum, Stockholm University, Box 6730, SE-113 85 Stockholm, Sweden

ay Dipartimento di Fisica Sperimentale, Università di Torino and INFN, Via P. Giuria 1, IT-10125 Turin, Italyaz INFN, Sezione di Torino, and Dipartimento di Fisica Teorica, Università di Torino, Via P. Giuria 1, IT-10125 Turin, Italy

ba Dipartimento di Fisica, Università di Trieste and INFN, Via A. Valerio 2, IT-34127 Trieste, Italybb Istituto di Fisica, Università di Udine, IT-33100 Udine, Italy

bc Universidad Federal do Rio de Janeiro, C.P. 68528, Cidade Universidad, Ilha do Fundão, BR-21945-970 Rio de Janeiro, Brazilbd Department of Radiation Sciences, University of Uppsala, P.O. Box 535, SE-751 21 Uppsala, Sweden

be IFIC, Valencia-CSIC, and D.F.A.M.N., U. de Valencia,Avda. Dr. Moliner 50, ES-46100 Burjassot (Valencia), Spainbf Institut für Hochenergiephysik, Österr. Akad. d. Wissensch., Nikolsdorfergasse 18, AT-1050 Vienna, Austria

bg Institute Nuclear Studies and University of Warsaw, Ul. Hoza 69, PL-00681 Warsaw, Polandbh Fachbereich Physik, University of Wuppertal, Postfach 100 127, DE-42097 Wuppertal, Germany


datag from 189

Received 6 October 2003; accepted 4 March 2004

Available online 28 April 2004

Editor: M. Doser

Abstract

A search for single top production (e+e− → t c) via flavour changing neutral currents (FCNC) was performed using thetaken by the DELPHI detector at LEP2. The data analyzed have been accumulated at center-of-mass energies ranginto 208 GeV. Limits at 95% confidence level were obtained on the anomalous coupling parametersκγ andκZ . 2004 Published by Elsevier B.V.

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1. Introduction

Flavour changing neutral currents (FCNC) are hily suppressed in the Standard Model (SM) due toGlashow–Iliopoulos–Maiani (GIM) mechanism [1However, small contributions appear at one-loop le(Br(t → (γ, g,Z) + c(u)) < 10−10) due to the Cabibbo–Kobayashi–Maskawa (CKM) mixing matrix [2Many extensions of the SM, such as supersymm[3] and multi-Higgs doublet models [4], predict thpresence of FCNC already at tree level. Some spemodels [5] give rise to detectable FCNC amplitude

The most prominent signature for direct obsertion of FCNC processes at LEP is the production otop quark together with a charm or an up quark inprocesse+e− → t c.1 The strength of the transitionγ → ff ′ andZ → ff ′ can be described in terms of thLagrangian given in [6]:

(1)Γ γµ = κγ

eeq

Λσµν(g1Pl + g2Pr )q

ν,

(2)Γ Zµ = κZ

e

sin2ΘW

γµ(z1Pl + z2Pr ),

where e is the electron charge,eq the top quarkcharge,ΘW is the weak mixing angle andPl (Pr )is the left (right) handed projector. Theκγ and κZ

are the anomalous couplings to theγ andZ bosons,respectively.Λ is the new physics scale. A valuof 175 GeV was used for numerical calculatiothroughout the Letter. The relative contributions of tleft and right handed currents are determined by thgi

E-mail address:[email protected] (A. Stocchi).1 Throughout this Letter the notationt c stands fort c + t u and

includes the charge conjugate contribution as well.

andzi constants which obey the constraints:

(3)g21 + g2

2 = 1, z21 + z2

2 = 1.

In the approach which gives the most conservalimits on the couplings, the interference term, whdepends ongi and zi , gives a negative contributioto the cross-section of the processe+e− → t c. Thiscorresponds to the requirement [6]:

(4)g1z1 + g2z2 = −1.

