Scholarly article on topic 'Search for excited electrons in ep collisions at HERA'

Search for excited electrons in ep collisions at HERA Academic research paper on "Physical sciences"

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Abstract of research paper on Physical sciences, author of scientific article — F.D. Aaron, C. Alexa, V. Andreev, B. Antunovic, S. Aplin, et al.

Abstract A search for excited electrons is performed using the full e ± p data sample collected by the H1 experiment at HERA, corresponding to a total luminosity of 475 pb−1. The electroweak decays of excited electrons e ∗ → e γ , e ∗ → e Z and e ∗ → ν W with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited electron production is found. Mass dependent exclusion limits on e ∗ production cross sections and on the ratio f / Λ of the coupling to the compositeness scale are derived within gauge mediated models. These limits extend the excluded region compared to previous excited electron searches. The e ∗ production via contact interactions is also addressed for the first time in ep collisions.

Academic research paper on topic "Search for excited electrons in ep collisions at HERA"

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Physics Letters B

Search for excited electrons in ep collisions at HERA

H1 Collaboration

F.D. Aaronei1, C. Alexae, V. Andreevy, B. Antunovick, S. Aplink, A. Asmoneag, A. Astvatsatourovd, A. Bacchettak, S. Backovicad, A. Baghdasaryanal, P. Baranovy'*, E. Barreletac, W. Bartelk, M. Beckinghamk, K. Begzsurenai,

O. Behnken, A. Belousovy, N. Bergeran, J.C. Bizotaa, M.-O. Boenigh, V. Boudryab, I. Bozovic-Jelisavcicb, J. Bracinikc,

G. Brandtk, M. Brinkmannk, V. Brissonaa, D. Brunckop, A. Bunyatyanm al, G. Buschhornz, L. Bystritskayax,

A.J. Campbellk, K.B. Cantun Avilav, F. Cassol-Brunneru, K. Cernyaf, V. Cernyp'2, V. Chekelianz, A. Cholewak, J.G. Contrerasv, J.A. Coughlanf, G. Cozzikaj, J. Cvachae, J.B. Daintonr, K. Daumak 3, M. Deâkk, Y. de Boerk,

B. Delcourtaa, M. Del Deganan, J. Delvaxd, A. De Roeckk'4, E.A. De Wolfd, C. Diaconuu'*, V. Dodonovm, A. Dossanovz, A. Dubakad'5, G. Eckerlink, V. Efremenkox, S. Egliaj, A. Eliseevy, E. Elsenk, S. Essenovx, A. Falkiewiczg, P.J.W. Faulknerc, L. Favartd, A. Fedotovx, R. Felstk, J. Feltessej'6, J. Ferenceip, L. Finkek,

M. Fleischerk, A. Fomenkoy, E. Gabathulerr, J. Gaylerk, S. Ghazaryanal, A. Glazovk, I. Glushkovam, L. Goerlichg, M. Goettlichl, N. Gogitidzey, M. Gouzevitchab, C. Graban, T. Greenshawr, B.R. Grellk, G. Grindhammerz, S. Habib''7, D. Haidtk, M. Hanssont, C. Helebrantk, R.C.W. Hendersonq, H. Henschelam, G. Herreraw, M. Hildebrandtaj, K.H. Hilleram, D. Hoffmann", R. Horisbergeraj, A. Hovhannisyanal, T. Hreusd'8, M. Jacquetaa, M.E. Janssenk, X. Janssend, V. Jemanov^ L. Jönssont, D.P. Johnsond'*, A.W. Jungo, H. Jungk, M. Kapichinei, J. Katzyk, I.R. Kenyonc,

C. Kieslingz, M. Kleinr, C. Kleinwortk, T. Klimkovich, T. Kluger, A. Knutssonk, R. Koglerz, V. Korbelk, P. Kostka am, M. Kraemerk, K. Krastevk, J. Kretzschmarr, A. Kropivnitskayax, K. Krügero, K. Kutakk, M.P.J. Landons, W. Lange am, G. Lastovicka-Medinad, P. Laycockr, A. Lebedevy, G. Leibenguthan, V. Lendermanno, S. Levoniank, G. Liaa, K. Lipka^ A. Liptajz, B. Listl, J. Listk, N. Loktionovay, R. Lopez-Fernandez w, V. Lubimovx, A.-I. Lucaci-Timocek, L. Lytkinm,

A. Makankinei, E. Malinovskiy, P. Maraged, Ll. Martik, H.-U. Martyna, S.J. Maxfieldr, A. Mehtar, K. Meiero, A.B. Meyerk, H. Meyerk, H. Meyerak, J. Meyerk, V. Michelsk, S. Mikockig, I. Milcewicz-Mikag, F. Moreauab, A. Morozovi, J.V. Morrisf, M.U. Mozerd, M. Mudrinicb, K. Müllerao, P. Murinp'8, K. Nankovah, B. Naroskal'*, Th. Naumannam, P.R. Newmanc, C. Niebuhrk, A. Nikiforovk, G. Nowakg, K. Nowakao, M. Nozickak, B. Olivierz, J.E. Olssonk, S. Osmant, D. Ozerovx, V. Palichiki, I. Panagouliask'9'10, M. Pandurovicb, Th. Papadopoulouk'9'10,

