At the HERA electron-proton collider an event has been observed in the H1 detector which shows an isolated muon recoiling against a hadronic system, both of high transverse momentum. The event was registered in a total integrated luminosity of 4 pb ?1 .

A measurement is presented, using data taken with the H1 detector at HERA, of the contribution of diiractive interactions to deep{inelastic electron{proton (ep) scattering in the kinematic range 8:5 < Q 2 < 50 GeV 2 , 2:410 ?4 < Bjorken{x < 0:0133, and 3:7 10 ?4 < x IP < 0:043. The diiractive contribution to the proton structure function F 2 (x; Q 2) is evaluated as a function of the appropriate deep{inelastic scattering variables x IP , Q 2 , and (= x=x IP) using a class of deep{inelastic ep scattering events with no hadronic energy ow in an interval of pseudo{rapidity adjacent to the proton beam direction. The dependence of this contribution on x IP is measured to be x ?n IP with n = 1:19 0:06(stat:) 0:07(syst:) independent of and Q 2 , which is consistent with both a diiractive interpretation and a factorisable ep diiractive cross section. A rst measurement of the deep{inelastic structure of the pomeron in the form of the Q 2 and dependences of a factorised structure function is presented. For all measured , this structure function is observed to be consistent with scale invariance.

We report results of the first measurements of Λ and ¯ Λ polarization produced in deep inelastic polarized muon scattering on the nucleon. The results are consistent with an expected trend towards positive polarization with increasing x F. The polarizations of Λ and ¯ Λ appear to have opposite signs. A large negative polarization for Λ at low positive x F is observed and is not explained by existing models. A possible interpretation is presented.

