Search for single-top production in ep collisions at HERA
ZEUS Collaboration, H. Abramowicz, I. Abt, L. Adamczyk, M. Adamus, R. Aggarwal, S. Antonelli, P. Antonioli, A. Antonov, M. Arneodo, V. Aushev, Y. Aushev, O. Bachynska, A. Bamberger, A. N. Barakbaev, G. Barbagli, G. Bari, F. Barreiro, N. Bartosik, D. Bartsch, M. Basile, O. Behnke, J. Behr, U . Behrens, L. Bellagamba, A. Bertolin, S. Bhadra, M. Bindi, C. Blohm, V. Bokhonov, T. Bołd, K. Bondarenko, E. G. Boos, K. Borras, D. Boscher ini, D. Bot, I. Brock, E. Brownson, R. Brugnera, N. Brümmer, A. Bruni, G. Bruni, B. Brzozowska, P. J. Bussey, B. Bylsma, A. Caldwell, M. Capua, R. Carlin, C. D. Catterall, S. Chekanov, J. Chwastowski, J. Ciborowski, R . Ciesielski, L. Cifarelli, F. Cindolo, A. Contin, A. M. Cooper-Sarkar, N. Coppola, M. Corradi, F. Corriveau, M. Costa, G. D'Agostini, F. Dal Corso, J. del Peso, R. K. Dementiev, S. De Pasquale, M. Derrick, R. C. E. Devenish, D. Dobur, B. A. Dolgoshein, G. Dolinska, A. T. Doyle, V. Drugakov, L. S. Durkin, S. Dusini, Y. Eisenberg, P. F. Ermolov, A. Eskreys, S. Fang, S. Fazio, J. Ferrando, M. I. Ferrero, J. Figiel, M. Forrest, B. Foster, G. Gach, A. Galas, E. Gallo, A. Garfagnini, A. Geiser, I. Gialas, L. K. Gladilin, D. Gladkov, C. Glasman, O. Gogota, Yu. A. Golubkov, P. Göttlicher, I. Grabowska-Bołd, J. Grebenyuk, I. Gregor, et al. (203 additional authors not shown)
aa r X i v : . [ h e p - e x ] F e b Search for single-top production in ep collisions at HERA ZEUS Collaboration
Abstract
A search for single-top production, ep → etX , has been performed with the ZEUSdetector at HERA using data corresponding to an integrated luminosity of . fb − .No evidence for top production was found, consistent with the expectation fromthe Standard Model. Limits were computed for single-top production via flavourchanging neutral current transitions involving a neutral electroweak vector boson, γ or Z . The result was combined with a previous ZEUS result yielding a total luminosityof . fb − . A credibility level upper limit of . pb was obtained for the crosssection at the centre-of-mass energy of √ s = 315 GeV.
Published in Physics Letters B 708 (2012) 27-36 he ZEUS Collaboration
H. Abramowicz ,ah , I. Abt , L. Adamczyk , M. Adamus , R. Aggarwal ,c , S. Antonelli ,P. Antonioli , A. Antonov , M. Arneodo , V. Aushev , ,z , Y. Aushev, ,z,aa , O. Bachynska ,A. Bamberger , A.N. Barakbaev , G. Barbagli , G. Bari , F. Barreiro , N. Bartosik ,ab ,D. Bartsch , M. Basile , O. Behnke , J. Behr , U. Behrens , L. Bellagamba , A. Bertolin ,S. Bhadra , M. Bindi , C. Blohm , V. Bokhonov ,z , T. Bołd , K. Bondarenko , E.G. Boos ,K. Borras , D. Boscherini , D. Bot , I. Brock , E. Brownson , R. Brugnera , N. Brümmer ,A. Bruni , G. Bruni , B. Brzozowska , P.J. Bussey , B. Bylsma , A. Caldwell , M. Capua ,R. Carlin , C.D. Catterall , S. Chekanov , J. Chwastowski ,e , J. Ciborowski ,al , R. Ciesielski ,g ,L. Cifarelli , F. Cindolo , A. Contin , A.M. Cooper-Sarkar , N. Coppola ,h , M. Corradi ,F. Corriveau , M. Costa , G. D’Agostini , F. Dal Corso , J. del Peso , R.K. Dementiev ,S. De Pasquale ,a , M. Derrick , R.C.E. Devenish , D. Dobur ,s , B.A. Dolgoshein , † , G. Dolinska , ,A.T. Doyle , V. Drugakov , L.S. Durkin , S. Dusini , Y. Eisenberg , P.F. Ermolov , † ,A. Eskreys , † , S. Fang ,i , S. Fazio , J. Ferrando , M.I. Ferrero , J. Figiel , M. Forrest ,v ,B. Foster ,ad , G. Gach , A. Galas , E. Gallo , A. Garfagnini , A. Geiser , I. Gialas ,w ,L.K. Gladilin ,ac , D. Gladkov , C. Glasman , O. Gogota , , Yu.A. Golubkov , P. Göttlicher ,j ,I. Grabowska-Bołd , J. Grebenyuk , I. Gregor , G. Grigorescu , G. Grzelak , O. Gueta ,M. Guzik , C. Gwenlan ,ae , T. Haas , W. Hain , R. Hamatsu , J.C. Hart , H. Hartmann ,G. Hartner , E. Hilger , D. Hochman , R. Hori , K. Horton ,af , A. Hüttmann , Z.A. Ibrahim ,Y. Iga , R. Ingbir , M. Ishitsuka , H.-P. Jakob , F. Januschek , T.W. Jones , M. Jüngst ,I. Kadenko , B. Kahle , S. Kananov , T. Kanno , U. Karshon , F. Karstens ,t , I.I. Katkov ,k ,M. Kaur , P. Kaur ,c , A. Keramidas , L.A. Khein , J.Y. Kim , D. Kisielewska , S. Kitamura ,aj ,R. Klanner , U. Klein ,l , E. Koffeman , P. Kooijman , Ie. Korol , , I.A. Korzhavina ,ac ,A. Kotański ,f , U. Kötz , H. Kowalski , O. Kuprash , M. Kuze , A. Lee , B.B. Levchenko ,A. Levy , V. Libov , S. Limentani , T.Y. Ling , M. Lisovyi , E. Lobodzinska , W. Lohmann ,B. Löhr , E. Lohrmann , K.R. Long , A. Longhin , D. Lontkovskyi , O.Yu. Lukina , J. Maeda ,ai ,S. Magill , I. Makarenko , J. Malka , R. Mankel , A. Margotti , G. Marini , J.F. Martin ,A. Mastroberardino , M.C.K. Mattingly , I.