C. Pascaudaa, G.D. Patelr, O. Pejchalaf, H. Pengk, E. Perezj 4, A. Petrukhinx, I. Picuricad, S. Piecam, D. Pitzlk, R. Placakyte k, R. Polif^ca af, B. Povhm, T. Predae, V. Radescuk, A.J. Rahmatr, N. Raicevicad, A. Raspiarezaz,

T. Ravdandorjai, P. Reimerae, E. Rizvis, P. Robmannao, B. Roland d, R. Roosend, A. Rostovtsevx, M. Rotarue, J.E. Ruiz Tabascov, Z. Rurikovak, S. Rusakovy, D. Salekaf, F. Salvairek, D.P.C. Sankeyf, M. Sauteran, E. Sauvanu, S. Schmidtk, S. Schmittk, C. Schmitzao, L. Schoeffelj, A. Schöningk'ao, H.-C. Schultz-Coulono, F. Sefkowk, R.N. Shaw-Westc, I. Sheviakovy, L.N. Shtarkovy, S. Shushkevichz, T. Sloanq, I. Smiljanicb, P. Smirnovy, Y. Solovievy, P. Sopickig, D. Southh, V. Spaskovi, A. Speckaab, Z. Staykovak, M. Stederk, B. Stellaag, U. Straumannao, D. Sunard, T. Sykorad, V. Tchoulakovi, G. Thompsons, P.D. Thompsonc, T. Tollk, F. Tomaszp, T.H. Tranaa, D. Traynors, T.N. Trinhu, P. Truölao, I. Tsakovah, B. Tseepeldorjai11, I. Tsurinam, J. Turnaug, E. Tzamariudakiz, K. Urbano, A. Valkârovâ af, C. Vallée u, P. Van Mechelend, A. Vargas Trevinok, Y. Vazdiky, S. Vinokurovak, V. Volchinskial,

D. Wegenerh, M. Wesselsk, Ch. Wissingk, E. Wünsch k, V. Yeganoval, J. Zâcekaf, J. Zâlesâkae, Z. Zhangaa, A. Zhelezovx, A. Zhokinx, Y.C. Zhuk, T. Zimmermannan, H. Zohrabyanal, F. Zomeraa

a I. Physikalisches Institut der RWTH, Aachen, Germany12 b Vinca Institute of Nuclear Sciences, Belgrade, Serbia

c School of Physics and Astronomy, University of Birmingham, Birmingham, UK13 d Inter-University Institute for High Energies ULB-VUB, Universiteit Antwerpen, Antwerpen, Belgium14 e National Institute for Physics and Nuclear Engineering (NIPNE), Bucharest, Romania f Rutherford Appleton Laboratory, Chilton, Didcot, UK13 g Institute for Nuclear Physics, Cracow, Poland15

0370-2693/$ - see front matter © 2008 Elsevier B.V. AH rights reserved. doi:10.1016/j.physletb.2008.07.014

h Institut für Physik, TU Dortmund, Dortmund, Germany12 1 Joint Institute for Nuclear Research, Dubna, Russia j CEA, DSM/DAPNIA, CE-Saclay, Gif-sur-Yvette, France k DESY, Hamburg, Germany

l Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany12 m Max-Planck-Institut für Kernphysik, Heidelberg, Germany n Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany12

0 Kirchhoff-Institut für Physik, Universität Heidelberg, Heidelberg, Germany12

p Institute of Experimental Physics, Slovak Academy of Sciences, KoSice, Slovak Republic16 q Department of Physics, University of Lancaster, Lancaster, UK13 r Department of Physics, University of Liverpool, Liverpool, UK13 s Queen Mary and Westfield College, London, UK13

1 Physics Department, University of Lund, Lund, Sweden17

u CPPM, CNRS/IN2P3, Univ. Mediterranee, Marseille, France

v Departamento de Fisica Aplicada, CINVESTAV, Mérida, Yucatan, Mexico18

w Departamento de Fisica, CINVESTAV, Mexico18

x Institute for Theoretical and Experimental Physics, Moscow, Russia

y Lebedev Physical Institute, Moscow, Russia19

z Max-Planck-Institut für Physik, München, Germany

aa LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France

ab LLR, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France

ac LPNHE, Universités Paris VI and VII, IN2P3-CNRS, Paris, France

ad Faculty of Science, University of Montenegro, Podgorica, Montenegro19

ae Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic20

af Faculty of Mathematics and Physics, Charles University, Praha, Czech Republic20

ag Dipartimento di Fisica, Université di Roma Tre and INFN Roma 3, Roma, Italy

ah Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria19

a1 Institute ofPhysics and Technology ofthe Mongolian Academy ofSciences, Ulaanbaatar, Mongolia

aj Paul Scherrer Institut, Villigen, Switzerland

ak Fachbereich C, Universität Wuppertal, Wuppertal, Germany

a1 Yerevan Physics Institute, Yerevan, Armenia

am DESY, Zeuthen, Germany

an Institut für Teilchenphysik, ETH, Zürich, Switzerland21 a0 Physik-Institut der Universität Zürich, Zürich, Switzerland21

ARTICLE INFO ABSTRACT

A search for excited electrons is performed using the full e± p data sample collected by the H1 experiment at HERA, corresponding to a total luminosity of 475 pb-1. The electroweak decays of excited electrons e*^ ey, e*^ eZ and e*^v W with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited electron production is found. Mass dependent exclusion limits on e* production cross sections and on the ratio f /A of the coupling to the compositeness scale are derived within gauge mediated models. These limits extend the excluded region compared to previous excited electron searches. The e* production via contact interactions is also addressed for the first time in ep collisions.

