A Search for Excited Neutrinos in e-p Collisions at HERA
aa r X i v : . [ h e p - e x ] F e b DESY 08-009 ISSN 0418-9833January 2008
A Search for Excited Neutrinos in e − p Collisions at HERA
H1 Collaboration
Abstract
A search for excited neutrinos is performed using the full e − p data sample collected bythe H1 experiment at HERA at a centre-of-mass energy of GeV, corresponding to a to-tal luminosity of pb − . The electroweak decays of excited neutrinos ν ∗ → νγ , ν ∗ → νZ and ν ∗ → eW with subsequent hadronic or leptonic decays of the W and Z bosons are con-sidered. No evidence for excited neutrino production is found. Mass dependent exclusionlimits on ν ∗ production cross sections and on the ratio of the coupling to the compositenessscale f / Λ are derived within gauge mediated models. A limit on f / Λ , independent of therelative couplings to the SU( ) and U( ) gauge bosons, is also determined. These limitsextend the excluded region to higher masses than has been possible in previous excitedneutrino searches. Submitted to
Phys. Lett. B .D. Aaron , , C. Alexa , V. Andreev , B. Antunovic , S. Aplin , A. Asmone ,A. Astvatsatourov , S. Backovic , A. Baghdasaryan , P. Baranov , † , E. Barrelet ,W. Bartel , S. Baudrand , M. Beckingham , K. Begzsuren , O. Behnke , A. Belousov ,N. Berger , J.C. Bizot , M.-O. Boenig , V. Boudry , I. Bozovic-Jelisavcic , J. Bracinik ,G. Brandt , M. Brinkmann , V. Brisson , D. Bruncko , A. Bunyatyan , , G. Buschhorn ,L. Bystritskaya , A.J. Campbell , K.B. Cantun Avila , F. Cassol-Brunner , K. Cerny ,V. Cerny , , V. Chekelian , A. Cholewa , J.G. Contreras , J.A. Coughlan , G. Cozzika ,J. Cvach , J.B. Dainton , K. Daum , , M. De´ak , Y. de Boer , B. Delcourt ,M. Del Degan , J. Delvax , A. De Roeck , , E.A. De Wolf , C. Diaconu , V. Dodonov ,A. Dossanov , A. Dubak , , G. Eckerlin , V. Efremenko , S. Egli , F. Eisele ,A. Eliseev , E. Elsen , S. Essenov , A. Falkiewicz , P.J.W. Faulkner , L. Favart ,A. Fedotov , R. Felst , J. Feltesse , , J. Ferencei , L. Finke , M. Fleischer ,A. Fomenko , G. Franke , T. Frisson , E. Gabathuler , J. Gayler , S. Ghazaryan ,A. Glazov , I. Glushkov , L. Goerlich , M. Goettlich , N. Gogitidze , M. Gouzevitch ,C. Grab , T. Greenshaw , B.R. Grell , G. Grindhammer , S. Habib , , D. Haidt ,M. Hansson , C. Helebrant , R.C.W. Henderson , H. Henschel , G. Herrera ,M. Hildebrandt , K.H. Hiller , D. Hoffmann , R. Horisberger , A. Hovhannisyan ,T. Hreus , , M. Jacquet , M.E. Janssen , X. Janssen , V. Jemanov , L. J¨onsson ,D.P. Johnson , † , A.W. Jung , H. Jung , M. Kapichine , J. Katzy , I.R. Kenyon ,C. Kiesling , M. Klein , C. Kleinwort , T. Klimkovich, T. Kluge , A. Knutsson ,R. Kogler , V. Korbel , P. Kostka , M. Kraemer , K. Krastev , J. Kretzschmar ,A. Kropivnitskaya , K. Kr¨uger , K. Kutak , M.P.J. Landon , W. Lange ,G. Laˇstoviˇcka-Medin , P. Laycock , A. Lebedev , G. Leibenguth , V. Lendermann ,S. Levonian , G. Li , K. Lipka , A. Liptaj , B. List , J. List , N. Loktionova ,R. Lopez-Fernandez , V. Lubimov , A.-I. Lucaci-Timoce , L. Lytkin , A. Makankine ,E. Malinovski , P. Marage , Ll. Marti , H.-U. Martyn , S.J. Maxfield , A. Mehta ,K. Meier , A.B. Meyer , H. Meyer , H. Meyer , J. Meyer , V. Michels , S. Mikocki ,I. Milcewicz-Mika , F. Moreau , A. Morozov , J.V. Morris , M.U. Mozer , M. Mudrinic ,K. M ¨uller , P. Mur´ın , , K. Nankov , B. Naroska , Th. Naumann , P.R. Newman ,C. Niebuhr , A. Nikiforov , G. Nowak , K. Nowak , M. Nozicka , B. Olivier ,J.E. Olsson , S. Osman , D. Ozerov , V. Palichik , I. Panagoulias l, , , M. Pandurovic ,Th. Papadopoulou l, , , C. Pascaud , G.D. Patel , O. Pejchal , H. Peng , E. Perez , ,A. Petrukhin , I. Picuric , S. Piec , D. Pitzl , R. Plaˇcakyt˙e , R. Polifka , B. Povh ,T. Preda , V. Radescu , A.J. Rahmat , N. Raicevic , A. Raspiareza , T. Ravdandorj ,P. Reimer , C. Risler , E. Rizvi , P. Robmann , B. Roland , R. Roosen , A. Rostovtsev ,M. Rotaru , J.E. Ruiz Tabasco , Z. Rurikova , S. Rusakov , D. Salek , F. Salvaire ,D.P.C. Sankey , M. Sauter , E. Sauvan , S. Schmidt , S. Schmitt , C. Schmitz ,L. Schoeffel , A. Sch¨oning , H.-C. Schultz-Coulon , F. Sefkow , R.N. Shaw-West ,I. Sheviakov , L.N. Shtarkov , T. Sloan , I. Smiljanic , P. Smirnov , Y. Soloviev ,D. South , V. Spaskov , A. Specka , Z. Staykova , M. Steder , B. Stella , U. Straumann ,D. Sunar , T. Sykora , V. Tchoulakov , G. Thompson , P.D. Thompson , T. Toll ,F. Tomasz , T.H. Tran , D. Traynor , T.N. Trinh , P. Tru¨ol , I. Tsakov ,B. Tseepeldorj , , I. Tsurin , J. Turnau , E. Tzamariudaki , K. Urban , A. Valk´arov´a ,C. Vall´ee , P. Van Mechelen , A. Vargas Trevino , Y. Vazdik , S. Vinokurova ,V. Volchinski , D. Wegener , M. Wessels , Ch. Wissing , R. Wolf , E. W¨unsch ,V. Yeganov , J. ˇZ´aˇcek , J. Z´aleˇs´ak , Z. Zhang , A. Zhelezov , A. Zhokin , Y.C. Zhu ,1. Zimmermann , H. Zohrabyan , and F. Zomer I. Physikalisches Institut der RWTH, Aachen, Germany a Vinca Institute of Nuclear Sciences, Belgrade, Serbia School of Physics and Astronomy, University of Birmingham, Birmingham, UK b Inter-University Institute for High Energies ULB-VUB, Brussels; Universiteit Antwerpen,Antwerpen; Belgium c National Institute for Physics and Nuclear Engineering (NIPNE) , Bucharest, Romania Rutherford Appleton