The existence of anomalous top couplings to gabosons allows the top to decay throught → cγ andt → cZ in addition to the dominant decay modt → bW . This effect was taken into accountthe evaluation of results. Numerical estimates ofexpected number of events taking into accountlimits on anomalous vertices set by CDF Collaborat[7] can be found in [6].

This Letter is devoted to the search for FCNprocesses associated to single top production at(e+e− → t c). Limits are set on the anomalous coplings κγ andκZ in the most conservative approacThe t quark is expected to decay predominantly intoWb, giving distinct signatures for the leptonic anhadronicW decays. For each decay mode a dedicaanalysis was developed. In thesemileptonic channetwo jets and one isolated lepton (from theW lep-tonic decays,W → lνl ) were searched for. In thhadronic channelfour jets were required in the eve(two of them from theW hadronic decays,W → qq ′).A nearly background-free signature is obtained insemileptonic channel, but the branching ratio is retively low. In the hadronic channel, theW decays givean event rate about two times higher, but the baground conditions are less favourable.


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Table 1Luminosity collected by DELPHI and used in this analysis for eacenter-of-mass energy (see text for details)√

s (GeV) 189 192 196 200 202 205 207Luminosity (pb−1) 151.8 25.9 76.4 83.4 40.1 78.8 84.3

2. The DELPHI data and simulated samples

The data collected with the DELPHI detector [at

√s = 189–208 GeV, well above thet c production

threshold, were used in this analysis. The integraluminosity used for each center-of-mass energy bigiven in Table 1. The data collected in the year 20at energies up to 208 GeV are split into two enebins 205 and 207 GeV for center-of-mass energieslow and above 206 GeV, respectively. The 189, 1196, 200, 202, 205 and 207 GeV energy bins crespond to average center-of-mass energies of 18191.6, 195.5, 199.5, 201.6, 204.8 and 206.6 GeV,spectively. While for the semileptonic channel the tlast energy bins were considered separately, they wconsidered together in the hadronic channel.

The background processe+e− → Z/γ → qq(γ )

was generated with PYTHIA 6.125 [9]. Forµ+µ−(γ )

and τ+τ−(γ ), DYMU3 [10] and KORALZ 4.2 [11]were used, respectively, while the BHWIDE genetor [12] was used for Bhabha events. Simulationfour-fermion final states was performed using ECALIBUR [13] and GRC4F [14]. Two-photon interactions giving hadronic final states were generausing TWOGAM [15]. Signal events were generaby a standalone simulation program interfaced wPYTHIA 6.125 [9] for quark hadronization. The geeration of the signal events was performed withdiative corrections included. The SM contributionknown to be very small (Br(t → (γ, g,Z) + c(u)) <

10−10 [2]) and was not taken into account. Both tsignal and background events were passed througdetailed simulation of the DELPHI detector and thprocessed with the same reconstruction and anaprograms as the real data.

3. Hadronic channel

In the hadronic channel, the final state correspoing to the single top production is characterized

,

four jets: ab jet from the top decay, a spectatorc jetand two other jets from theW hadronic decay.

In this analysis the reconstructed charged parttracks were required to fulfill the following criteria:2

– momentump > 0.4 GeV/c;– momentum errorp/p < 1;– Rφ impact parameter< 4 cm;– z impact parameter< 10 cm.

Tracks seen by only the central tracking devic(vertex detector and inner detector) were rejectedNeutral clusters were required to have an energy oleast 400 MeV. Events with the visible energy> 100GeV and at least 8 charged tracks were selectedfurther processing.

The information of the DELPHI calorimeters antracking devices was used to classify charged pacles as electrons or muons according to standard DPHI algorithms [8]. A well-identified lepton was deignated as a “standard” lepton. Whenever some amguity persisted the lepton was called a “loose” leptTo each lepton tag there corresponds a given detecefficiency and misidentification probability [8]. Evenwith leptons with momenta above 20 GeV/c, identi-fied as at least “standard” electrons or “loose” muowere rejected.