© 2008 Elsevier B.V. All rights reserved.

Article history:

Received 29 May 2008

Received in revised form 8 July 2008

Accepted 8 July 2008

Available online 11 July 2008

Editor: W.-D. Schlatter

* Corresponding author.

E-mail address: diaconu@cppm.in2p3.fr (C. Diaconu).

1 Also at Faculty of Physics, University of Bucharest, Bucharest, Romania.

2 Also at Comenius University, Bratislava, Slovak Republic.

3 Also at Rechenzentrum, Universität Wuppertal, Wuppertal, Germany.

4 Also at CERN, Geneva, Switzerland.

5 Also at Max-Planck-Institut für Physik, München, Germany.

6 Also at DESY and University Hamburg, Helmholtz Humboldt Research Award.

7 Supported by a scholarship of the World Laboratory Björn Wiik Research Project.

8 Also at University of P.J. Safarik, Kosice, Slovak Republic.

9 Also at Physics Department, National Technical University, Zografou Campus, GR-15773 Athens, Greece.

10 This project is co-funded by the European Social Fund (75%) and National Resources (25%)—(EPEAEK II)-PYTHAGORAS II.

11 Also at Ulaanbaatar University, Ulaanbaatar, Mongolia.

12 Supported by the Bundesministerium für Bildung und Forschung, FRG, under contract numbers 05 H1 1GUA /1, 05 H1 1PAA /1, 05 H1 1PAB /9, 05 H1 1PEA /6, 05 H1 1VHA /7 and 05 H1 1VHB /5.

13 Supported by the UK Science and Technology Facilities Council, and formerly by the UK Particle Physics and Astronomy Research Council.

14 Supported by FNRS-FWO-Vlaanderen, IISN-IIKW and IWT and by Interuniversity Attraction Poles Programme, Belgian Science Policy.

15 Partially Supported by Polish Ministry of Science and Higher Education, grant PBS/DESY/70/2006.

16 Supported by VEGA SR grant No. 2/7062/27.

1. Introduction

The three-family structure and mass hierarchy of the known fermions is one of the most puzzling characteristics of the Standard Model (SM) of particle physics. Attractive explanations are provided by models assuming composite quarks and leptons [1]. The existence of excited states of leptons and quarks is a natural consequence of these models and their discovery would provide convincing evidence of a new scale of matter. Electron22-proton interactions at very high energies provide good conditions to search for excited states of first generation fermions. For instance, excited electrons (e*) could be singly produced through the exchange of a Y or a Z boson in the t-channel.

17 Supported by the Swedish Natural Science Research Council.

18 Supported by CONACYT, México, grant 48778-F.

19 Supported by the Deutsche Forschungsgemeinschaft.

20 Supported by the Ministry of Education of the Czech Republic under the projects LC527 and INGO-1P05LA259.

21 Supported by the Swiss National Science Foundation. ^ Deceased.

22 In this Letter the term "electron" refers to both electron and positrons, if not otherwise stated.

In this Letter a search for excited electrons using the complete j^ = nLF*YpFL + n'LFLYpFL + nLF*YpF* + h.c. + (L ^ R).

e± p HERA collider data of the H1 experiment is presented. Elec-troweak decays into a SM lepton (e, ve) and a SM gauge boson (y, W and Z) are considered and hadronic as well as leptonic decays of the W and Z are analysed.

The data are recorded at electron beam energy of 27.6 GeV and proton beam energies of 820 GeV and 920 GeV, corresponding to centre-of-mass energies «fs of 301 GeV and 319 GeV, respectively. The total integrated luminosity of the data is 475 pb-1. The data comprise 184 pb-1 recorded in p collisions and 291 pb-1 in e+ p collisions, of which 35 pb-1 were recorded at fs = 301 GeV. With a four-fold increase in statistics, this analysis supersedes the result of the previous H1 search for excited electrons [2]. It complements the search for excited neutrinos [3].

2. Excited electron models

In the present study a model [4-6] is considered in which excited fermions are assumed to have spin 1 /2 and isospin 1 /2. The left-handed and right-handed components of the excited fermions form weak iso-doublets F** and F*.

Interactions between excited and ordinary fermions may be mediated by gauge bosons, as described by the effective Lagrangian [5,6]:

¿CM = ¿ F * avv

T a Y —a

gf y WIV + g f' - B vv + gsfs y Gau

Fl + h.c.

Only the right-handed component of the excited fermion FR is allowed to couple to light fermions, in order to protect the light leptons from radiatively acquiring a large anomalous magnetic moment [7,8]. The matrix avv is the covariant bilinear tensor, Wa v, B^v and GaIJvV are the field-strength tensors of the SU(2), U(1) and SU(3)C gauge fields, ta, Y and -a are the Pauli matrices, the weak hypercharge operator and the Cell-Mann matrices, respectively. The standard electroweak and strong gauge couplings are denoted by g, g and gs, respectively. The parameter A has units of energy and can be regarded as the compositeness scale which reflects the range of the new confinement force. The constants f, f' and fs are coupling parameters associated to the three gauge groups and are determined by the yet unknown composite dynamics.