Laboratory, Chilton, Didcot, UK b Institute for Nuclear Physics, Cracow, Poland d Institut f¨ur Physik, TU Dortmund, Dortmund, Germany a Joint Institute for Nuclear Research, Dubna, Russia CEA, DSM/DAPNIA, CE-Saclay, Gif-sur-Yvette, France DESY, Hamburg, Germany Institut f¨ur Experimentalphysik, Universit¨at Hamburg, Hamburg, Germany a Max-Planck-Institut f¨ur Kernphysik, Heidelberg, Germany Physikalisches Institut, Universit¨at Heidelberg, Heidelberg, Germany a Kirchhoff-Institut f¨ur Physik, Universit¨at Heidelberg, Heidelberg, Germany a Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovak Republic f Department of Physics, University of Lancaster, Lancaster, UK b Department of Physics, University of Liverpool, Liverpool, UK b Queen Mary and Westfield College, London, UK b Physics Department, University of Lund, Lund, Sweden g CPPM, CNRS/IN2P3 - Univ. Mediterranee, Marseille - France Departamento de Fisica Aplicada, CINVESTAV, M´erida, Yucat´an, M´exico j Departamento de Fisica, CINVESTAV, M´exico j Institute for Theoretical and Experimental Physics, Moscow, Russia Lebedev Physical Institute, Moscow, Russia e Max-Planck-Institut f¨ur Physik, M¨unchen, Germany LAL, Univ Paris-Sud, CNRS/IN2P3, Orsay, France LLR, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France LPNHE, Universit´es Paris VI and VII, IN2P3-CNRS, Paris, France Faculty of Science, University of Montenegro, Podgorica, Montenegro e Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic h Faculty of Mathematics and Physics, Charles University, Praha, Czech Republic h Dipartimento di Fisica Universit`a di Roma Tre and INFN Roma 3, Roma, Italy Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria e Institute of Physics and Technology of the Mongolian Academy of Sciences , Ulaanbaatar,Mongolia Paul Scherrer Institut, Villigen, Switzerland Fachbereich C, Universit¨at Wuppertal, Wuppertal, Germany Yerevan Physics Institute, Yerevan, Armenia DESY, Zeuthen, Germany Institut f¨ur Teilchenphysik, ETH, Z¨urich, Switzerland i Physik-Institut der Universit¨at Z¨urich, Z¨urich, Switzerland i Also at Physics Department, National Technical University, Zografou Campus, GR-15773Athens, Greece Also at Rechenzentrum, Universit¨at Wuppertal, Wuppertal, Germany Also at University of P.J. ˇSaf´arik, Koˇsice, Slovak Republic Also at CERN, Geneva, Switzerland Also at Max-Planck-Institut f¨ur Physik, M¨unchen, Germany Also at Comenius University, Bratislava, Slovak Republic Also at DESY and University Hamburg, Helmholtz Humboldt Research Award Also at Faculty of Physics, University of Bucharest, Bucharest, Romania Supported by a scholarship of the World Laboratory Bj¨orn Wiik Research Project Also at Ulaanbaatar University, Ulaanbaatar, Mongolia † Deceased a Supported by the Bundesministerium f¨ur Bildung und Forschung, FRG, under contractnumbers 05 H1 1GUA /1, 05 H1 1PAA /1, 05 H1 1PAB /9, 05 H1 1PEA /6, 05 H1 1VHA /7 and05 H1 1VHB /5 b Supported by the UK Particle Physics and Astronomy Research Council, and formerly by theUK Science and Engineering Research Council c Supported by FNRS-FWO-Vlaanderen, IISN-IIKW and IWT and by Interuniversity AttractionPoles Programme, Belgian Science Policy d Partially Supported by Polish Ministry of Science and Higher Education, grantPBS/DESY/70/2006 e Supported by the Deutsche Forschungsgemeinschaft f Supported by VEGA SR grant no. 2/7062/ 27 g Supported by the Swedish Natural Science Research Council h Supported by the Ministry of Education of the Czech Republic under the projects LC527 andINGO-1P05LA259 i Supported by the Swiss National Science Foundation j Supported by CONACYT, M´exico, grant 48778-F l This project is co-funded by the European Social Fund (75%) and National Resources (25%)- (EPEAEK II) - PYTHAGORAS II Introduction
The three-family structure and mass hierarchy of the known fermions is one of the most puz-zling characteristics of the Standard Model (SM) of particle physics. Attractive explanationsare provided by models assuming composite quarks and leptons [1]. The existence of excitedstates of leptons and quarks is a natural consequence of these models and their discovery wouldprovide convincing evidence of a new scale of matter. Electron-proton interactions at very highenergies provide a good environment in which to search for excited states of first generationfermions. In particular, excited neutrinos ( ν ∗ ) could be singly produced through the exchangeof a W boson in the t -channel.In this letter a search for excited neutrinos using the complete e − p HERA collider data ofthe H1 experiment is presented. Electroweak decays into a SM lepton ( e , ν e ) and a SM gaugeboson ( γ , W and Z ) are considered and hadronic as well as leptonic decays of the W and Z areanalysed.The data, collected at electron and proton beam energies of . GeV and