The LUCLUS [9] algorithm withdjoin = 6.5 GeV/c

was then applied to cluster the event into jets. Evewith 4, 5, or 6 jets were selected and forced into ajet topology. Each of the three most energetic jets mcontain at least one charged particle. The preselecwas completed by requiring the event visible eneand combined b-tag parameter [16] to be greater t130 GeV and−1.5, respectively. The energies and mmenta of the jets were then rescaled by applying a cstrained fit with NDF= 4 imposing four-momentumconservation [17].

The assignment of jets to quarks is not straigforward as the kinematics of the event varies stronwith the energy. Near thet c production threshold botquarks are produced at rest and the subsequent to

2 The DELPHI coordinate system has thez-axis aligned alongthe electron beam direction, thex-axis pointing toward the centeof LEP andy-axis vertical. R is the radius in the(x, y)-plane.The polar angleΘ is measured with respect to thez-axis and theazimuthal angleφ is aboutz.


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cay (t → Wb) produces a high momentumb quark.However, at higher LEP center-of-mass energies thc

quark becomes more energetic with momentum vaup to 30 GeV/c. Four different methods of jet assigment were considered:

(1) the jet with highest b-tag parameter [16] wasb jet candidate and the least energetic jet (amthe three remaining jets) was thec jet candidate;

(2) the most energetic jet was theb jet candidate andthe least energetic one was thec jet candidate;

(3) the jet with highest b-tag parameter was theb jetcandidate and two jets were assigned to theW

according to the probability of the 5-C constrainfit;

(4) the most energetic jet was theb jet candidate andtwo jets were assigned to theW according to theprobability of the 5-C constrained fit.

All the above studies were performed and the hiest efficiency for the signal and strongest backgrosuppression was obtained with the first method. Tmethod was used in the hadronic analysis forcenter-of-mass energies. Method 2, well suited atkinematic threshold of single-top production, was leefficient at the highest LEP energies because theergy of theb jet becomes comparable to the energof the other jets.

After the preselection, signal and background-lprobabilities were assigned to each event basedprobability density functions (PDF) constructed wthe following variables:

• the event thrust value [18];• the event sphericity [18];• the event b-tag calculated with the combin

algorithm [16];• the energy of the jet assigned asb jet (Eb);• the energy of the most energetic jet in the ev

(Emax);• the ratio of the energies of the least and m

energetic jets (Emin/Emax);• the invariant mass of the two jets assigned

originating from theW decay (MW );• the absolute value of the reconstructedW momen-

tum (PW ).

Examples of these distributions are shown in Figsand 2, after the preselection.

All eight PDF were estimated for the signal (Psignali )

and background (Pbacki ) distributions. They were use

to construct the signalLS = ∏8i=1P

signali and back-

groundLB = ∏8i=1Pback

i likelihoods. A discriminantvariable

(5)W = ln

(LSLB

)

based on the ratio of the likelihoods was then cstructed for each event.

Fig. 3 shows the discriminant variable distributiand the number of accepted events, at

√s = 205–

207 GeV, as function of signal efficiency for a tomass of 175 GeV/c2. Events were selected by appling a cut on the discriminant variable ln(LS/LB), de-pendent on the center-of-mass energy. Its valuechosen to maximize the efficiency for a low bacground contamination. The number of data eventsexpected background from the SM processes (moWW background) passing the likelihood ratio seletion are shown in Table 2 for all center-of-mass engies, together with the signal efficiencies convolutedwith theW hadronic branchingratio. A general goodagreement with the Standard Model expectationobserved.

4. Semileptonic channel

In the semileptonic channel, the final state corsponding to single top production is characterizedtwo jets (ab jet from the top decay and a spectac jet) and at least one isolated lepton from theW lep-tonic decay.