-A. Melzer-Pellmann , S. Mergelmeyer , S. Miglioranzi ,m ,F. Mohamad Idris , V. Monaco , A. Montanari , J.D. Morris ,b , K. Mujkic ,n , B. Musgrave ,K. Nagano , T. Namsoo ,o , R. Nania , A. Nigro , Y. Ning , T. Nobe , U. Noor , D. Notz ,R.J. Nowak , A.E. Nuncio-Quiroz , B.Y. Oh , N. Okazaki , K. Oliver , K. Olkiewicz ,Yu. Onishchuk , K. Papageorgiu , A. Parenti , E. Paul , J.M. Pawlak , B. Pawlik , P. G. Pelfer ,A. Pellegrino , W. Perlański ,am , H. Perrey , K. Piotrzkowski , P. Pluciński ,an , N.S. Pokrovskiy ,A. Polini , A.S. Proskuryakov , M. Przybycień , A. Raval , D.D. Reeder , B. Reisert ,Z. Ren , J. Repond , Y.D. Ri ,ak , A. Robertson , P. Roloff ,m , I. Rubinsky , M. Ruspa ,R. Sacchi , A. Salii , U. Samson , G. Sartorelli , A.A. Savin , D.H. Saxon , M. Schioppa ,S. Schlenstedt , P. Schleper , W.B. Schmidke , U. Schneekloth , V. Schönberg , T. Schörner-Sadenius , J. Schwartz , F. Sciulli , L.M. Shcheglova , R. Shehzadi , S. Shimizu ,m , I. Singh ,c ,I.O. Skillicorn , W. Słomiński , W.H. Smith , V. Sola , A. Solano , D. Son , V. Sosnovtsev ,A. Spiridonov ,p , H. Stadie , L. Stanco , A. Stern , T.P. Stewart , A. Stifutkin , P. Stopa ,S. Suchkov , G. Susinno , L. Suszycki , J. Sztuk-Dambietz , D. Szuba , J. Szuba ,q , A.D. Tapper ,E. Tassi ,d , J. Terrón , T. Theedt , H. Tiecke , K. Tokushuku ,x , O. Tomalak , J. Tomaszewska ,r , I . Tsurugai , M. Turcato , T. Tymieniecka ,ao , M. Vázquez ,m , A. Verbytskyi , O. Viazlo , ,N.N. Vlasov ,u , O. Volynets , R. Walczak , W.A.T. Wan Abdullah , J.J. Whitmore ,ag ,L. Wiggers , M. Wing , M. Wlasenko , G. Wolf , H. Wolfe , K. Wrona , A.G. Yagües-Molina , S. Yamada , Y. Yamazaki ,y , R. Yoshida , C. Youngman , A.F. Żarnecki , L. Zawiejski ,O. Zenaiev , W. Zeuner ,m , B.O. Zhautykov , N. Zhmak ,z , C. Zhou , A. Zichichi , Z. Zolkapli ,M. Zolko , D.S. Zotkin II Argonne National Laboratory, Argonne, Illinois 60439-4815, USA A Andrews University, Berrien Springs, Michigan 49104-0380, USA INFN Bologna, Bologna, Italy B University and INFN Bologna, Bologna, Italy B Physikalisches Institut der Universität Bonn, Bonn, Germany C H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom D Panjab University, Department of Physics, Chandigarh, India Calabria University, Physics Department and INFN, Cosenza, Italy B Institute for Universe and Elementary Particles, Chonnam National University,Kwangju, South Korea Jabatan Fizik, Universiti Malaya, 50603 Kuala Lumpur, Malaysia E Nevis Laboratories, Columbia University, Irvington on Hudson, New York 10027,USA F The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy ofSciences, Krakow, Poland G AGH-University of Science and Technology, Faculty of Physics and Applied Com-puter Science, Krakow, Poland H Department of Physics, Jagellonian University, Cracow, Poland Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany Deutsches Elektronen-Synchrotron DESY, Zeuthen, Germany INFN Florence, Florence, Italy B University and INFN Florence, Florence, Italy B Fakultät für Physik der Universität Freiburg i.Br., Freiburg i.Br., Germany School of Physics and Astronomy, University of Glasgow, Glasgow, United King-dom D Department of Engineering in Management and Finance, Univ. of the Aegean, Chios,Greece Hamburg University, Institute of Experimental Physics, Hamburg, Germany I Imperial College London, High Energy Nuclear Physics Group, London, United King-dom D Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan J Institute of Physics and Technology of Ministry of Education and Science of Kaza-khstan, Almaty, Kazakhstan Institute for Nuclear Research, National Academy of Sciences, Kyiv, Ukraine Department of Nuclear Physics, National Taras Shevchenko University of Kyiv, Kyiv,Ukraine Kyungpook National University, Center for High Energy Physics, Daegu, SouthKorea K Institut de Physique Nucléaire, Université Catholique de Louvain, Louvain-la-Neuve,Belgium L Departamento de Física Teórica, Universidad Autónoma de Madrid, Madrid,Spain M Department of Physics, McGill University, Montréal, Québec, Canada H3A 2T8 N Meiji Gakuin University, Faculty of General Education, Yokohama, Japan J Moscow Engineering Physics Institute, Moscow, Russia O Moscow State University, Institute of Nuclear Physics, Moscow, Russia P Max-Planck-Institut für Physik, München, Germany NIKHEF and University of Amsterdam, Amsterdam, Netherlands Q Physics Department, Ohio State University, Columbus, Ohio 43210, USA A Department of Physics, University of Oxford, Oxford, United Kingdom D III INFN Padova, Padova, Italy B Dipartimento di Fisica dell’ Università and INFN, Padova, Italy B Department of Physics, Pennsylvania State University, University Park,Pennsylvania 16802, USA F Polytechnic University, Sagamihara, Japan J Dipartimento di Fisica, Università ’La Sapienza’ and INFN, Rome, Italy B Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom D Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics,Tel Aviv University, Tel Aviv, Israel R Department of Physics, Tokyo Institute of Technology, Tokyo, Japan J Department of Physics, University of Tokyo, Tokyo, Japan J Tokyo Metropolitan University, Department of Physics, Tokyo, Japan J Università di Torino and INFN, Torino, Italy B Università del Piemonte Orientale, Novara, and INFN, Torino, Italy B Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S1A7 N Physics and Astronomy Department, University College London, London, UnitedKingdom D Faculty of Physics, University of Warsaw, Warsaw, Poland National Centre for Nuclear Research, Warsaw, Poland Department of Particle Physics and Astrophysics, Weizmann Institute, Rehovot, Is-rael Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA A Department of Physics, York University, Ontario, Canada M3J 1P3 N IV supported by the US