GeV, respec-tively, correspond to a total integrated luminosity of pb − . During the HERA II runningperiod, the electron beam was longitudinally polarised, at a level of typically . For thisanalysis the periods with left-handed and right-handed beams are combined and the analyseddata sample has a residual polarisation of left-handed. With more than a ten-fold increasein statistics, this analysis supersedes the result of the previous H1 search for excited neutrinosbased on a data sample corresponding to a luminosity of pb − [2]. In the present study a model [3–5] is considered in which excited fermions are assumed tohave spin / and isospin / . Both left-handed and right-handed components of the excitedfermions form weak iso-doublets F ∗ L and F ∗ R . In order to prevent the light leptons from radia-tively acquiring a large anomalous magnetic moment [6, 7], only the right-handed componentof the excited fermions takes part in the generalised magnetic de-excitation. The interaction be-tween excited fermions, gauge bosons and ordinary fermions is then described by the effectiveLagrangian [4]: L int. = 12Λ ¯ F ∗ R σ µν (cid:20) gf τ a W aµν + g ′ f ′ Y B µν + g s f s λ a G aµν (cid:21) F L + h.c. . (1)The matrix σ µν is the covariant bilinear tensor, W aµν , B µν and G aµν are the field-strengthtensors of the SU( ), U( ) and SU( ) C gauge fields, τ a , Y and λ a are the Pauli matrices, theweak hypercharge operator and the Gell-Mann matrices, respectively. The standard electroweakand strong gauge couplings are denoted by g , g ′ and g s , respectively. The parameter Λ hasunits of energy and can be regarded as the compositeness scale which reflects the range of anew confinement force. The constants f , f ′ and f s are form factors associated to the threegauge groups. They can be interpreted as parameters setting different scales Λ i = Λ /f i for the4ifferent gauge groups, thus allowing the composite fermion to have arbitrary coupling strengthswith the three gauge bosons.Following this model, single production of excited neutrinos in ep collisions may result fromthe t -channel exchange of a W boson. Due to the helicity dependence of the weak interactionand given the valence quark composition and density distribution of the proton, the ν ∗ produc-tion cross section is predicted to be much larger for e − p collisions than for e + p . For a ν ∗ mass M ν ∗ of GeV the ratio of the production cross sections is of order . The ν ∗ productioncross section is expected to scale linearly with the polarisation of the incident electron beam,similarly to the SM charged current process. The excited neutrino may decay into a leptonand an electroweak gauge boson via ν ∗ → νγ , ν ∗ → eW and ν ∗ → νZ . For a given M ν ∗ valueand assuming a numerical relation between f and f ′ , the ν ∗ branching ratios are fixed and theproduction cross section depends only on f / Λ . The ν ∗ is expected not to have strong interac-tions and therefore this search is insensitive to f s . Two complementary coupling assignments f = + f ′ and f = − f ′ are studied in detail. For f = + f ′ the excited neutrino has no tree-levelelectromagnetic coupling [8] and therefore the photonic decay of the ν ∗ is forbidden whereasfor f = − f ′ decays into νγ , νZ and eW are allowed. In addition, arbitrary ratios of f ′ /f areconsidered in the range − to +5 . A Monte Carlo (MC) program developed for this analysis is used for the calculation of the ν ∗ production cross section and the simulation of signal events. The events are simulated usingthe cross section calculated from the Lagrangian described in equation (1) using the CompHEPpackage [9]. Initial state radiation from the incident electron is included using the Weizs¨acker-Williams approximation [10]. The proton parton densities are taken from the CTEQ5L [11]parametrisation and are evaluated at the scale p Q , where Q is the four-momentum transfersquared. The parton shower approach [12] is applied to simulate Quantum Chromodynamics(QCD) corrections in the initial and final states. The hadronisation is performed using Lundstring fragmentation as implemented in PYTHIA [12].In the MC generator the full transition matrix including the production and the decay isimplemented. This is important if the natural width of the ν ∗ is large, which is typically the caseat high mass where factorisation of the ν ∗ production and its decay no longer holds. Events usedin the determination of signal efficiencies are generated with the coupling f / Λ corresponding,for each ν ∗ mass, to the expected boundary of the probed domain in the plane defined by M ν ∗ and f / Λ .The Standard Model background processes that may mimic a ν ∗ signal are neutral current(NC) and charged current (CC) deep-inelastic scattering (DIS) and to a lesser extent photopro-duction, lepton pair production and real W boson production.The RAPGAP [13] event generator, which implements the Born, QCD Compton and BosonGluon Fusion matrix elements, is used to model NC DIS events. The QED radiative effectsarising from real photon emission from both the incoming and outgoing electrons are simulatedusing the HERACLES [14] program. Direct and resolved photoproduction of jets and prompt5hoton production are simulated using the PYTHIA event generator. The simulation is basedon Born level hard scattering matrix elements with radiative QED corrections. In RAPGAP andPYTHIA, jet production from higher order QCD radiation is simulated using leading logarith-mic parton showers and hadronisation is modelled with Lund string fragmentation. The leadingorder MC prediction of NC DIS and photoproduction processes with two or more high