At the preselection level, events with an energythe detector greater than 20% of the center-of-menergy and at least 7 charged particles were seleThe identification of muons relies on the associatof charged particles to signals in the muon chamband in the hadronic calorimeter and was providedstandard DELPHI algorithms [8].

The identification of electrons and photons was pformed by combining information from the electromagnetic calorimeter and the tracking system. Raation and interaction effects were taken into accoby an angular clustering procedure around the mshower [19].


e,, (e) thethick

Fig. 1. Distributions of relevant variables for the hadronic decay channel after the preselection, for√

s = 205–207 GeV: (a) the b-tag variabl(b) the reconstructedW mass, (c) the ratio between the minimal and the maximal jet energies, (d) the energy of the most b-like jetsphericity of the event and (f) the energy of the most energetic jet. The dots show the data, the shaded region the SM simulation and theline the expected signal behaviour (with arbitrary normalization) for a top mass of 175 GeV/c2.

on-s oflf-

re-
Isolated leptons (photons) were defined by c
structing double cones centered around the axithe charged particle track (neutral cluster) with ha

opening angles of 5◦ and 25◦ (5◦ and 15◦), and re-quiring that the average energy density in thegion between the two cones was below 150 MeV/deg


tion with an

gion)und

cieslso

Fig. 2. Distributions of relevant variables for the hadronic decay channel after the preselection for√

s = 205–207 GeV: (a) the reconstructedW

momentum and (b) the event thrust. The dots show the data and the shaded histograms show the SM simulation. The signal distribuarbitrary normalization is shown by the thick line for a top quark mass of 175 GeV/c2.

Fig. 3. (a) distributions of the discriminant variable at√

s = 205–207 GeV for data (dots), SM background simulation (shadowed reand signal (thick line) with arbitrary normalization and (b) number of accepted data events (dots) together with the expected SM backgrosimulation (full line) as a function of the signal efficiency (convoluted with theW hadronic branching ratio) for a top mass of 175 GeV/c2.

Table 2Number of events in the hadronic analysis at the preselection and final selection levels, for different center-of-mass energies. The efficienconvoluted with theW hadronic branching ratio (Br) are shown for a top-quark mass of 175 GeV/c2. Statistical and systematic errors are agiven (see Section 5)√

s (GeV) Preselection Final selection

Data Back± stat Data Back± stat± syst ε × Br (%)

189 568 530.6± 3.3 37 37.1 ± 1.4 ± 1.2 17.6 ± 0.5 ± 0.4192 106 91.4± 1.2 3 3.4 ± 0.4 ± 0.3 17.7 ± 0.5 ± 0.4196 266 253.1± 1.5 17 10.7 ± 0.4 ± 0.4 17.9 ± 0.6 ± 0.5200 251 265.0± 1.7 12 11.9 ± 0.5 ± 0.7 16.7 ± 0.5 ± 0.4202 134 133.3± 0.9 5 6.9 ± 0.3 ± 0.3 17.9 ± 0.6 ± 0.5205–207 486 544.1± 2.7 25 30.1 ± 0.9 ± 1.2 17.5 ± 0.5 ± 0.6


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(100 MeV/deg), to assure isolation. In the caseneutral deposits, no charged particle with more th250 MeV/c was allowed inside the inner cone. Tenergy of the isolated particle was then re-evaluaas the sum of the energies inside the inner cone.well identified leptons or photons the above requments were weakened. In this case only the extecone was used and the angleα was varied accordingto the energy of the lepton (photon) candidate, doto 2◦ for Plep � 70 GeV/c (3◦ for Eγ � 90 GeV),with the allowed energy inside the cone reducedsinα/sin25◦ (sinα/sin15◦).

Events with only one charged lepton and no ilated photons were selected. No other specific crria were additionally applied to perform lepton flavoidentification.