Department of Energy B supported by the Italian National Institute for Nuclear Physics (INFN) C supported by the German Federal Ministry for Education and Research (BMBF),under contract No. 05 H09PDF D supported by the Science and Technology Facilities Council, UK E supported by an FRGS grant from the Malaysian government F supported by the US National Science Foundation. Any opinion, findings and con-clusions or recommendations expressed in this material are those of the authors anddo not necessarily reflect the views of the National Science Foundation. G supported by the Polish Ministry of Science and Higher Education as a scientificproject No. DPN/N188/DESY/2009 H supported by the Polish Ministry of Science and Higher Education and its grants forScientific Research I supported by the German Federal Ministry for Education and Research (BMBF),under contract No. 05h09GUF, and the SFB 676 of the Deutsche Forschungsge-meinschaft (DFG) J supported by the Japanese Ministry of Education, Culture, Sports, Science andTechnology (MEXT) and its grants for Scientific Research K supported by the Korean Ministry of Education and Korea Science and EngineeringFoundation L supported by FNRS and its associated funds (IISN and FRIA) and by an Inter-University Attraction Poles Programme subsidised by the Belgian Federal SciencePolicy Office M supported by the Spanish Ministry of Education and Science through funds providedby CICYT N supported by the Natural Sciences and Engineering Research Council of Canada(NSERC) O partially supported by the German Federal Ministry for Education and Research(BMBF) P supported by RF Presidential grant N 4142.2010.2 for Leading Scientific Schools,by the Russian Ministry of Education and Science through its grant for ScientificResearch on High Energy Physics and under contract No.02.740.11.0244 Q supported by the Netherlands Foundation for Research on Matter (FOM) R supported by the Israel Science Foundation V now at University of Salerno, Italy b now at Queen Mary University of London, United Kingdom c also funded by Max Planck Institute for Physics, Munich, Germany d also Senior Alexander von Humboldt Research Fellow at Hamburg University, Insti-tute of Experimental Physics, Hamburg, Germany e also at Cracow University of Technology, Faculty of Physics, Mathemathics andApplied Computer Science, Poland f supported by the research grant No. 1 P03B 04529 (2005-2008) g now at Rockefeller University, New York, NY 10065, USA h now at DESY group FS-CFEL-1 i now at Institute of High Energy Physics, Beijing, China j now at DESY group FEB, Hamburg, Germany k also at Moscow State University, Russia l now at University of Liverpool, United Kingdom m now at CERN, Geneva, Switzerland n also affiliated with Universtiy College London, UK o now at Goldman Sachs, London, UK p also at Institute of Theoretical and Experimental Physics, Moscow, Russia q also at FPACS, AGH-UST, Cracow, Poland r partially supported by Warsaw University, Poland s now at Istituto Nucleare di Fisica Nazionale (INFN), Pisa, Italy t now at Haase Energie Technik AG, Neumünster, Germany u now at Department of Physics, University of Bonn, Germany v now at Biodiversität und Klimaforschungszentrum (BiK-F), Frankfurt, Germany w also affiliated with DESY, Germany x also at University of Tokyo, Japan y now at Kobe University, Japan z supported by DESY, Germany † deceased aa member of National Technical University of Ukraine, Kyiv Polytechnic Institute,Kyiv, Ukraine ab member of National University of Kyiv - Mohyla Academy, Kyiv, Ukraine ac partly supported by the Russian Foundation for Basic Research, grant 11-02-91345-DFG_a ad Alexander von Humboldt Professor; also at DESY and University of Oxford ae STFC Advanced Fellow af nee Korcsak-Gorzo ag This material was based on work supported by the National Science Foundation,while working at the Foundation. ah also at Max Planck Institute for Physics, Munich, Germany, External Scientific Mem-ber ai now at Tokyo Metropolitan University, Japan aj now at Nihon Institute of Medical Science, Japan ak now at Osaka University, Osaka, Japan al also at Łódź University, Poland am member of Łódź University, Poland an now at Department of Physics, Stockholm University, Stockholm, Sweden VI o also at Cardinal Stefan Wyszyński University, Warsaw, Poland VIIIII
Introduction
The dominant production process of single top quarks in the Standard Model (SM) in ep collisions at HERA is the charged current (CC) reaction ep → νtX [1], which hasa cross section of less than fb [2]. Flavour changing neutral current (FCNC) processescould enhance single-top production, but they are strongly suppressed in the SM by theGIM mechanism [3]. This mechanism forbids FCNCs at the tree level, allowing only forsmall contributions at the one-loop level, exploiting the flavour mixing due to the CKMmatrix [4]. Several extensions of the SM predict FCNC contributions already at the treelevel [5]. The search for such new interactions involving the top quark ( ut or ct transitionsmediated by neutral vector bosons, γ or Z ) opens an interesting window to look for effectsbeyond the SM [6].The FCNC couplings tuV and tcV , with