trans-verse momentum jets is scaled by a factor of . in order to normalise to next-to-leading orderQCD calculations [15]. Charged current DIS events are simulated using the DJANGO [16] pro-gram, which includes first order leptonic QED radiative corrections based on HERACLES. Theproduction of two or more jets in DJANGO is accounted for using the colour-dipole-model [17].Contributions from elastic and quasi-elastic QED Compton scattering are simulated withthe WABGEN [18] generator. Contributions arising from the production of W bosons andmulti-lepton events are modelled using the EPVEC [19] and GRAPE [20] event generators,respectively.All processes are generated with an integrated luminosity significantly higher than that ofthe data sample. Generated events are passed through the full GEANT [21] based simulation ofthe H1 apparatus, which takes into account the running conditions of the different data takingperiods, and are reconstructed and analysed using the same program chain as for the data. A detailed description of the H1 experiment can be found in [22]. Only the detector compo-nents relevant to the present analysis are briefly described here. The origin of the H1 coordinatesystem is the nominal ep interaction point, with the direction of the proton beam defining thepositive z -axis (forward region). Transverse momentum ( P T ) is measured in the xy plane. Thepseudorapidity η is related to the polar angle θ by η = − ln tan( θ/ . The Liquid Argon (LAr)calorimeter [23] is used to measure electrons, photons and hadrons. It covers the polar anglerange ◦ < θ < ◦ with full azimuthal acceptance. Electromagnetic shower energies aremeasured with a precision of σ ( E ) /E = 12% / p E/ GeV ⊕ and hadronic energies with σ ( E ) /E = 50% / p E/ GeV ⊕ , as measured in test beams [24, 25]. In the backward re-gion, energy measurements are provided by a lead/scintillating-fiber (SpaCal) calorimeter [26]covering the angular range ◦ < θ < ◦ . The central ( ◦ < θ < ◦ ) and forward( ◦ < θ < ◦ ) tracking detectors are used to measure charged particle trajectories, to re-construct the interaction vertex and to complement the measurement of hadronic energy. TheLAr and inner tracking detectors are enclosed in a super-conducting magnetic coil with a fieldstrength of . T. The return yoke of the coil is the outermost part of the detector and isequipped with streamer tubes forming the central muon detector ( ◦ < θ < ◦ ). In the for-ward region of the detector ( ◦ < θ < ◦ ) a set of drift chambers detects muons and measurestheir momenta using an iron toroidal magnet. The luminosity is determined from the rate ofthe Bethe-Heitler process ep → epγ , measured using a photon detector located close to the beampipe at z = −
103 m , in the backward direction.6
Data Analysis
The triggers used in this analysis are based on the detection of energy deposits in the LArcalorimeter [27]. Events containing an electromagnetic deposit (electron or photon) with anenergy greater than GeV are triggered with an efficiency close to %. For events withmissing transverse energy above GeV, the trigger efficiency is ∼ %.In order to remove background events induced by cosmic showers and other non- ep sources,the event vertex is required to be reconstructed within cm in z of the nominal interactionpoint. In addition, topological filters and timing vetoes are applied.The identification of electrons or photons relies on the measurement of a compact and iso-lated electromagnetic shower in the LAr calorimeter. In addition, the hadronic energy withina distance in the pseudorapidity-azimuth ( η − φ ) plane R = p ∆ η + ∆ φ < . around theelectron (photon) is required to be below % of the electron (photon) energy. Muon iden-tification is based on a track measured in the inner tracking systems associated with signalsin the muon detectors [28]. A muon candidate should have no more than GeV depositedin a cylinder, centred on the muon track direction, of radius cm and cm in the elec-tromagnetic and hadronic sections of the LAr calorimeter, respectively. Calorimeter energydeposits and tracks not previously identified as electron, photon or muon candidates are used toform combined cluster-track objects, from which the hadronic energy is reconstructed [29, 30].Jets are reconstructed from these combined cluster-track objects using an inclusive k T algo-rithm [31, 32] with a minimum transverse momentum of . GeV. The missing transverse mo-mentum P miss T of the event is derived from all identified particles and energy deposits in theevent. The P miss T is assumed to originate from a single neutrino. The four-vector of this neu-trino candidate is reconstructed assuming transverse momentum conservation and the relation P i ( E i − P iz ) + ( E ν − P νz ) = 2 E e = 55 . GeV, where the sum runs over all detected particles; P z is the momentum along the beam axis and E e is the electron beam energy.Specific selection criteria applied in each decay channel are presented in the following sub-sections. A detailed description of the analysis can be found in [33]. νγ Resonance Search