Quasi-elastic (z > 0.95) photo-production of ψ(2S) mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The ψ(2S) mesons were identified through their decays to l+l− and to J/ψ π+π−, where the J/ψ subsequently decays to l+l−, the lepton l being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be (18.0±2.8(stat)±3.0(syst)) nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the ψ(2S) to J/ψ quasi-elastic cross-sections is (0.150 ± 0.027(stat) ± 0.022(syst)). Submitted to Physics Letters B C. Adloff35, S. Aid13, M. Anderson23, V. Andreev26, B. Andrieu29, V. Arkadov36, C. Arndt11, I. Ayyaz30, A. Babaev25, J. Bähr36, J. Bán18, P. Baranov26, E. Barrelet30, R. Barschke11, W. Bartel11, U. Bassler30, M. Beck14, H.-J. Behrend11, C. Beier16, A. Belousov26, Ch. Berger1, G. Bernardi30, G. Bertrand-Coremans4, R. Beyer11, P. Biddulph23, J.C. Bizot28, K. Borras8, V. Boudry29, S. Bourov25, A. Braemer15, W. Braunschweig1, V. Brisson28, D.P. Brown23, W. Brückner14, P. Bruel29, D. Bruncko18, C. Brune16, J. Bürger11, F.W. Büsser13, A. Buniatian4, S. Burke19, G. Buschhorn27, D. Calvet24, A.J. Campbell11, T. Carli27, M. Charlet11, D. Clarke5, B. Clerbaux4, S. Cocks20, J.G. Contreras8, C. Cormack20, J.A. Coughlan5, M.-C. Cousinou24, B.E. Cox23, G. Cozzika9, D.G. Cussans5, J. Cvach31, S. Dagoret30, J.B. Dainton20, W.D. Dau17, K. Daum40, M. David9, A. De Roeck11, E.A. De Wolf4, B. Delcourt28, M. Dirkmann8, P. Dixon19, W. Dlugosz7, K.T. Donovan21, J.D. Dowell3, A. Droutskoi25, J. Ebert35, T.R. Ebert20, G. Eckerlin11, V. Efremenko25, S. Egli38, R. Eichler37, F. Eisele15, E. Eisenhandler21, E. Elsen11, M. Erdmann15, A.B. Fahr13, L. Favart28, A. Fedotov25, R. Felst11, J. Feltesse9, J. Ferencei18, F. Ferrarotto33, K. Flamm11, M. Fleischer8, M. Flieser27, G. Flügge2, A. Fomenko26, J. Formánek32, J.M. Foster23, G. Franke11, E. Gabathuler20, K. Gabathuler34, F. Gaede27, J. Garvey3, J. Gayler11, M. Gebauer36, R. Gerhards11, A. Glazov36, L. Goerlich6, N. Gogitidze26, M. Goldberg30, B. Gonzalez-Pineiro30 , I. Gorelov25, C. Grab37, H. Grässler2, T. Greenshaw20, R.K. Griffiths21, G. Grindhammer27, A. Gruber27, C. Gruber17, T. Hadig1, D. Haidt11, L. Hajduk6, T. Haller14, M. Hampel1, W.J. Haynes5, B. Heinemann11, G. Heinzelmann13, R.C.W. Henderson19, S. Hengstmann38, H. Henschel36, R. Heremans4, I. Herynek31, K. Hewitt3, K.H. Hiller36, C.D. Hilton23, J. Hladký31, M. Höppner8, D. Hoffmann11, T. Holtom20, R. Horisberger34, V.L. Hudgson3, M. Hütte8, M. Ibbotson23, Ç. İşsever8, H. Itterbeck1, M. Jacquet28, M. Jaffre28, J. Janoth16, D.M. Jansen14, L. Jönsson22, D.P. Johnson4, H. Jung22, P.I.P. Kalmus21, M. Kander11, D. Kant21, U. Kathage17, J. Katzy15, H.H. Kaufmann36, O. Kaufmann15, M. Kausch11, S. Kazarian11, I.R. Kenyon3, S. Kermiche24, C. Keuker1, C. Kiesling27, M. Klein36, C. Kleinwort11, G. Knies11, J.H. Köhne27, H. Kolanoski39, S.D. Kolya23, V. Korbel11, P. Kostka36, S.K. Kotelnikov26, T. Krämerkämper8, M.W. Krasny6,30, H. Krehbiel11, D. Krücker27, A. Küpper35, H. Küster22, M. Kuhlen27, T. Kurča36, B. Laforge9, R. Lahmann11, M.P.J. Landon21, W. Lange36, U. Langenegger37, A. Lebedev26, F. Lehner11, V. Lemaitre11, S. Levonian29, M. Lindstroem22, J. Lipinski11, B. List11, G. Lobo28, G.C. Lopez12, V. Lubimov25, D. Lüke8,11, L. Lytkin14, N. Magnussen35, H. Mahlke-Krüger11, E. Malinovski26, R. Maraček18 , P. Marage4, J. Marks15, R. Marshall23, J. Martens35, G. Martin13, R. Martin20, H.-U. Martyn1, J. Martyniak6, S.J. Maxfield20, S.J. McMahon20, A. Mehta5, K. Meier16, P. Merkel11, F. Metlica14 , A. Meyer13, A. Meyer11, H. Meyer35, J. Meyer11, P.-O. Meyer2, A. Migliori29, S. Mikocki6, D. Milstead20, J. Moeck27, F. Moreau29, J.V. Morris5, E. Mroczko6, D. Müller38, K. Müller11, P. Muŕın18, V. Nagovizin25, R. Nahnhauer36, B. Naroska13, Th. Naumann36, I. Négri24, P.R. Newman3, D. Newton19, H.K. Nguyen30, T.C. Nicholls3, F. Niebergall13, C. Niebuhr11, Ch. Niedzballa1, H. Niggli37, G. Nowak6, T. Nunnemann14, H. Oberlack27, J.E. Olsson11, D. Ozerov25, P. Palmen2, E. Panaro11, A. Panitch4, C. Pascaud28, S. Passaggio37, G.D. Patel20, H. Pawletta2, E. Peppel36, E. Perez9, J.P. Phillips20, A. Pieuchot24, D. Pitzl37, R. Pöschl8, G. Pope7, B. Povh14, K. Rabbertz1, P. Reimer31, H. Rick8, S. Riess13, E. Rizvi11, P. Robmann38, R. Roosen4, K. Rosenbauer1, A. Rostovtsev30, F. Rouse7, C. Royon9, K. Rüter27, S. Rusakov26, K. Rybicki6, D.P.C. Sankey5, P. Schacht27, J. Scheins1, S. Schiek11, S. Schleif16, W. von Schlippe21, D. Schmidt35, G. Schmidt11, L. Schoeffel9, A. Schöning11, V. Schröder11, E. Schuhmann27, H.