All other particles were then forced into jets usithe Durham jet algorithm [20], which is based onscaled transverse momentum method. Two-jet evwere selected by a cut on the value of the correspoing resolution variabley at the transition between onand two jets:− log10(y2→1) � 0.45. The most energetic particle in each jet had to be charged. It wasquired that the momenta of the lepton and jets wgreater than 10 GeV/c and 5 GeV/c, respectively. Polar angles of the lepton and of the two jets werequired to be in the region 20◦ � θlep � 160◦ and 10◦ �θj1,j2 � 170◦, respectively. The missing momentupolar angle had to be above 20◦ and below 160◦ andthe combined b-tag parameter [16] of the most engetic jet was required to be greater than−1.1.

The energies and momenta of the jets, the lepand the momentum of the undetected neutrinosumed to be the missing momentum) were calculafrom four-momentum conservation with a constrainfit (NDF = 1). Events withχ2 lower than 7 were accepted, provided the invariant mass of the neutrinothe isolated lepton was below 125 GeV/c2. The mostenergetic jet was assigned to theb quark and the second jet to thec quark. The top mass was reconstrucas the invariant mass of theb jet, the isolated leptonand the neutrino four-momenta.

Figs. 4 and 5 show some relevant distributionsdata and MC, after the preselection and for

√s = 205–

207 GeV. The number of events at preselectionfinal selection levels are given in Table 3 for eacenter-of-mass energy. Most of the background cofrom SM e+e− → WW events.

After the preselection, signal and background-lprobabilities were assigned to each event (as forhadronic channel) based on PDF constructed withfollowing variables:

• momentum of the less energetic jet;• more energetic jet b-tag variable [16];• reconstructed mass of the two jets;• reconstructed top mass;• angle between the two jets;• lepton–neutrino invariant mass;• ql ·cosθl , whereql is the charge andθl is the polar

angle of the lepton;• qj1 · cosθj1, whereqj1 = −ql andθj1 is the polar

angle of the more energetic jet;• pj1 · [√s − pj1(1 − cosθj1j2)], wherepj1 is the

momentum of the more energetic jet andθj1j2 isthe angle between the two jets. This variableproportional to(m2

t − m2W)/2, i.e., not dependen

on the center-of-mass energy.

The signal (LS ) and background (LB) likelihoodswere used on an event-by-event basis to compudiscriminant variable defined as ln(LS/LB). A loosecut on the signal likelihood was applied to the evenFig. 6 presents, after this cut, the discriminant variadistribution and the number of events accepted afunction of signal efficiency for

√s = 205–207 GeV

(assuming a top mass of 175 GeV/c2 for the signal).There is a general good agreement between theand the SM predictions. The background distributhas a tail for higher values of the discriminant variawhich goes below every data event. Correlatiobetween the variables were studied. Their effect onlikelihood ratio is small.

Events were further selected by applying aon the discriminant variable ln(LS/LB), dependenon the center-of-mass energy. Table 3 showsnumber of data and background events which pasthe cut for the different center-of-mass energies.efficiencies convoluted with theW leptonic branchingratio are also shown. The dominant backgrounds cfrom SM e+e− → WW ande+e− → qq events.

5. Systematic errors and limit derivation

Studies of systematic errors were performedtheir effect evaluated at the final selection level. T


t (e)iour

Fig. 4. Distributions of relevant variables for the semileptonic decay channel at the preselection level for√

s = 205–207 GeV. The momentumof the most energetic jet (a) and its polar angle (b), the lepton momentum (c) and its polar angle (d), the momentum of the least energetic jeand its polar angle (f) are shown. The dots show the data, the shaded region the SM simulation and the thick line the expected signal behav(with arbitrary normalization) for a top mass of 175 GeV/c2.

onnt

stability of the results with respect to variationsthe selection criteria, the PDF definition, the differe

hadronization schemes and the uncertainty in topquark mass were studied.