The signature of the decay channel ν ∗ → νγ consists of an isolated electromagnetic clusterin events with missing transverse momentum. Background arises from CC DIS events withan isolated π or a radiated photon. Events with substantial missing transverse momentum P miss T > GeV are selected. In each event, a photon candidate with transverse momentum P γT > GeV in a polar angle range ◦ < θ γ < ◦ is required. This polar angle range isrestricted to θ γ < ◦ in events with P miss T below GeV, in order to reduce background fromNC DIS. The photon is required to be isolated from jets by a distance
R > . to any jet axis.In the central region ( θ γ > ◦ ), photon candidates are selected only if no well measured trackpoints to the electromagnetic cluster within a distance of closest approach (DCA) of cm. Forevents with P miss T below GeV, this condition is tightened by accepting only photon candi-dates having no track with a DCA to the cluster below cm or within R < . . The energyand polar angle of the photon are combined into one discriminant variable ξ γ = E γ cos ( θ γ / .7adiative CC DIS events are suppressed by requiring that ξ γ > GeV. For signal events, inmost cases the final state contains a recoil jet, due to ν ∗ production via t -channel W bosonexchange. Hence, in the final selection the presence of at least one jet with P jet T > GeV isalso required.Seven events are selected in the data, compared to a SM expectation of . ± . , which isdominated by CC DIS events. The invariant mass of the excited neutrino candidate is calculatedfrom the four-vectors of the neutrino and the photon. The invariant mass distributions of the ν ∗ candidates and of the expected SM background are presented in figure 1(a). The resultingselection efficiency is % for M ν ∗ = 120 GeV, increasing to % for M ν ∗ = 260 GeV. FromMonte Carlo studies, the total width of the reconstructed ν ∗ mass distribution is GeV for agenerated ν ∗ mass of GeV, increasing to GeV for a ν ∗ mass of GeV. νq ¯ q Resonance Search
The signature of the ν ∗ → νZ → νq ¯ q decay channel consists of two jets with high transversemomentum in events with large P miss T . The SM background is dominated by multi-jet CCDIS events and contains a moderate contribution from photoproduction. Events with missingtransverse momentum P miss T > GeV are selected. In each event at least two jets are requiredin the polar angle range ◦ < θ jet < ◦ with transverse momenta larger than and GeV,respectively. Additionally, the hadronic final state must exhibit a polar angle γ h , as definedin [34], larger than ◦ , in order to remove photoproduction events. Events with P miss T < GeVare selected if the ratio V ap /V p of transverse energy flow anti-parallel and parallel to the hadronicfinal state [34] is above . . This condition reduces the contribution of CC DIS processes.Photoproduction and NC DIS backgrounds typically have low volues of x h , the Bjorken scalingvariable calculated from the hadronic system using the Jacquet-Blondel method [34, 35], andare thus suppressed by requiring x h > . . Finally, to further reduce the background from CCDIS, a jet multiplicity greater than or equal to three is required for events with P miss T < GeV.In each event, a Z candidate is reconstructed from the combination of the two jets with aninvariant mass closest to the nominal Z boson mass. The reconstructed Z candidate is requiredto have an invariant mass above GeV.After this selection, events are found in the data compared to a SM expectation of ± events. The invariant mass of the ν ∗ candidate is calculated from the neutrino and Z candidatefour-vectors. For this calculation, the Z candidate four-vector is scaled such that its mass is setequal to the nominal Z boson mass. The invariant mass distributions of the ν ∗ candidates andof the expected SM background are presented in figure 1(b). The selection efficiency in thischannel is % for M ν ∗ = 120 GeV, increasing to % for M ν ∗ = 260 GeV. From Monte Carlostudies, the total width of the reconstructed ν ∗ mass distribution is GeV for a generated ν ∗ mass of GeV, increasing to GeV for a ν ∗ mass of GeV. eq ¯ q Resonance Search
The signature of the ν ∗ → eW → eq ¯ q decay channel consists of one electron and two high P T jets. Multi-jet NC DIS events constitute the main background contribution from SM processes.8vents are selected with an isolated electron in the LAr calorimeter in the polar angle range ◦ < θ e < ◦ . The electron variable ξ e = E e cos ( θ e / is required to be above GeVor the electron should have a transverse momentum P eT greater than GeV. These conditionsremove a large part of the NC DIS contribution. In addition, the electron should be isolatedfrom jets by a distance