V = γ, Z , have been investigated in pp collisions atthe Tevatron, where searches for the top-quark decays t → uV and t → cV [7,8] were carriedout. The Tevatron experiments also constrained the couplings tug and tcg [9] which induceFCNC transitions mediated by the gluon. The couplings tuV and tcV were also investigatedin e + e − interactions at LEP2 by searching for single-top production through the reactions e + e − → tu (+c . c . ) and e + e − → tc (+c . c . ) [10, 11]. No evidence for such interactions wasfound and limits were set on the branching ratios Br( t → qγ ) and Br( t → qZ ) , with q = u, c .The same FCNC couplings could induce single-top production in ep collisions, ep → etX [12], in which the incoming lepton exchanges a γ or Z with an up quark in theproton, yielding a top quark in the final state, see Fig. 1. Owing to the large Z mass, thisprocess is more sensitive to a coupling of the type tqγ . Furthermore, large values of x , thefraction of the proton momentum carried by the struck quark, are needed to produce a topquark. Since the u -quark parton distribution function (PDF) of the proton is dominant atlarge x , the production of single top quark is most sensitive to the tuγ coupling.In the present study, the top signal was searched for by looking for the decays t → beν e and t → bµν µ . At HERA, such event topologies with one lepton with high transversemomentum, p T , and large missing transverse momentum originate predominantly fromsingle- W production, which has a cross section of about pb [13] and is the most importantbackground to any top signal. The present analysis extends the previously published ZEUSresults [14] which used data from the HERA I running period , corresponding to a totalintegrated luminosity of . fb − . The integrated luminosity used in this analysis is aboutthree times larger. A combination of the results from the two running periods (totalintegrated luminosity . fb − ) has been performed. Here and in the following, e denotes both the electron and the positron. Data collected between 1994 and 2000. Theoretical framework
The effects of the FCNC transitions induced by couplings of the type tuV are parameterisedusing the following effective Lagrangian [15]: ∆ L eff = e e t t iσ µν p ν Λ κ γ u A µ + g θ W tγ µ v Z uZ µ + h . c . (1)where κ γ and v Z are two FCNC couplings mediating ut transitions, e ( e t ) is the electron(top quark) electric charge, g is the weak coupling constant, θ W is the weak mixing angle, σ µν = ( γ µ γ ν − γ ν γ µ ) , Λ is an effective cut-off parameter which, by convention, is set tothe mass of the t quark, M t , p is the momentum of the gauge boson and A µ ( Z µ ) is thephoton ( Z ) field. In the following, it is assumed that the magnetic coupling κ γ and thevector coupling v Z are real and positive.The cross section for the process ep → etX was evaluated at the leading order (LO) usingthe package CompHEP-4.5.1 [16] and was parameterised in terms of three parametersdescribing the effects of the two FCNC couplings, A σ and B σ , and their interference, C σ : σ ep → etX = A σ κ γ + B σ v Z + C σ κ γ v Z . (2)The decay widths of the top in the different channels were also evaluated using CompHEP-4.5.1: Γ t → uγ = A Γ κ γ , Γ t → uZ = B Γ v Z , Γ t → qW = C Γ , (3)where A Γ and B Γ are the partial width of the top corresponding to uγ and uZ unitaryFCNC couplings, respectively, and C Γ is the SM top width.The above parameters, summarised in Table 1, were evaluated using the top mass M t =172 . ± . GeV [17] and the PDF set CTEQ6L1 [18]. The interference parameter C σ has only a small effect, producing a cross section variation of less than . in the wholerange of the couplings considered in this analysis, and was therefore neglected. The QCDcorrections to the LO cross-section were evaluated at the approximate next-to-leadingorder (NLO) and next-to-next-to-leading order (NNLO) [12, 19] for magnetic couplingsboth at the γ and Z vertices. Since we considered a different coupling (vector coupling)at the Z vertex, we used such corrections only to evaluate the limits for the γ exchange(see Sect. 7.1). Such corrections increase the LO cross-section by and slightly reducesthe uncertainties due to the QCD factorisation-scale (see Sect. 6). The limits involvingboth coupling (see Sect. 7.2) were evaluated using the LO cross-section. The analysis is based on ep collisions recorded with the ZEUS detector during the HERA IIrunning period , using an integrated luminosity of . fb − , divided into two approximately Data collected between 2004 and 2007. e + p and e − p collisions. The lepton beams were polarised, with roughlyequal luminosities for positive and negative polarisation, such that the average polarisationwas negligible for this analysis.A detailed description of the ZEUS detector can be found elsewhere [20]. A brief outlineof the components that are most relevant for this analysis is given below.Charged particles were tracked in the central tracking detector (CTD) [21] which operatedin a magnetic field of . T provided by a thin superconducting solenoid. The CTDconsisted of 72 cylindrical drift chamber layers, organised in nine superlayers