-C. Schultz-Coulon11, B. Schwab15, F. Sefkow38, A. Semenov25, V. Shekelyan11, I. Sheviakov26, L.N. Shtarkov26, G. Siegmon17, U. Siewert17, Y. Sirois29, I.O. Skillicorn10, T. Sloan19, P. Smirnov26, M. Smith20, V. Solochenko25, Y. Soloviev26, A. Specka29, J. Spiekermann8, S. Spielman29, H. Spitzer13, F. Squinabol28, P. Steffen11, R. Steinberg2, J. Steinhart13, B. Stella33, A. Stellberger16, J. Stiewe16, K. Stolze36, U. Straumann15, W. Struczinski2, J.P. Sutton3, M. Swart16, S. Tapprogge16, M. Taševský32, V. Tchernyshov25, S. Tchetchelnitski25, J. Theissen2, G. Thompson21, P.D. Thompson3, N. Tobien11, R. Todenhagen14, P. Truöl38, G. Tsipolitis37, J. Turnau6, E. Tzamariudaki11, P. Uelkes2, A. Usik26, S. Valkár32, A. Valkárová32, C. Vallée24 , P. Van Esch4, P. Van Mechelen4, D. Vandenplas29, Y. Vazdik26, P. Verrecchia9, G. Villet9, K. Wacker8, A. Wagener2, M. Wagener34, R. Wallny15, T. Walter38, B. Waugh23, G. Weber13, M. Weber16, D. Wegener8, A. Wegner27, T. Wengler15, M. Werner15, L.R. West3, S. Wiesand35, T. Wilksen11, S. Willard7, M. Winde36, G.-G. Winter11, C. Wittek13, M. Wobisch2, H. Wollatz11 , E. Wünsch11, J. Žáček32, J. Zálešák32, D. Zarbock12, Z. Zhang28, A. Zhokin25, P. Zini30, F. Zomer28, J. Zsembery9, and M. zurNedden38, 1 I. Physikalisches Institut der RWTH, Aachen, Germany 2 III. Physikalisches Institut der RWTH, Aachen, Germany 3 School of Physics and Space Research, University of Birmingham, Birmingham, UK 4 Inter-University Institute for High Energies ULB-VUB, Brussels; Universitaire Instelling Antwerpen, Wilrijk; Belgium 5 Rutherford Appleton Laboratory, Chilton, Didcot, UK 6 Institute for Nuclear Physics, Cracow, Poland 7 Physics Department and IIRPA, University of California, Davis, California, USA 8 Institut für Physik, Universität Dortmund, Dortmund, Germany 9 DSM/DAPNIA, CEA/Saclay, Gif-sur-Yvette, France 10 Department of Physics and Astronomy, University of Glasgow, Glasgow, UK 11 DESY, Hamburg, Germany 12 I. Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany 13 II. Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany 14 Max-Planck-Institut für Kernphysik, Heidelberg, Germany 15 Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany 16 Institut für Hochenergiephysik, Universität Heidelberg, Heidelberg, Germany 17 Institut für Reine und Angewandte Kernphysik, Universität Kiel, Kiel, Germany 18 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic 19 School of Physics and Chemistry, University of Lancaster, Lancaster, UK 20 Department of Physics, University of Liverpool, Liverpool, UK 21 Queen Mary and Westfield College, London, UK 22 Physics Department, University of Lund, Lund, Sweden 23 Physics Department, University of Manchester, Manchester, UK 24 CPPM, Université d’Aix-Marseille II, IN2P3-CNRS, Marseille, France 25 Institute for Theoretical and Experimental Physics, Moscow, Russia 26 Lebedev Physical Institute, Moscow, Russia 27 Max-Planck-Institut für Physik, München, Germany 28 LAL, Université de Paris-Sud, IN2P3-CNRS, Orsay, France 29 LPNHE, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France 30 LPNHE, Universités Paris VI and VII, IN2P3-CNRS, Paris, France 31 Institute of Physics, Czech Academy of Sciences of the Czech Republic, Praha, Czech Republic 32 Nuclear Center, Charles University, Praha, Czech Republic 33 INFN Roma 1 and Dipartimento di Fisica, Università Roma 3, Roma, Italy 34 Paul Scherrer Institut, Villigen, Switzerland 35 Fachbereich Physik, Bergische Universität Gesamthochschule Wuppertal, Wuppertal, Germany 36 DESY, Institut für Hochenergiephysik, Zeuthen, Germany 37 Institut für Teilchenphysik, ETH, Zürich, Switzerland 38 Physik-Institut der Universität Zürich, Zürich, Switzerland 39 Institut für Physik, Humboldt-Universität, Berlin, Germany 40 Rechenzentrum, Bergische Universität Gesamthochschule Wuppertal, Wuppertal, Germany a Supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, FRG, under contract numbers 7AC17P, 7AC47P, 7DO55P, 7HH17I, 7HH27P, 7HD17P, 7HD27P, 7KI17I, 6MP17I and 7WT87P b Supported by the UK Particle Physics and Astronomy Research Council, and formerly by the UK Science and Engineering Research Council c Supported by FNRS-NFWO, IISN-IIKW d Partially supported by the Polish State Committee for Scientific Research, grant no. 115/E343/SPUB/P03/002/97 and grant no. 2P03B 055 13 e Supported in part by US DOE grant DE F603 91ER40674 f Supported by the Deutsche Forschungsgemeinschaft g Supported by the Swedish Natural Science Research Council h Supported by GA ČR grant no. 202/96/0214, GA AV ČR grant no. A1010619 and GA UK grant no. 177 i Supported by the Swiss National Science Foundation j Supported by VEGA SR grant no. 2/1325/96 k Supported by Russian Foundation for Basic Researches grant no. 96-02-00019