Following this model of gauge mediated (CM) interactions, single e* production in ep collisions may result from the t-channel exchange of a y or Z boson. Since the e* is expected not to have strong interactions, the present search is insensitive to fs. The produced e* may decay into a lepton and an electroweak gauge boson via e*^ey, e*^vW and e*^eZ. For a given e* mass value Me* and assuming a numerical relation between f and f', the e* branching ratios are fixed and the production cross section depends only on f /A. In most analyses the assumption is made that the coupling parameters f and f' are of comparable strength and only the relationships f = —f' and f = +f' are considered. In the case f = —f', the excited electron does not couple to the photon and therefore the e* production cross section at HERA is small. Therefore, only the case f = +f' is considered in this analysis.

In addition to CM interactions, novel composite dynamics may be visible as contact interactions (CI) between excited fermions and SM quarks and leptons. Such interactions can be described by the effective four-fermion Lagrangian [5]:

where A is assumed to be the same parameter as in the Lagrangian (1) and jp is the fermion current

Conventionally, the n factors are set to one for the left-handed and to zero for the right-handed current.

Contact interactions may induce changes in the cross section of neutral current (NC) deep-inelastic scattering (DIS) ep ^ eX. Searches for deviations from the SM cross section at the highest squared momentum transfers Q2 in NC DIS processes have excluded values of A between 1.6 TeV and 5.5 TeV, depending on the chiral structure considered [9]. Contact interactions may also mediate the resonant production of excited electrons in ep collisions as well as their decays into an electron and a pair of SM fermions. The e* production and decay by both gauge and contact interactions is also considered in this analysis. In this case the total e* production cross section ctgm+ci is the sum of pure GM and CI cross sections and of the interference between the two processes [10]. For simplicity, the relative strength of gauge and contact interactions are fixed by setting the parameters f and f' of the gauge interaction to one. The ratio of the GM + CI and GM cross sections ctgm+ci/ogm then depends only on A and on the e* mass. For Me* = 150 GeV and A = 1 TeV, ctgm+ci/ctgm is equal to 8.4, but reduces to 1.3 for A = 4 TeV. Relative branching ratios of GM and CI decays are determined by the e* partial widths in each decay channel [5]. In the sensitive domain of the present analysis (A ~ 4 TeV and 100 GeV < Me* < 200 GeV), more than 95% of e* decays are gauge mediated. Therefore, only GM decay channels are used for the present search.

3. Simulation of signal and background processes

The Monte Carlo (MC) event generator COMPOS [11] is used for the calculation of the e* production cross section and to determine the signal detection efficiencies. It is based on the cross section formulae for gauge mediated interactions [4,5]. Cross section formulae for contact interaction production and for the interference between contact and gauge interactions [10] have also been incorporated into COMPOS. Only e* decays via gauge mediated interactions are simulated. Initial state radiation of a photon from the incident electron is included using the Weizsacker-Williams approximation [12]. The proton parton densities are taken from the CTEQ5L [13] parametrisation and are evaluated at the scale yQ2. The parton shower approach [14] is applied in order to simulate Quantum Chromodynamics (QCD) corrections in the initial and final states. Hadronisation is performed using Lund string fragmentation as implemented in PYTHIA [14]. The COMPOS generator uses the narrow width approximation (NWA) for the calculation of the production cross section and takes into account the natural width of the excited electron for the e* decay. The NWA is valid for e* masses below 290 GeV and the couplings f /A relevant to this analysis, as the total e* width is less than 10% of the e* mass.

The Standard Model (SM) processes which may mimic the e* signal are QED Compton scattering, neutral current and charged current (CC) deep-inelastic scattering and to a lesser extent photoproduction, lepton pair production and real W boson production. All SM processes are generated with at least 10 times the integrated luminosity of the data sample.

The RAPGAP [15] event generator, which implements the Born, QCD Compton and Boson Gluon Fusion matrix elements, is used to model NC DIS events. The QED radiative effects arising from real photon emission from both the incoming and outgoing electrons are simulated using the HERACLES [16] program. Direct and resolved photoproduction of jets and prompt photon production are simulated using the PYTHIA event generator. The simulation is based on Born level hard scattering matrix elements with radiative QED corrections. In RAPGAP and PYTHIA, jet production from higher order QCD radiation is simulated using leading log-

arithmic parton showers and hadronisation is modelled with Lund string fragmentation. The leading order MC prediction of NC DIS and photoproduction processes with two or more high transverse momentum jets is scaled by a factor of 1.2 to account for missing higher order QCD contributions in the MC generators [19,20]. Charged current DIS events are simulated using the DJANGO [17] program, which includes first order leptonic QED radiative corrections based on HERACLES. The production of two or more jets in DJANGO is accounted for using the colour-dipole model [18]. Contributions from elastic and quasi-elastic QED Compton scattering are simulated with the WABGEN [21] generator. Contributions arising from the production of W bosons and multi-lepton events are modelled using the EPVEC [22] and GRAPE [23] event generators, respectively.

Generated events are passed through the full GEANT [24] based simulation of the H1 apparatus, which takes into account the actual running conditions of the data taking, and are reconstructed and analysed using the same program chain as for the data.