covering thepolar-angle region ◦ < θ < ◦ . The CTD was complemented by a silicon microvertexdetector (MVD) [22], consisting of three active layers in the barrel and four disks in theforward region. For CTD-MVD tracks that pass through all nine CTD superlayers, themomentum resolution was σ ( p T ) /p T = 0 . p T ⊕ . ⊕ . /p T with p T in GeV.The high-resolution uranium–scintillator calorimeter (CAL) [23] consisted of three parts:the forward (FCAL), the barrel (BCAL) and the rear (RCAL) calorimeters. Each partwas subdivided transversely into towers and longitudinally into one electromagnetic sec-tion (EMC) and either one (in RCAL) or two (in BCAL and FCAL) hadronic sections(HAC). The smallest subdivision of the calorimeter was called a cell. The CAL energy res-olutions, as measured under test-beam conditions, were σ ( E ) /E = 0 . / √ E for electronsand σ ( E ) /E = 0 . / √ E for hadrons, with E in GeV.The luminosity was measured using the Bethe-Heitler reaction ep → eγp by a luminositydetector which consisted of a lead–scintillator calorimeter [24] and an independent magneticspectrometer [25]. The fractional uncertainty on the measured luminosity was . . Samples of events were generated using Monte Carlo (MC) simulations to determine theselection efficiency for single-top events produced through FCNC processes and to estim-ate background rates from SM processes. The generated events were passed through the
Geant-3.21 [26] ZEUS detector- and trigger-simulation programs [20]. They were recon-structed and analysed by the same program chain as the data.Single-top samples were generated with
Comphep
Pythia
Comphep was set to M t = 175 GeV. Different sets were produced for the two differentproduction processes ( γ - and Z -mediated) and for the two decay modes ( t → bW and t → uZ ).Alternative sets were also generated, only for the γ -mediated process, with the Hexf gen-erator [28] assuming top-quark masses of and
GeV. These sets were used to study The ZEUS coordinate system is a right-handed Cartesian system, with the Z axis pointing in the protonbeam direction, referred to as the “forward direction”, and the X axis pointing towards the centre ofHERA. The coordinate origin is at the nominal interaction point. The pseudorapidity is defined as η = − ln (cid:0) tan θ (cid:1) , where the polar angle, θ , is measured with respect to the proton beam direction. M t variation, in order to correct the selection efficiency, evaluated usingthe Comphep samples, for the different M t values used in the generation and in the cross-section calculation (see Sect. 2). Initial-state radiation from the lepton beam was includedusing the Weizsäcker-Williams approximation [29]. The hadronic final state was simulatedusing the matrix-element and parton-shower model of Lepto [30] for the QCD cascadeand the Lund string model [31] as implemented in
Jetset [32] for the hadronisation. Theresults for
Comphep and the alternative samples agree within uncertainties.Standard Model single- W production is the most significant background to top production.Another important background in the electron-decay channel of the W ( t → bW → beν )arises from neutral current (NC) deep inelastic scattering (DIS). In addition, two-photonprocesses provide a source of high- p T leptons that are a significant background in the muon-decay channel of the W ( t → bW → bµν ). The CC DIS is a minor source of backgroundfor both channels.The following MC programs were used to simulate the different background processes.Single- W production was simulated using the event generator Epvec [33] which did notinclude hard QCD radiation. The ep → eW X and ep → νW X events from Epvec werescaled by a factor dependent on the transverse momentum and rapidity of the W , suchthat the resulting cross section corresponded to a calculation including QCD correctionsat next-to-leading order [34].Neutral current and CC DIS events were simulated using the Lepto
Heracles
Djangoh
Heracles program in-cludes photon and Z exchanges and first-order electroweak radiative corrections. TheQCD cascade was modelled with the colour-dipole model [37] by using the Ariadne
Grape γγ , Zγ and ZZ processes and considers both elastic and inelasticproduction at the proton vertex. The event selection was optimised for single-top production via photon exchange, lookingfor the dominant decay t → bW and subsequent W decay to e and µ and their respectiveneutrinos. The selection is based on requiring an isolated high- p T lepton and a large missingtransverse momentum.Cosmic background, relevant especially for the muon channel, was suppressed using timingcuts based on calorimeter measurements and the track impact parameter with respect tothe beam spot. Further cosmic background overlapping with ep interactions was rejectedby applying a cut E − p Z < GeV, E − p Z being the sum of the total and longitudinalenergy deposits of the cells in the calorimeter. For fully contained events, E − p Z is twicethe electron-beam energy and peaks at GeV.4vents from beam-gas interactions were rejected on the basis of the ratio of the number oftracks pointing to the vertex to the total number of tracks in an event.