R > . to any jet axis. The events are required to have at least two jetsin the polar angle range ◦ < θ jet < ◦ with transverse momenta larger than and GeV,respectively. In each event, a W candidate is reconstructed from the combination of the two jetswith an invariant mass closest to the nominal W boson mass. The reconstructed mass of the W candidate is required to be larger than GeV. To further reduce the NC DIS background itis required that the polar angle of the jet with the highest P T associated to the W candidate beless than ◦ and that events with P eT < GeV contain at least three jets with a P T larger than GeV.After the selection, events are observed compared to a SM expectation of ± .The invariant mass of the ν ∗ candidate is calculated from the electron and W candidate four-vectors. For this calculation, the W candidate four-vector is scaled such that its mass is setequal to the nominal W boson mass. The invariant mass distributions of the ν ∗ candidates andof the expected SM background are presented in figure 1(c). The selection efficiency in thischannel is % for M ν ∗ = 120 GeV, increasing to % for M ν ∗ = 260 GeV. From Monte Carlostudies, the total width of the reconstructed ν ∗ mass distribution is GeV for a generated ν ∗ mass of GeV, increasing to GeV for a ν ∗ mass of GeV. eνµ and eνe
Resonance Searches
In the search for ν ∗ → eW → eνµ , events with P miss T > GeV, one electron with P eT > GeVand one muon with P µT > GeV are selected. The electron and the muon have to be detectedin the polar angle ranges ◦ < θ e < ◦ and ◦ < θ µ < ◦ , respectively. Furthermore,the electron and the muon must be isolated from jets by minimum distances of R e > . and R µ > , respectively. The contribution from NC DIS processes is reduced by requiring ξ e > GeV. After this selection no data event remains, while . ± . SM backgroundevents are expected. The selection efficiency for ν ∗ with masses above GeV is ∼ %.The signatures of the ν ∗ → eW → eνe and ν ∗ → νZ → νee channels are similar and consistof two high P T electrons in events with large missing transverse momentum. Events with P miss T > GeV are selected. In each event two isolated electromagnetic clusters are required,with a transverse momentum larger than and GeV, respectively. The highest P T electronshould be detected in the polar angle range ◦ < θ e < ◦ and the second electron in therange ◦ < θ e < ◦ . To reduce the background from Compton processes, a track has to beassociated to each electromagnetic cluster in the central region ( θ e > ◦ ). Events in which theinvariant mass of the two electromagnetic clusters is within GeV of the nominal Z bosonmass are attributed to the Z → ee decay channel. Events from the W → νe decay channel areselected by requiring the invariant mass of the ν and one of the electromagnetic clusters tobe compatible with the W boson mass within GeV. In this channel, the variable ξ e definedfrom the highest P T electron is required to be above GeV. No data candidate is observed in This variable is proportional to the four-momentum transfer squared Q for NC DIS. Z or W decay channels compared to SM expectations of . ± . and . ± . ,respectively. In both channels, the selection efficiency for ν ∗ with masses above GeV is ∼ %. The following experimental systematic uncertainties are considered: • The uncertainty on the electromagnetic energy scale varies between % and % depend-ing on the polar angle. The polar angle measurement uncertainty of electromagneticclusters is mrad. • The efficiency to identify photons is known with a precision of % for high P T photons. • The scale uncertainty on the transverse momentum of high P T muons amounts to %.The uncertainty on the reconstruction of the muon polar angle is mrad. • The hadronic energy scale is known within %. The uncertainty on the jet polar angledetermination is mrad. • The uncertainty on the trigger efficiency is %. • The luminosity measurement has an uncertainty of . %.The effect of the above systematic uncertainties are determined by varying the experimentalquantities by ± standard deviation in the MC samples and propagating these variations throughthe whole analysis chain.Additional model systematic uncertainties are attributed to the SM background MC gen-erators described in section . An error of % on the simulation of NC DIS, CC DIS andphotoproduction processes with at least two high P T jets is considered to account for the uncer-tainty on higher order QCD corrections. An uncertainty of % on the simulation of radiativeCC DIS events is considered to account for the lack of QED radiation from the quark line inthe DJANGO generator. This uncertainty is estimated in the specific phase space of the anal-ysis by a comparison of the DJANGO result to the calculated cross section of the e − p → ν e γX process [36]. The error on the QED Compton cross section is estimated to be %. The errorsattributed to multi-lepton and W production are % and %, respectively. The total error onthe SM background prediction is determined by adding the effects of all model and experimentalsystematic uncertainties in quadrature.The theoretical uncertainty on the ν ∗ production cross section is dominated by the uncer-tainty on the scale at which the proton parton densities are evaluated. It is estimated by varyingthis scale from p Q / to p Q . The resulting uncertainty depends on the ν ∗ mass and is %at M ν ∗ = 100 GeV, increasing to % at M ν ∗ = 300 GeV.10
Interpretation