4. Experimental conditions

A detailed description of the H1 experiment can be found in [25]. Only the detector components relevant to the present analysis are briefly described here. The origin of the H1 coordinate system is the nominal ep interaction point, with the direction of the proton beam defining the positive z-axis (forward region). Transverse momentum (PT) is measured in the xy plane. The pseudorapidity n is related to the polar angle 6 by n = —lntan(6/2). The Liquid Argon (LAr) calorimeter [26] is used to measure electrons, photons and hadrons. It covers the polar angle range 4° <6 < 154° with full azimuthal acceptance. Electromagnetic shower energies are measured with a precision of a(E)/E = 12%/VE/GeV ® 1% and hadronic energies with a(E)/E = 50%/VE/GeV ® 2%, as measured in test beams [27,28]. In the backward region, energy measurements are provided by a lead/scintillating-fiber (SpaCal) calorimeter [29] covering the angular range 155° <6 < 178°. The central (20° <6 < 160°) and forward (7° <6 < 25°) tracking detectors are used to measure charged particle trajectories, to reconstruct the interaction vertex and to complement the measurement of hadronic energy. The LAr and inner tracking detectors are enclosed in a super-conducting magnetic coil with a field strength of 1.16 T. The return yoke of the coil is the outermost part of the detector and is equipped with streamer tubes forming the central muon detector (4° < 6 < 171 °). In the forward region of the detector (3° < 6 < 17°) a set of drift chambers detects muons and measures their momenta using an iron toroidal magnet. The luminosity is determined from the rate of the Bethe-Heitler process ep^epy, measured using a photon detector located close to the beam pipe at z = —103 m, in the backward direction.

5. Data analysis

The triggers employed for collecting the data used in this analysis are based on the detection of electromagnetic deposits or missing transverse energy in the LAr calorimeter [30]. The trigger efficiency is ~ 90% for events with missing transverse energy of 20 GeV, and increases above 95% for missing transverse energy above 30 GeV. Events containing an electromagnetic deposit (electron or photon) with an energy greater than 10 GeV are triggered with an efficiency close to 100%.

In order to remove background events induced by cosmic showers and other non-ep sources, the event vertex is required to be reconstructed within 35 cm in z of the nominal interaction point. In addition, topological filters and timing vetoes are applied.

The identification of electrons or photons relies on the measurement of a compact and isolated electromagnetic shower in

the LAr calorimeter. The hadronic energy within a distance in the pseudorapidity-azimuth (n-$) plane R = y7An2 + A02 < 0.5 around the electron (photon) is required to be below 3% of the electron (photon) energy. Furthermore, each electron (photon) candidate must be isolated from jets by a minimum distance in pseudorapidity-azimuth of R > 0.5 to any jet axis. The electron and photon energy and angular direction are measured by the calorimeters. Muon identification is based on a track measured in the inner tracking systems associated with signals in the muon detectors [31]. A muon candidate is required to have no more than 5 GeV deposited in a cylinder, centred on the muon track direction, of radius 25 cm and 50 cm in the electromagnetic and hadronic sections of the LAr calorimeter, respectively. Additionally, the muon candidate is required to be separated from the closest jet and from any track by R > 1 and R > 0.5, respectively. Calorimeter energy deposits and tracks not previously identified as electron, photon or muon candidates are used to form combined cluster-track objects, from which the hadronic energy is reconstructed [32,33]. Jets are reconstructed from these combined cluster-track objects using an inclusive kT algorithm [34,35] with a minimum transverse momentum of 2.5 GeV. The missing transverse momentum Pm'ss of the event is derived from all detected particles and energy deposits in the event. In events with large PTmlss, the only non-detected particle in the event is assumed to be a neutrino. The four-vector of this neutrino candidate is reconstructed assuming transverse momentum conservation and the relation (E' — P'z) + (Ev — Pv) = 2E0e = 55.2 GeV, where the sum runs over all detected particles, P z is the momentum along the proton beam axis and E0 is the electron beam energy.

Specific selection criteria applied in each decay channel are presented in the following subsections. A detailed description of the analysis can be found in [36].

5.1. ey resonance search

The signature of the e* ^ ey decay channel consists of two high PT isolated electromagnetic clusters. SM background arises mainly from elastic and inelastic QED Compton events. Two isolated electromagnetic clusters are required, each with transverse momentum PT > 15 GeV and polar angle 5° < 6 < 130°. No explicit electron and photon identification based on tracking conditions is performed in order to retain a high selection efficiency. To reduce contributions from QED Compton processes, the sum of the energies of the two electromagnetic clusters is required to be greater than 110 GeV and the sum of their total transverse momenta has to be larger than 75 GeV.

After this selection, the SM background from elastic QED Comp-ton events is smaller than that from inelastic QED Compton processes. Since about half of the e* production cross section is expected from elastic e* production [4], the analysis is separated into two parts. Events with a total hadronic energy Eh < 5 GeV are used to search for elastic e* production, whereas the other events are attributed to possible inelastic e* production.

In the elastic channel 42 events are selected in the data compared to a SM expectation of 48 ± 4. In the inelastic channel 65 events are found for 65 ± 8 expected. The errors on the SM prediction include model and experimental systematic errors added in quadrature (see Section 5.5). The invariant mass of the e* candidate is calculated from the four-vectors of the electron and photon candidates. The invariant mass distribution of the e* candidates and the SM background expectations are presented in Fig. 1(a) and (b) for the elastic and inelastic channels, respectively. The selection efficiency is 60% for Me* = 120 GeV, increasing to 70% for Me* = 260 GeV. From Monte Carlo studies, the experimental resolution on the reconstructed e* mass distribution is 3 GeV for a

Fig. 1. Invariant mass distribution of the e* candidates in the elastic e* ^ ey (a), inelastic e* ^ ey (b), e* ^ vW ^ vqq (c), and e* ^ eZ ^ eqq (d) search channels. The points correspond to the observed data events and the histograms to the SM expectation after the final selections. The error bands on the SM prediction include model uncertainties and experimental systematic errors added in quadrature. The dashed line represents with an arbitrary normalisation the reconstructed mass distribution of e* events with Me* = 240 GeV.

generated e* mass of 120 GeV, increasing to 6 GeV for an e* mass of 260 GeV.