t

our

Fig. 5. Distributions of relevant variables at the preselection level in the semileptonic decay channel, for√

s = 205–207 GeV: (a) the mosenergetic jet b-tag parameter, (b) the reconstructed two jet system mass, (c) top mass, (d) the angle between the jets, (e)ql cos(θl) (see text forexplanation) and (f )−ql cos(θj ). The dots show the data, the shaded region the SM simulation and the thick line the expected signal behavi

(with arbitrary normalization) for a top quark mass of 175 GeV/c2.

en-tionles

m-

he

Concerning the stability of the results, an indepdent (and large, compared to the resolution) variaon the selection criteria applied to analysis variab

like the missing momentum polar angle, the cobined b-tag of the most energetic jet, theW mass,the Durham resolution variable, etc., was allowed. T


There

Table 3Number of events in the semileptonic analysis at the preselection and finalselection levels, for the different center-of-mass energies.efficiencies convoluted with theW leptonic branching ratio are also shown for a top mass of 175 GeV/c2. Statistical and systematic errors agiven (see the systematic errors and limit derivation section)√

s (GeV) Preselection Final selection

Data Back± stat Data Back± stat± syst ε × Br (%)

189 102 120.7± 4.3 1 2.4 ± 0.7 ± 0.8 8.0 ± 0.3 ± 0.5192 24 21.5± 0.8 1 0.5 ± 0.1 ± 0.1 7.7 ± 0.9 ± 0.5196 72 76.2± 2.5 2 0.9 ± 0.3 ± 0.1 7.1 ± 0.9 ± 0.5200 95 87.6± 2.8 1 2.0 ± 0.5 ± 0.3 6.9 ± 0.3 ± 0.3202 40 42.2± 1.3 1 1.7 ± 0.3 ± 0.1 7.9 ± 0.4 ± 0.3205 90 90.0± 2.9 2 1.4 ± 0.4 ± 0.1 6.2 ± 0.3 ± 0.3207 71 90.2± 2.6 2 1.9 ± 0.5 ± 0.2 6.2 ± 0.3 ± 0.4

er--th-ni-forer-ent)n-

er-thectslsonalforeen

ions 2

undels,yere

entnic

itsss-

withit

ion

Fig. 6. (a) the discriminant variable distribution for√

s

= 205–207 GeV is shown. The dots show the data, the shaded regionthe SM simulation and the thick line the expected signal behaviour(with arbitrary normalization) for a top quark mass of 175 GeV/c2.(b) number of accepted data events (dots) together with the expectedSM background simulation (full line) as a function of the signal ef-ficiency (convoluted with theW leptonic branching ratio) for a topmass of 175 GeV/c2.

most significant contributions gave a maximumror of 0.5 events and 0.3% for the expected background and efficiency, respectively. Different smooing procedures were performed for the PDF defition and their effect is at most 0.5 events (0.4%)the expected background (signal efficiency). Diffent hadronization schemes (string and independ[9] were studied for the signal and their effect cotributes at most 0.1% for the signal efficiencyror. The uncertainty on the top quark mass ismost important source of systematic errors. It affenot only the total production cross-section but athe kinematics of signal events. In terms of sigefficiency, its effect could be as high as 0.9%the semileptonic channel (in the mass range betw170 GeV/c2 and 180 GeV/c2). The effects of suchvariations (added quadratically) on the final selectcriteria are quoted as a systematic error in Tableand 3.

The number of data and expected SM backgroevents for the hadronic and semileptonic channthe respective signal efficiencies and data luminositcollected at the various center-of-mass energies wcombined to derive limits in the (κγ , κZ) plane usinga Bayesian approach [21]. In total, 13 independchannels (6 in the hadronic and 7 in the semileptomodes) correspond to different

√s values. These

channels are fitted simultaneously to extract the limon the FCNC parameters. The total production crosection and top FCNC decay widths dependenceκγ andκZ were properly considered [6] in the limderivation.