The event yields observed in all decay channels are in agreement with the corresponding SMexpectations and are summarised in table 1. The SM predictions are dominated by NC DIS pro-cesses for the eq ¯ q resonance search and by CC DIS for the νγ and νq ¯ q resonance searches. Thedistributions of the invariant mass of the data events are in agreement with those of the expectedSM background as shown in figure 1. No data event is observed in channels corresponding toleptonic decays of the W or Z bosons, in agreement with the low SM expectations.Since no evidence for the production of excited neutrinos is observed, upper limits on the ν ∗ production cross section and on the coupling f / Λ are derived as a function of the mass of theexcited neutrino. Limits are presented at the % confidence level (CL) and are obtained fromthe mass spectra using a modified frequentist approach which takes statistical and systematicuncertainties into account [37].Upper limits on the product of the ν ∗ production cross section and decay branching ratio areshown in figure 2. The analysed decay channels of the W and Z gauge bosons are combined.The resulting limits on f / Λ after combination of all ν ∗ decay channels are displayed as a func-tion of the ν ∗ mass in figure 3, for the conventional assumptions f = − f ′ and f = + f ′ . Limitsare derived for ν ∗ masses up to GeV. The total fraction of ν ∗ decays covered in this analysisis ∼ % and ∼ % in the two cases f = − f ′ and f = + f ′ , respectively. In the case f = − f ′ ,the limit on f / Λ is dominated at low mass by the ν ∗ → νγ channel, while the ν ∗ → eW channelstarts to contribute significantly for masses above GeV. Under the assumption f = + f ′ , thelimit on f / Λ is driven mainly by the ν ∗ → eW channel. These new results improve significantlythe previously published limits by H1 [2] and ZEUS [38]. For comparison, the most stringentlimits obtained in e + e − collisions at LEP for the two cases f = − f ′ and f = + f ′ , determinedby L3 [39] and DELPHI [40], respectively, are also shown in figure 3. The H1 measurementprovides the most stringent constraints for masses larger than ∼ GeV. With the assump-tion f /
Λ = 1 /M ν ∗ excited neutrinos with masses up to GeV (
GeV) are excluded for f = − f ′ ( f = + f ′ ).Limits with less model dependence can be derived if arbitrary ratios f ′ /f are considered.The dependence of the limits on this ratio for different ν ∗ masses is displayed in figure 4(a).Limits which are independent of f ′ /f are derived for f ′ /f ∈ [ −
5; 5] by choosing in figure 4(a)the point with the weakest limit for each mass hypothesis. The result is shown in figure 4(b)and is found to be almost equal to the limit obtained under the assumption f = + f ′ . Using the full e − p data sample collected by the H1 experiment at HERA with an integratedluminosity of pb − a search for the production of excited neutrinos is performed. Theexcited neutrino decay channels ν ∗ → νγ , ν ∗ → νZ and ν ∗ → eW with subsequent hadronic orleptonic decays of the W and Z bosons are considered and no indication of a ν ∗ signal is found.New limits on the production cross section of excited neutrinos are obtained. Previous HERAresults are improved by a factor three to four. Upper limits on the coupling f / Λ as a function of11he excited neutrino mass are established for specific relations between the couplings ( f = + f ′ and f = − f ′ ) and independent of the ratio f ′ /f . Assuming f = − f ′ and f / Λ = 1 /M ν ∗ ,excited neutrinos with a mass lower than GeV are excluded at % confidence level. Theresults presented in this letter greatly extend the previously excluded domain and demonstratethe unique sensitivity of HERA to excited neutrinos with masses beyond the LEP reach. Acknowledgements
We are grateful to the HERA machine group whose outstanding efforts have made this ex-periment possible. We thank the engineers and technicians for their work in constructing andmaintaining the H1 detector, our funding agencies for financial support, the DESY technicalstaff for continual assistance and the DESY directorate for the hospitality which they extendto the non DESY members of the collaboration. We would like to thank M. Spira for helpfuldiscussions.
References [1] H. Harari, Phys. Rept. (1984) 159.[2] C. Adloff et al. [H1 Collaboration], Phys. Lett. B (2002) 9 [hep-ex/0110037].[3] K. Hagiwara, D. Zeppenfeld and S. Komamiya, Z. Phys. C (1985) 115.[4] F. Boudjema, A. Djouadi and J. L. Kneur, Z. Phys. C (1993) 425.[5] U. Baur, M. Spira and P. M. Zerwas, Phys. Rev. D (1990) 815.[6] S. J. Brodsky and S. D. Drell, Phys. Rev. D (1980) 2236.[7] F. M. Renard, Phys. Lett. B (1982) 264.[8] F. Boudjema and A. Djouadi, Phys. Lett. B (1990) 485.[9] E. Boos et al. [CompHEP Collaboration], Nucl. Instrum. Meth. A (2004) 250[hep-ph/0403113];A. Pukhov et al. , “CompHEP - a package for evaluation of Feynman diagrams and inte-gration over multi-particle phase space”, hep-ph/9908288(available at http://theory.sinp.msu.ru/comphep).[10] C. Berger and W. Wagner, Phys. Rept. (1987) 1.[11] J. Pumplin et al. , JHEP (2002) 012 [hep-ph/0201195].[12] T. Sj¨ostrand et al. , PYTHIA version 6.1, Comput. Phys. Commun. (2001) 238[hep-ph/0010017].[13] H. Jung, RAPGAP version 3.1, Comput. Phys. Commun. (1995) 147.1214] A. Kwiatkowski, H. Spiesberger and H. J. M ¨ohring, Comput. Phys. Commun. (1992)155.[15] C. Adloff et al. [H1 Collaboration], Eur. Phys. J. C (2002) 13 [hep-ex/0201006].[16] G. A. Schuler and H. Spiesberger, DJANGOH version 1.4, “Django: The Interface for TheEvent Generators Heracles and Lepto”, Proc. of the Workshop “Physics at HERA” (1991),Eds. W. Buchm ¨uller and G. Ingelman, Vol. 3, p. 1419.