5.2. vqq resonance search

The signature of the e* ^ v W ^ vqq decay channel consists of two high transverse momentum jets in events with large Pm'ss. The SM background is dominated by multi-jet CC DIS events and contains moderate contributions from NC DIS and photoproduction. Events with missing transverse momentum Pm'ss > 20 GeV are selected. In each event at least two jets with transverse momenta larger than 20 and 15 GeV, respectively, are required in the polar angle range 5° <6 < 130

The ratio Vap/Vp of transverse energy flow anti-parallel and parallel to the hadronic final state [37] is used to suppress photoproduction events. Events with Vap/Vp > 0.3 are rejected. Photoproduction and NC DIS backgrounds typically have low values of Xh, the Bjorken scaling variable calculated from the hadronic

system using the Jacquet-Blondel method [37,38], and are thus suppressed by requiring xh > 0.04. In each event, a W candidate is reconstructed from the combination of those two jets with invariant mass closest to the nominal W boson mass. The reconstructed W candidate is required to have an invariant mass above 60 GeV. In order to further reduce the background from CC DIS, the invariant mass of all jets and hadrons in the event not associated to the decay of the W boson candidate is required to be below 15 GeV.

After this selection, 129 events are found compared to a SM expectation of 133 ± 32 events which is dominated by CC DIS events. The CC DIS cross section is smaller in e+p collisions than in e—p, in contrast to the e* cross section which is comparable in both collision modes. Therefore, e+ p data have a larger sensitivity to a potential e* signal in this channel than e—p data. In the e+p (e— p) data sample, 33 (96) events are observed compared to a SM expectation of 51 ± 13 (82 ± 19). A significant excess is observed neither in e+ p nor in e— p data. The invariant mass of the e* candi-

date is calculated from the neutrino and W candidate four-vectors. For this calculation, the W candidate four-vector is scaled such that its mass is set to the nominal W boson mass. The invariant mass distributions of the e* candidates and the SM background are presented in Fig. 1(c). The selection efficiency in this channel is 20% for Me* = 120 GeV, increasing to 55% for Me* = 260 GeV. From Monte Carlo studies, the experimental resolution on the reconstructed e* mass distribution is 9 GeV for a generated e* mass of 120 GeV, increasing to ~ 20 GeV for an e* mass of 260 GeV.

5.3. eqq resonance search

The signature of the e* ^ eZ ^ eqq decay channel consists of one electron and two high PT jets. Multi-jet NC DIS events constitute the main background contribution from SM processes. Events are selected with an isolated electron in the LAr calorimeter in the polar angle range 5° <6e < 90°. The electron should have either a transverse momentum PeT greater than 25 GeV or the variable23 %e = Ee cos2 (6e/2) above 23 GeV. These conditions remove a large part of the NC DIS contribution. The events are required to have at least two jets in the polar angle range 5° < 6jet < 130° with transverse momenta larger than 20 and 15 GeV, respectively. In each event, a Z candidate is reconstructed from the combination of those two jets with invariant mass closest to the nominal Z boson mass. The reconstructed mass of the Z candidate is required to be larger than 70 GeV. To further reduce the NC DIS background the polar angle of the jet with the highest PT associated to the Z candidate is required to be less than 80°. The polar angle of the second jet is required to be greater than 10° in events with Pf2 < 25 GeV.

After this selection, 286 events are observed while 277 ± 62 are expected from the SM. The invariant mass of the e* candidate is calculated from the electron and Z candidate four-vectors. For this calculation, the Z candidate four-vector is scaled such that its mass is set to the nominal Z boson mass. The invariant mass distributions of the e* candidates and the SM background are presented in Fig. 1(d). The selection efficiency in this channel is 20% for Me* = 120 GeV, increasing to 55% for Me* = 260 GeV. From Monte Carlo studies, the experimental resolution on the reconstructed e* mass distribution is 2 GeV for a generated e* mass of 120 GeV, increasing to 8 GeV for an e* mass of 260 GeV.

5.4. eee, epp and evv resonance searches

In the search for e* ^ eZ ^ eee, events with three electrons of high transverse momenta are selected. The electrons must be detected in the polar angle range 5° <6e < 150° and have transverse momenta larger than 25, 20 and 15 GeV, respectively. To reduce the background from QED Compton processes, each electron in the central region (6e > 35°) must be associated to a charged track. A Z candidate is reconstructed from the combination of the two electrons with an invariant mass closest to the nominal Z boson mass. The reconstructed mass of the Z candidate is required be compatible with the nominal Z boson mass within 7 GeV. After this selection no data event remains, while 0.72 ± 0.06 SM background events are expected. The selection efficiency for e* with masses above 120 GeV is ~ 60%.