The effect of systematic errors on the (κγ , κZ)plane limits was considered. Initial State Radiat


yereTheark

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Fig. 7. Limits at 95% confidence level in theκγ –κZ plane. Thedifferent curved and filled areasrepresent the regions allowed bDELPHI for different top quark masses. Radiative corrections wtaken into account in the total production cross-section at LEP.CDF and ZEUS allowed regions are also shown for a top qumass of 175 GeV/c2. The ZEUS limits are scaled by a factor√

2 because of the difference in the Lagrangian definitions.

Table 495% C.L. upper limits derived from the combined hadronic asemileptonic channels at

√s = 189–208 GeV forΛ = 175 GeV

mt (GeV/c2) 170 175 180

κZ(κγ = 0) 0.340 0.411 0.527κγ (κZ = 0) 0.402 0.486 0.614

(ISR) and QCD corrections [22] were also taken inaccount in thet c total production cross-section.

Fig. 7 shows the 95% confidence level (C.L.) uper limits in the (κγ , κZ) plane obtained by this analysis. The different filled areas correspond to thelowed regions obtained for different top mass valuandΛ = 175 GeV. Due to thes-channelZ dominance,the LEP2 data are less sensitive to theκγ parameterthan toκZ . The upper limits obtained by CDF Collaboration [7] and ZEUS [23] are also shown in the figufor comparison. The 95% C.L. upper limits on eacoupling parameter, setting the other coupling to zeare summarized in Table 4. For comparison theues atmt = 175 GeV/c2 areκZ(κγ = 0) = 0.434 andκγ (κZ = 0) = 0.505 if the Born level cross-sectio(without radiative corrections) is taken into account.

Upper limits were also obtained by using onthe hadronic and the semileptonic channels separa

when radiative corrections to the total producticross-section were taken into account. The valuemt = 175 GeV/c2 are κZ(κγ = 0) = 0.491 (0.547)and κγ (κZ = 0) = 0.568 (0.625) for the hadronic(semileptonic) channel alone.

6. Summary

The data collected by the DELPHI detectorcenter-of-mass energies ranging from 189 to 208 Gwere used to perform a search for FCNCt c produc-tion, in the hadronic and semileptonic topologies.deviation with respect to the SM expectations wfound. Upper limits on the anomalous couplingsκγ

andκZ were derived. A comparison with CDF [7] anZEUS [23] is also shown. Results on the searchsingle-top production were also obtained by the otexperiments at LEP [24].

Acknowledgements

We are greatly indebted to our technical collabotors, to the members of the CERN-SL Division for texcellent performance of the LEP collider, and tofunding agencies for their support in building and oerating the DELPHI detector.

We acknowledge in particular the support of

– Austrian Federal Ministry of Education, Scienand Culture, GZ 616.364/2-III/2a/98;

– FNRS–FWO, Flanders Institute to encourageentific and technological research in the indtry (IWT), Federal Office for Scientific, Technicaand Cultural affairs (OSTC), Belgium;

– FINEP, CNPq, CAPES, FUJB and FAPERJ, Bzil;

– Czech Ministry of Industry and Trade, GA C202/99/1362;

– Commission of the European Communities (DXII);

– Direction des Sciences de la Matière, CEA, Frce;

– Bundesministerium für Bildung, WissenschaForschung und Technologie, Germany;

– General Secretariat for Research and TechnolGreece;


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– National Science Foundation (NWO) and Fountion for Research on Matter (FOM), The Nethelands;

– Norwegian Research Council;– State Committee for Scientific Research, Pola

SPUB-M/CERN/PO3/DZ296/2000, SPUB-MCERN/PO3/DZ297/2000 and 2P03B 104 19 a2P03B 69 23(2002–2004);

– JNICT–Junta Nacional de Investigação Científie Tecnológica, Portugal;

– Vedecka grantova agentura MS SR, Slovakia,95/5195/134;

– Ministry of Science and Technology of the Repulic of Slovenia;

– CICYT, Spain, AEN99-0950 and AEN99-0761– The Swedish Natural Science Research Coun– Particle Physics and Astronomy Research Co

cil, UK;– Department of Energy, USA, DE-FG02-01E

41155;– EEC RTN contract HPRN-CT-00292-2002.