[17] L. L¨onnblad, Comput. Phys. Commun. (1992) 15.[18] C. Berger and P. Kandel, “A New Generator For Wide Angle Bremsstrahlung”, Preparedfor Workshop on Monte Carlo Generators for HERA Physics Hamburg, Germany, 27-30April 1998.[19] U. Baur, J. A. Vermaseren and D. Zeppenfeld, EPVEC version 1.1, Nucl. Phys. B (1992) 3.[20] T. Abe, GRAPE-Dilepton version 1.1, Comput. Phys. Commun. (2001) 126[hep-ph/0012029].[21] R. Brun et al. , “Geant3”, CERN-DD/EE/84-1.[22] I. Abt et al. [H1 Collaboration], Nucl. Instrum. Meth. A (1997) 310;I. Abt et al. [H1 Collaboration], Nucl. Instrum. Meth. A (1997) 348.[23] B. Andrieu et al. [H1 Calorimeter Group Collaboration], Nucl. Instrum. Meth. A (1993) 460.[24] B. Andrieu et al. [H1 Calorimeter Group Collaboration], Nucl. Instrum. Meth. A (1993) 499.[25] B. Andrieu et al. [H1 Calorimeter Group Collaboration], Nucl. Instrum. Meth. A (1994) 57.[26] R. D. Appuhn et al. [H1 SPACAL Group Collaboration], Nucl. Instrum. Meth. A (1997) 397.[27] C. Adloff et al. [H1 Collaboration], Eur. Phys. J. C (2003) 1 [hep-ex/0304003].[28] V. Andreev et al. [H1 Collaboration], Phys. Lett. B (1993) 3160 [hep-ph/9305266].[32] S. Catani et al. , Nucl. Phys. B et al. [H1 Collaboration], Eur. Phys. J. C (2000) 609 [hep-ex/9908059].[35] A. Blondel, F. Jacquet, Proceedings of the Study of an ep Facility for Europe, ed. U.Amaldi, DESY 79/48 (1979) 391.[36] T. Helbig and H. Spiesberger, Nucl. Phys. B (1992) 73.[37] T. Junk, Nucl. Instrum. Meth. A (1999) 435 [hep-ex/9902006].[38] S. Chekanov et al. [ZEUS Collaboration], Phys. Lett. B (2002) 32 [hep-ex/0109018].[39] P. Achard et al. [L3 Collaboration], Phys. Lett. B (2003) 23 [hep-ex/0306016].[40] J. Abdallah et al. [DELPHI Collaboration], Eur. Phys. J. C (2006) 277[hep-ex/0603045]. 14 earch for ν ∗ at HERA ( e − p , pb − ) Channel Data SM Signal Efficiency [%] ν ∗ → νγ . ± . – ν ∗ → eW → eq ¯ q
220 223 ±
47 40 – ν ∗ → eW → eνµ . ± .
05 35 ν ∗ → eW → eνe . ± . ν ∗ → νZ → νq ¯ q
89 95 ±
21 25 – ν ∗ → νZ → νee . ± .
05 45
Table 1: Observed and predicted event yields for the studied ν ∗ decay channels. The analyseddata sample corresponds to an integrated luminosity of pb − . The error on the SM predic-tions includes model and experimental systematic errors added in quadrature. Typical selectionefficiencies for ν ∗ masses ranging from to GeV are also indicated.15
Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s * Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s SMH1 Data ) -1 p, 184 pb - * at HERA (e n Search for g n fi * n H1 (a) * Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s * Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s SMH1 Data ) -1 p, 184 pb - * at HERA (e n Search for qq n fi Z n fi * n H1 (b) * Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s * Mass [GeV] n
50 100 150 200 250 300 350 E ve n t s SMH1 Data ) -1 p, 184 pb - * at HERA (e n Search for qeq fi e W fi * n H1 (c) Figure 1: Invariant mass distribution of the ν ∗ candidates for the ν ∗ → νγ (a), ν ∗ → νZ → νq ¯ q (b) and ν ∗ → eW → eq ¯ q (c) searches. The points correspond to the observed data events and thehistogram to the SM expectation after the final selections. The error bands on the SM predictioninclude model and experimental systematic errors added in quadrature.16 Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 BR [ pb ] · s -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 BR [ pb ] · s -1 ) -1 p, 184 pb - * at HERA (e n Search for H1 g n fi * n Z n fi * n e W fi * n Figure 2: Upper limits at % CL on the product of the ν ∗ production cross section and decaybranching ratio, σ × BR, in the three decay channels as a function of the excited neutrino mass.The different decay channels of the W and Z gauge bosons are combined. Areas above thecurves are excluded. * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 ) -1 p, 184 pb - * at HERA (e n Search for f = - f’ L3 H1 = 1 / M L f / * n H1 (a) * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 ) -1 p, 184 pb - * at HERA (e n Search for f = + f’
DELPHI H1 = 1 / M L f / * n H1 (b) Figure 3: Exclusion limits at % CL on the coupling f / Λ as a function of the mass of theexcited neutrino with the assumptions (a) f = − f ′ and (b) f = + f ′ . The excluded domainbased on all H1 e − p data is represented by the shaded area. Values of the couplings above thecurves are excluded. The dashed line corresponds to the exclusion limit obtained at LEP by theL3 Collaboration [39] in (a) and by the DELPHI Collaboration [40] in (b).17 ’ / f -6 -4 -2 0 2 4 6 ] - [ G e V L f / -3 -2 -1 * mass n
110 GeV160 GeV190 GeV210 GeV230 GeV250 GeV280 GeV ) -1 p, 184 pb - * at HERA (e n Search for H1 (a) * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 ) -1 p, 184 pb - * at HERA (e n Search for [-5 ; 5] ˛ f’ / f -1 H1 15 pb H1 H1 (b) Figure 4: (a) Exclusion limits at % CL on the coupling f / Λ as a function of the ratio f ′ /f .Each curve corresponds to a different ν ∗ mass. The circle indicates the weakest limit for eachmass. (b) Exclusion limit at % CL on the coupling f / Λ as a function of the mass of theexcited neutrino (shaded area). This limit corresponds to the weakest limit on f / Λ for f ′ /f values in the interval [ −
5; +5]5; +5]