In the search for e* ^ eZ ^ epp, events are selected with one electron with transverse momentum above 20 GeV and two muons with transverse momenta above 15 and 10 GeV, respectively. The electron and the muons must be detected in the polar angle ranges 5° <6e < 150° and 10° < 6p < 160°, respectively. A Z candidate

23 For NC DIS events, this variable is proportional to the four-momentum transfer squared Q2.

is reconstructed from the combination of the two muons and its reconstructed mass is required to be larger than 60 GeV. After this selection no data event remains, while 0.52 ± 0.05 SM background events are expected. The selection efficiency in this channel is ~ 40% for Me* = 120 GeV, decreasing to 15% for Me* = 260 GeV.

The signatures of the e* ^ vW ^ vev and e* ^ eZ ^ evv channels are similar and consist of one high PT electron in events with large missing transverse momentum. Events with Pm'ss > 25 GeV and one electron with PT > 20 GeV are selected. The electron is detected in the polar angle range 5° <6e < 100° and is required to be isolated from jets by a minimum distance of R > 1. To reduce the background from radiative CC DIS processes, a track must be associated to the electron in the central region (6e > 35°). Events from photoproduction are suppressed by requiring Vap/Vp < 0.1. Remaining NC DIS events are removed by requiring that the longitudinal momentum balance of the event be Ej (Ej - Pz,i) < 45 GeV, where the sum runs over all visible particles. In order to remove background arising from SM W production, the hadron system is required to have a total transverse momentum PT < 20 GeV and to exhibit a polar angle Yh, as defined in [37], below 80°. In each event, only one neutrino candidate can be reconstructed, from the total missing transverse momentum, as explained at the beginning of Section 5. The invariant mass of the e* candidate in the evv final state is therefore estimated from the four-vectors of the neutrino candidate and the electron candidate. To further remove background from W production, only events in which the reconstructed e* mass is above 90 GeV are considered. After this selection four data events remain, while 4.5 ± 0.7 SM background events are expected. The selection efficiency for the e* ^ vW ^ vev (e* ^ eZ ^ evv) signature is ~ 60% (~ 35%) for e* with masses above 120 GeV.

5.5. Systematic uncertainties

The following experimental systematic uncertainties are considered:

• The uncertainty on the electromagnetic energy scale varies between 0.7% and 2% depending on the polar angle. The polar angle measurement uncertainty is 3 mrad for electromagnetic clusters.

• The scale uncertainty on the transverse momentum of high PT muons amounts to 2.5%. The uncertainty on the reconstruction of the muon polar angle is 3 mrad.

• The hadronic energy scale is known within 2%. The uncertainty on the jet polar angle determination is 10 mrad.

• The uncertainty on the trigger efficiency is 3%.

• The luminosity measurement has an uncertainty of 3%.

The effect of the above systematic uncertainties on the SM expectation and the signal efficiency are determined by varying the experimental quantities by ±1 standard deviation in the MC samples and propagating these variations through the whole analysis chain.

Additional model systematic uncertainties are attributed to the SM background MC generators described in Section 3. An error of 20% on the normalisation of NC DIS, CC DIS and photoproduction processes with at least two high PT jets is considered to account for the uncertainty on higher order QCD corrections. The error on the elastic and quasi-elastic QED Compton cross sections is conservatively estimated to be 5%. The error on the inelastic QED Compton cross section is 10%. The errors attributed to lepton pair and W production are 3% and 15%, respectively. The total error on the SM background prediction is determined by adding the effects of all model and experimental systematic uncertainties in quadrature.

Table 1

Observed and predicted event yields for the studied e* decay channels. The analysed data sample corresponds to an integrated luminosity of 475 pb-1. The errors on the SM predictions include model and experimental systematic errors added in quadrature. Typical selection efficiencies for e* masses ranging from 120 to 260 GeV are also indicated

The theoretical uncertainty on the e* production cross section is dominated by the uncertainty on the scale at which the proton parton densities are evaluated. It is estimated by varying this scale from vQ2/2 to The resulting uncertainty depends on the

e* mass and is 10% at Me* = 100 GeV, increasing to 15% at Me* = 300 GeV.

6. Interpretation

The event yields observed in all decay channels are in agreement with the corresponding SM expectations and are summarised in Table 1. The SM predictions are dominated by QED Compton for the ey resonance search, by CC DIS in the vqq resonance search and by NC DIS processes for the eqq resonance search. The distributions of the invariant mass of the data events are in agreement with those of the expected SM background as shown in Fig. 1. Few or no data events are observed in channels corresponding to lep-tonic decays of the W or Z bosons, in agreement with the low SM expectations.

Since no evidence for the production of excited electrons is observed, upper limits on the e* production cross section and on the model parameters are derived as a function of the mass of the excited electron. Limits are presented at the 95% confidence level (CL) and are obtained from the mass spectra using a modified frequentist approach which takes statistical and systematic uncertainties into account [39].