References

[1] S.L. Glashow, J. Iliopoulos, L. Maiani, Phys. Rev. D 2 (1971285.

[2] B. Grzadkowski, J.F. Gunion, P. Krawczyk, Phys. Lett. B 2(1991) 106;G. Eilam, J.L. Hewett, A. Soni, Phys. Rev. D 44 (1991) 147G. Eilam, J.L. Hewett, A. Soni, Phys. Rev. D 59 (199039901, Erratum;M.E. Luke, M.J. Savage, Phys. Lett. B 307 (1993) 387.

[3] G.M. de Divitiis, R. Petronzio, L. Silvestrini, Nucl. Phys. B 50(1997) 45.

[4] D. Atwood, L. Reina, A. Soni, Phys. Rev. D 53 (1996) 1199[5] B.A. Arbuzov, M.Yu. Osipov, Phys. At. Nucl. 62 (1999) 485

B.A. Arbuzov, M.Yu. Osipov, Yad. Fiz. 62 (1999) 528.

[6] V.F. Obraztsov, S.R. Slabospitsky, O.P. Yushchenko, PLett. B 426 (1998) 393.

[7] CDF Collaboration, F. Abe, et al., Phys. Rev. Lett. 80 (1992525.

[8] DELPHI Collaboration, P. Aarnio, et al., Nucl. Instrum. Metods A 303 (1991) 233;DELPHI Collaboration, P. Abreu,et al., Nucl. Instrum. Meth-ods A 378 (1996) 57.

[9] T. Sjöstrand, Comput. Phys. Commun. 82 (1994) 74.[10] J.E. Campagne, R. Zitoun, Z. Phys. C 43 (1989) 469.[11] S. Jadach, B.F.L. Ward, Z. Was, Comput. Phys. Commun

(1994) 503.[12] S. Jadach, W. Placzek, B.F.L. Ward, Phys. Lett. B 390 (19

298.[13] F.A. Berends, R. Pittau, R. Kleiss, Comput. Phys. Commun

(1995) 437.[14] J. Fujimoto, et al., Comput. Phys. Commun. 100 (1997) 12[15] T. Alderweireld et al., CERN-2000-009, p. 219.[16] DELPHI Collaboration, J. Abdallah et al., b-tagging in DEL

PHI at LEP, CERN-EP/2002-088, Eur. Phys. J. C, submifor publication.

[17] DELPHI Collaboration, P. Abreu, et al., Eur. Phys. J. C(1998) 581.

[18] DELPHI Collaboration, P. Abreu, et al., Z. Phys. C 73 (19911.

[19] F. Cossutti, A. Tonazzo, F. Mazzucato, REMCLU: a packafor the reconstruction of electromagnetic clusters at LEP2DELPHI Note 2000-164, 2000.

[20] S. Catani, et al., Phys. Lett. B 269 (1991) 432.[21] V.F. Obraztsov, Nucl. Instrum. Methods A 316 (1992) 388;

V.F. Obraztsov, Nucl. Instrum. Methods A 399 (1997) 50Erratum.

[22] L.J. Reinders, H. Rubinstein, S. Yazaki, Phys. Rep. 127 (191.

[23] ZEUS Collaboration, S. Chekanov, Phys. Lett. B 559 (20153.

[24] ALEPH Collaboration, A. Heister, et al., Phys. Lett. B 54(2002) 173;OPAL Collaboration, G. Abbiendi, et al., Phys. Lett. B 521(2001) 181;L3 Collaboration, P. Achard, et al., Phys. Lett. B 549 (200290.

search for single top production via fcnc at lep at 189 ... · physics letters b 590 (2004) 21–34...

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