A three-level trigger system was used to select events online [40]. At the first level, coarsecalorimeter and tracking information were available. Events were selected using criteriabased on either the transverse energy or missing transverse momentum measured in theCAL. Events were accepted with a low threshold on these quantities when a coincidencewith CTD tracks from the event vertex was found, while a higher threshold was used forevents with no CTD tracks.At the second level, timing information from the CAL was used to reject events inconsistentwith an ep interaction. In addition, the topology of the CAL energy deposits was usedto reject non- ep background events. In particular, a tighter cut was made on missingtransverse momentum, since the resolution in this variable was better at the second thanat the first level.At the third level, track reconstruction and vertex finding were performed and used to rejectevents with a vertex inconsistent with ep interactions. Cuts were applied to calorimeterquantities and reconstructed tracks to further reduce beam-gas contamination. Jets, used in the selection to define lepton isolation, were reconstructed from CAL cellsusing the k T cluster algorithm [41] in the longitudinally invariant inclusive mode [42] andwere corrected for energy loss due to the dead material in front of the CAL. The jets wererequired to have a transverse energy E jet T > . GeV and pseudorapidity | η jet | < . . Muon selection
Muons were reconstructed by matching calorimeter cell-patterns compatible with a minimum-ionising particle to CTD tracks [43]. Events were selected as follows: • | Z vtx | < cm, Z vtx being the Z coordinate of the interaction vertex, to restrict toa region compatible with ep interactions; • E − p Z > GeV. The E − p Z of the CAL deposit associated with the muon wasreplaced by that of the muon track. This requirement rejected photoproductionevents, which populate the low E − p Z region; • P miss T > GeV, P miss T being the missing transverse momentum measured by theCAL; • at least one muon candidate with the following characteristics: – a track from the primary vertex matched with a CTD track with at least threehit superlayers and a transverse momentum, p µT , greater than GeV;5 the distance, ∆ R , of the muon candidate in the pseudorapidity-azimuth ( η - φ )plane with respect to any other track and jet in the event satisfying ∆ R = p (∆ η ) + (∆ φ ) > . .A total of events were selected, while ± (stat.) were expected from the SM, whichis dominated by the dimuon production from the γγ process. The quoted uncertainty isthe error on the expected SM prediction due to the MC statistics.Figure 2 shows the comparison between data and MC for the variables p µT , θ µ , acoplanarity( φ acop ), P miss T , transverse mass ( M T ), hadronic transverse momentum ( P had T ). Here P had T , M T and φ acop are defined as follows:- P had T = p ( P i P iX ) + ( P i P iY ) , where P iX and P iY are the X and Y components ofthe CAL energy deposits not associated with the lepton;- M T = p p lT p νT (1 − cos φ lν ) , where p lT is the lepton transverse momentum, p νT is themodulus of the missing P T vector obtained from the CAL and corrected using trackinformation to account for muons, φ lν is the azimuthal separation between the leptonand the missing P T vector;- φ acop is the angle between the lepton and the vector balancing the P had T and is definedfor events with P had T greater than GeV.Reasonable agreement is observed in all cases.
Electron selection
Electrons were reconstructed using an algorithm that combined information from thecluster of the energy deposits in the calorimeter with tracks [44]. Events were selectedas follows: • | Z vtx | < cm; • < E − p Z < GeV, to reject NC DIS and photoproduction background; • P miss T > GeV; • at least one electron candidate with the following characteristics:- p el T > GeV;- . < θ el < rad;- isolated from other tracks and jets in the event, ∆ R > . ;- the extrapolation of the track associated with the electron into the CAL shouldhave a distance of closest approach to the CAL cluster centre < cm and areconstructed momentum p > GeV; • M T > GeV, to reject events with P miss T along the electron direction; • . < φ acop < ( π − . rad, to reject badly reconstructed NC DIS events with P miss T in the direction of the electron or of the jet.6 total of events were selected, while ± (stat.) were expected from the SM, whichis dominated by the NC DIS process. The quoted uncertainty is the error on the expectedSM prediction due to the MC statistics.Figure 3 shows the comparison between data and MC for the variables p el T , θ el , φ acop , P miss T , M T , P had T . Reasonable agreement is observed in all cases. Since no excess of events above the SM expectation was observed, a further selection wasmade to maximise the sensitivity to a possible FCNC single top signal. A cut on P had T of GeV was applied to both decay channels while the cuts on φ acop and P miss T were optimisedseparately for the two channels: • P had T > GeV for both channels; muon channel:– φ acop > . rad; – events with more than one isolated muon were rejected; electron channel:– φ acop > . rad; – P miss T > GeV.One event survived the selection cuts in the electron channel while three events were foundin the muon channel. Table 2 summarises the results of the final selection. In order tocompare the MC to data, the P had T cut was relaxed to GeV. Figures 4 (a) and (b) showthe P had T behaviour for data and SM expectations for the muon and electron channels,respectively. Good agreement between data and predictions is observed for both channels.Also shown are the