Upper limits on the product of the e* production cross section and of the e* decay branching ratio are shown in Fig. 2. The analysed decay channels of the W and Z gauge bosons are combined. Considering pure gauge interactions, the resulting limit on f /A after combination of all decay channels is displayed as a function of the e* mass in Fig. 3, for the conventional assumption f = +f'. The total fraction of all possible e* gauge decay channels covered in this analysis is ~ 88%. The limit extends up to e* masses of 290 GeV. Considering the assumption f /A = 1/Me* excited electrons with masses up to 272 GeV are excluded. The relative contributions of the e* decay channels to the combined limit are shown in Fig. 3(a). At low mass, the combined limit on f /A is dominated by the e* ^ ey channel, while the e* ^ v W channel starts to contribute to the limit for masses above 200 GeV. These new results extend the previously published limits by H1 [2] and ZEUS [40] by more than a factor of two in f /A. Fig. 3(b) shows direct and indirect limits on e* production obtained in e+ e- collisions at LEP by the OPAL Collaboration [41] and DELPHI Collaboration [42], respectively. The result of the most recent search for e* production within gauge mediated models obtained at the Tevatron by the CDF Collaboration is also indicated [43]. The limit from the present analysis extends at high mass beyond the kinematic reach of LEP searches and to lower f /A values than are reached by Tevatron searches.

Search for e* at HERA (475 pb'1)

s1101---—-—-—

Q. l H1 ..... e*^ey

0C I ................ e*^eZ

1 A. -------- eMvW

10-2r —..........--------

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

100 120 140 160 180 200 220 240 260 280 300

e* Mass [GeV]

Fig. 2. Upper limits at 95% CL on the product of the e* cross section and decay branching ratio, a x BR, in the three e* decay channels as a function of the excited electron mass. The decay channels of the W and Z gauge bosons are combined. The limits correspond to e± p collision data with an effective V? = 318 GeV and relative fractions of e+ p and e-p data of 61% and 39%, respectively. Areas above the curves are excluded.

If e* production is considered via gauge and contact interactions together, an upper limit on 1 /A is also obtained, under the assumption f = f' = 1. Possible e* decays by either gauge or contact interactions are taken into account and the efficiency of the analysis to e* CI decays is conservatively assumed to be zero. The limit on 1 /A as a function of the e* mass is displayed in Fig. 4. For e* masses below 250 GeV, the additional contribution of CI to e* production changes the limit on A by a factor of 1.15 to 1.2. A limit on A as a function of the e* mass is also obtained at the Tevatron by considering single e* production via contact interactions only, followed by its gauge decay into an electron and a photon [44].

7. Conclusion

Using the full e±p data sample collected by the H1 experiment at HERA with an integrated luminosity of 475 pb-1 a search for the production of excited electrons is performed. The excited electron decay channels e* ^ ey, e* ^ eZ and e* ^ vW with subsequent hadronic or leptonic decays of the W and Z bosons are considered and no indication of a signal is found. New limits on the production cross section of excited electrons are obtained. Within gauge mediated models, an upper limit on the coupling f /A as a function of the excited electron mass is established for the specific relation f = +f' between the couplings. Assuming f = +f' and f /A = 1/Me* excited electrons with a mass lower than 272 GeV are excluded at 95% confidence level. For the first time in ep collisions, gauge and four-fermion contact interactions are also considered together for e* production and decays. In this scenario and assuming the same A parameter in contact and gauge interactions as well as f = +f' = 1, nL = 1 and nR = 0, the limit on 1 /A improves only slightly, demonstrating that the gauge interaction mechanism is dominant for excited electron processes at HERA. The results presented in this Letter extend previously excluded domain at HERA, LEP or Tevatron.

Acknowledgements

We are grateful to the HERA machine group whose outstanding efforts have made this experiment possible. We thank the

Channel Data SM Signal efficiency [%]

e* ^ ey (ela.) 42 48 ± 4 60-70

e* ^ ey (inel.) 65 65 ± 8 60-70

e* ^ v W ^ vqq 129 133 ± 32 20-55

e* ^ v W ^ vev e* ^ eZ ^ evv 4 4.5 ± 0.7 60 35

e* ^ eZ ^ eqq 286 277 ± 62 20-55

e* ^ eZ ^ eee 0 0.72 ± 0.06 60

e* ^ eZ ^ efifi 0 0.52 ± 0.05 40-15

Search for e* at HERA (475 pb"1)

Search for e* at HERA (475 pb'1)

! hi f = + f

^..f/A = 1 /Me*

Tevatron (202 pb"1) ! LEP OndirectT/

Ë ___^ I

— r / /

= * LEP (direct) "a J I I M I I 1 I 1 1 1 _l_1_L J i_lJ I 111 L_i_1_L l III j_L 1 1 1_1_1

100 120 140 160 180 200 220 240 260 280 300

e* Mass [GeV]

Fig. 3. Exclusion limits at 95% CL on the coupling f /A as a function of the mass of the excited electron considering gauge mediated interactions only, with the assumption f = +f'. The separate contributions of the different e* decay channels are presented in (a). Values of the couplings above the curves are excluded. The excluded domain based on all H1 e±p data is represented in (b) by the shaded area. It is compared to the direct (dashed line) and indirect (dotted line) exclusion limits obtained at LEP by the OPAL Collaboration [41] and by the DELPHI Collaboration [42], respectively. The result from the Tevatron obtained by the CDF experiment [43] is also shown (dashed-dotted line). The curve f /A = 1/Me* is indicated in (b).

engineers and technicians for their work in constructing and maintaining the H1 detector, our funding agencies for financial support, the DESY technical staff for continual assistance and the DESY directorate for the hospitality which they extend to the non-DESY members of the Collaboration. We also wish to thank M. Spira for many useful discussions and for providing the cross section calculation for excited electron production including contact interactions in ep collisions.

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X II 11 I I I I II 111 I I t = + f = 1 \ = 1 \ = °

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