expectations for top production through FCNC, normalised to the limiton the signal cross section obtained in Sect. 7.1. The data do not support a significantcontribution from this process. The following systematic uncertainties were taken into account: • the theoretical uncertainty on the W background normalisation was assumed to be ± [34]; • the statistical uncertainty on the total SM prediction after the final selection was ± and ± for the e - and µ -channel, respectively;7 the uncertainty on the NC DIS background, particularly relevant for the e -channel,was evaluated using a sample of events enriched in NC DIS by replacing the E − p Z and acoplanarity cuts by E − p Z > GeV and φ acop < . . A systematic uncertaintyof ± on this source was determined by the level of agreement between data andMC for such a selection. The effect of this uncertainty on the final selection SMprediction was ± for the e -channel and negligible for the µ -channel; • the uncertainty on the electromagnetic and the hadronic CAL energy scale was as-sumed to be ± and ± , respectively. The two scale uncertainties, summed inquadrature, produced a variation of ± and of ± on the final SM predictionsfor the e - and the µ -channel, respectively, while the effect on the signal selectionefficiencies was below and was therefore neglected; • the uncertainty on the top mass, . ± . GeV [17], produced a variation on theparameters of the signal cross section and decay widths as reported in Table 1 and avariation of ± on the signal selection efficiencies; • the uncertainties on the signal efficiency due to the statistics of the MC samples arereported in Table 3 for the different channels and decay processes; • the uncertainties on the PDFs gave a variation on the parameters of the signal crosssection as reported in Table 1. Such uncertainties were evaluated as suggested by theCTEQ group [18]; • the uncertainty due to the QCD factorisation-scale affected the signal cross section by ± for the LO calculation and by +8% − including the approximated NLO and NNLOQCD corrections (see Sect. 2). This effect was evaluated by varying the central value,set to M t , between M t / and M t ; • the uncertainty on the luminosity determination was ± . .The uncertainties due to the W normalisation, CAL energy scale, top mass, PDFs andluminosity were assumed to be correlated for the different channels and datasets. All theabove uncertainties were included in the limit calculation as explained in Sect. 7.1. Since no excess over the SM prediction was observed, limits on FCNC couplings of thetype tuV were evaluated using the results of Table 2. As a first step, limits were evaluatedon the signal cross section and on the κ γ coupling assuming v Z = 0 . In a second step, theeffect of a non-zero v Z coupling was accounted for. Limits on the anomalous top branchingratios, Br( t → uγ ) (Br uγ ) and Br( t → uZ ) (Br uZ ), were evaluated.8 .1 Limits on the cross section and κ γ The limit on the anomalous top-production cross section was evaluated using a Bayesianapproach and assuming a constant prior in the cross section, σ : f ( σ | data) = Q i P ( N obs i | σ ) f ( σ ) R ∞ Q i P ( N obs i | σ ) f ( σ ) dσ , (4) P ( N obs i | σ ) = µ N obs i i e − µ i N obs i ! , (5) µ i = N sig i + N bg i ,N sig i = σ L i ǫ i , where f ( σ | data) is the posterior probability density function (p.d.f.) of the signal crosssection, f ( σ ) its prior, i runs over the different channels and datasets, N obs i is the numberof events surviving the event selection, N sig i and N bg i are the number of signal events andthe expected SM background, L i is the integrated luminosity and ǫ i the signal efficiency in-cluding branching ratio for each decay channel (see the first row in Table 3). The branchingratio of the top to uγ was taken into account in the limits evaluation, the selection effi-ciency for such channel is expected to be low and was therefore set to zero. The systematicuncertainties were treated as nuisance parameters (NPs) and included in the limit calcula-tion, integrating out their dependence (marginalisation) assuming Gaussian priors . Themarginalisation over the NPs and the extraction of the posterior p.d.f. was performed usingthe package Bayesian Analysis Toolkit [45], which carries out multidimensional integrationusing the Markov Chain Monte Carlo technique.The
Credibility Level (C.L.) limit on the cross section was evaluated by integratingthe posterior p.d.f. σ Z f ( σ | data) dσ = 0 . , (6)and found to be σ < . pb (95% C . L . ) at √ s = 318 GeV . (7)The limit on the cross section was converted into a limit on the coupling κ γ , assuming avanishing v Z coupling and using the A σ parameter described in Sect. 2 taking into accountthe approximated NLO and NNLO QCD corrections (see Sect. 2): κ γ < .
17 (95% C . L . ) . (8)The limit is similar to that obtained by ZEUS from HERA I data [46] with an integratedluminosity of . fb − . In the HERA I data, no events were found in either the electron In case of unphysical values, the Gaussian priors were truncated.
9r muon channel and also the hadronic W -decay channel was exploited.The present result was combined with the HERA I limit for a total integrated luminosityof . fb − , using the same Bayesian approach as described above and assuming full cor-relation for the systematic uncertainties due to the W normalisation, CAL energy scale,top mass and PDFs.The combined cross-section and κ γ limits are: σ < . pb (95% C . L . ) at √ s = 315 GeV , (9) κ γ < .
12 (95% C . L . ) . (10)The combined cross-section limit corresponds to a centre-of-mass energy of GeV sincepart of the HERA I data was collected at √ s = 300 GeV.
Following the Bayesian approach described above, a two-dimensional posterior p.d.f., f (Br uγ , Br uZ | data) , (11)was evaluated combining the HERA I and HERA II datasets. Such a p.d.f. was built usingthe parameters described in Sect. 2 (no higher-order QCD corrections were applied in thiscase) to express the FCNC cross-section in terms of the anomalous top branching ratios.The signal efficiencies for the different production channels ( γ - or Z -mediated) and decaymodes ( bW or uZ ) were taken into account (see Table 3). The selection efficiency of the e -channel is larger for the Z -mediated process than the γ -mediated process, since in thiscase the final-state electron is scattered at a larger angle and is more often visible in thedetector.The decay channel t → uγ was not simulated since the branching ratio is very low for therange of couplings under consideration. In addition, the selection efficiency is expected tobe low for such events and was therefore set to zero.The
95% C . L . boundary in the (Br uγ , Br uZ ) plane was evaluated as the set of points f (Br uγ , Br uZ | data) = ρ , where ρ was chosen such that Z Z f (Br uγ , Br uZ | data) >ρ d Br uγ d Br uZ f (Br uγ , Br uZ | data) = 0 . . (12)Figure 5 shows the ZEUS boundary in the ( Br uγ , Br uZ ) plane compared to limits fromH1 [47] and from experiments at other colliders: ALEPH [10] at LEP (other LEP exper-iments [11] have similar results), CDF [7] and D0 [8] at Tevatron. The e + e − and hadroncolliders, contrary to HERA, have similar sensitivity to u - and c -quark; their limits arehence on both decays t → qV with q = u, c . The limits set by the ZEUS experiment in theregion where Br uZ is less than are the best to date.10 Conclusions
A search for possible deviations from the Standard Model predictions due to flavour-changing neutral current top production in events with high- p T leptons and high missingtransverse momentum was performed using an integrated luminosity of . fb − , collectedby the ZEUS detector in 2004–2007. Since no significant deviation from the expectationwas observed, the results were used to put limits on the anomalous production of singletop quarks at HERA.A credibility-level upper limit on the cross section of σ < . pb at a centre-of-mass energy of GeV was obtained. The limit was combined with a previous ZEUSresult, obtained using HERA I data, for a total integrated luminosity of . fb − , givinga combined credibility-level upper limit of σ < . pb at √ s = 315 GeV. Thislimit, assuming a vanishing coupling of the top quark to the Z boson, v Z , correspondsto a constraint on the coupling of the top to the γ , κ γ , of κ γ < . . Constraints onthe anomalous top branching ratios t → uγ and t → uZ were also evaluated assuming anon-zero v Z . For low values of v Z , resulting in branching ratios of t → uZ of less than ,this paper provides the current best limits. Acknowledgements
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Parameters used to evaluate single-top production cross sections and decaywidths for the different channels. The systematic effects due to the uncertainties on thetop mass and the parton distribution functions are also reported. N obs N pred W [%] electron channel e + p ± ± e + p ± ± e − p ± ± e − p ± ± ep ± ± ep ± ± Table 2:
Number of events passing the final selection cuts, N obs , compared to the SMprediction, N pred . The last column shows the W contribution as a percentage of the totalSM expectation. The uncertainties have been obtained by adding systematic and statisticalcontributions in quadrature.coupling decay e − channel µ − channel ǫ ∆ ǫ/ǫ ǫ ∆ ǫ/ǫκ γ t → bW . ± .
04 0 . ± . κ γ t → uZ . ± .
08 0 . ± . v Z t → bW . ± .
04 0 . ± . v Z t → uZ . ± .
03 0 . ± . Table 3:
Summary of selection efficiencies on signal samples for different production coup-lings and decay modes. The relative errors are due to the statistics of the MC samples.15 igure 1:
Anomalous single-top production via flavour changing neutral current trans-itions at HERA with subsequent decays t → bW + and W + → ν e ( ν µ ) e + ( µ + ) . EUS (GeV) µ T p
10 20 30 40 50 60 70 80 E ve n t s -1 -1 ZEUS 0.37 fbSMW (rad) µ θ E ve n t s -1 (rad) acop φ E ve n t s -1 (GeV) missT P E ve n t s -1 (GeV) T M E ve n t s -1 (GeV) hadT P E ve n t s -1 Figure 2:
Comparison between data and SM expectations for several variables in the muonchannel: p µT , θ µ , φ acop , P miss T , M T , P had T . The contribution of single- W production is alsoshown as the dark-shaded region. Any histogram overflows are included in the last bin. EUS (GeV) elT p E ve n t s -1 -1 ZEUS 0.37 fbSMW (rad) el θ E ve n t s -1 acop φ E ve n t s -1 (GeV) missT P E ve n t s -1 (GeV) T M E ve n t s -1 (GeV) hadT P E ve n t s -1 Figure 3:
Comparison between data and SM expectations for several variables in the elec-tron channel: p el T , θ el , φ acop , P miss T , M T , P had T . The contribution of single- W production isalso shown as the dark-shaded region. The last bin of the φ acop histogram contains eventswith P had T less than GeV for which φ acop was not evaluated. In the other cases, anyoverflows are included in the last bin. EUS (GeV) hadT P
10 20 30 40 50 60 70 80 90 100 E ve n t s -1 -channel µ (a) -1 ZEUS 0.37 fbSMWsingle top (GeV) hadT P
10 20 30 40 50 60 70 80 90 100 E ve n t s -1 (b) e-channel Figure 4:
Comparison between data and MC expectations for the P had T distribution ap-plying the final selection with a relaxed P had T cut at GeV for (a) the muon and (b) theelectron channel. The dots are the data, the solid histogram is the SM prediction includingthe W contribution, the dotted histogram the W contribution alone and the dashed histo-gram the single-top distribution normalized to the limit on the signal cross section of . pb(see Sect. 7.1). The final selection cut, P had T > GeV , is indicated. u Br -3 -2 -1 uZ B r -2 -1 exc l ud e d b y Z E U S ZEUS -1 ZEUS 0.5 fb ,u(c)Z γ u(c) → ALEPH t ,u(c)Z γ u(c) → CDF t γ u → H1 t u(c)Z → D0 t γ u Br -3 -2 -1 uZ B r -2 -1 Figure 5:
ZEUS boundary in the ( Br uγ , Br uZ ) plane. Also shown are boundaries of H1 [47] , CDF [7] , D0 [8] and
ALEPH [10] . The shaded area is excluded. The dark shadedregion denotes the area uniquely excluded by ZEUS.. The shaded area is excluded. The dark shadedregion denotes the area uniquely excluded by ZEUS.