Characteristics of hadron production in diffractive deep-inelastic positron-proton scattering are studied using data collected in 1994 by the Hi experiment at HERA. The following distributions are measured in the centre-of-mass frame of the photon dissociation system: the hadronic energy flow, the Feynman-x (x(F),) variable for charged particles, the squared transverse momentum of charged particles (p(T)*(2)), and the mean p(T)*(2) as a function of x(F),. These distributions are compared with results in the gamma *p centre-of-mass frame from inclusive deep-inelastic scattering in the fixed-target experiment EMC, and also with the predictions of several Monte Carlo calculations. The data are consistent with a picture in which the partonic structure of the diffractive exchange is dominated at low Q(2) by hard gluons. (C) 1998 Elsevier Science B.V. All rights reserved.

Low x deep-inelastic ep scattering data, taken in 1994 at the H1 detector at HERA, are analysed in the Breit frame of reference. The evolution of the peak and width of the current hemisphere fragmentation function is presented as a function of Q and compared with e+e− results at equivalent centre of mass energies. Differences between the average charged multiplicity and the multiplicity of e+e− annihilations at low energies are analysed. Invariant energy spectra are compared with MLLA predictions. Distributions of multiplicity are presented as functions of Bjorken-x and Q2, and KNO scaling is discussed.

The results of a measurement of the proton structure function F2(x,Q2) and the virtual photon-proton cross section are reported for momentum transfers squared Q2 between 0.35 GeV2 and 3.5 GeV2 and for Bjorken-x values down to 6 × 10−6 using data collected by the HERA experiment H1 in 1995. The data represent an increase in kinematic reach to lower x and Q2 values of about a factor of 5 compared to previous H1 measurements. Including measurements from fixed target experiments the rise of F2 with decreasing x is found to be less steep for the lowest Q2 values measured. Phenomenological models at low Q2 are compared with the data.

Results obtained by the H1 Collaboration in deep-inelastic e+p scattering up to 4-momentum transfers Q2 up to 35000 GeV2 are presented. At Q2 > 15000 GeV2 12 events are observed in neutral current (NC) interactions, where only 4.71±0.76 events are expected. In charged current (CC) interactions in the same range of Q2 4 events are observed, while the standard expectation is 1.77±0.87 events.

The electroproduction of rho mesons with proton diffractive dissociation for Q^2 > 7 GeV^2 and the elastic electroproduction of Phi mesons for Q^2 > 6 Gev^2 are studied in e^+ p collisions at HERA with the H1 detector, for an integrated luminosity of 2.8 pb-1. The dependence of the cross sections on P_t^2 and Q^2 is measured, and the vector meson polarisation obtained. The cross section ratio between proton dissociative and elastic production of rho mesons is measured and discussed in the framework of the factorisation hypothesis of diffractive vertices. The ratio of the elastic cross section for Phi and rho meson production is investigated as a function of Q^2.

A measurement of the inclusive cross section for the deep-inelastic scattering of positrons off protons at HERA is presented at momentum transfers 8.5 ≤ Q2 ≤ 35 GeV2 and large inelasticity = 0.7, i.e. for the Bjorken-x range 0.00013 ≤ x ≤ 0.00055. Using a next-to-leading order QCD fit to the structure function F2 at lower y values, the contribution of F2 to the measured cross section at high y is calculated and, by subtraction, the longitudinal structure function FL is determined for the first time with an average value of FL = 0.52 ± 0.03 (statt)−0.22+0.25 (syst) at Q2 = 15.4 GeV2 and x = 0.000243.

The transition between photoproduction and deep-inelastic scattering is investigated in jet production at the HERA ep collider, using data collected by the H1 experiment. Measurements of the differential inclusive jet cross-sections d sigma(ep)/dE(t)* and d sigma(ep)/d eta*, where E-t* and eta* are the transverse energy and the pseudorapidity of the jets in the virtual photon-proton centre of mass frame, are presented for 0 < Q(2) < 49 GeV2 and 0.3 < y < 0.6. The interpretation of the results in terms of the structure of the virtual photon is discussed. The data are best described by QCD calculations which include a partonic structure of the virtual photon that evolves with Q(2). (C) 1997 Published by Elsevier Science B.V.

The diffractive production of ρ0(770 @#@) mesons in muon-proton interactions is studied in the kinematic region 0.15 GeV2 < Q2 < 20 GeV2 and 20 GeV < ? < 420 GeV. The data were obtained in the Fermilab fixed-target experiment E665 with primary muons of 470 GeV energy. Results are presented on the Q2, x and ? dependence of the cross section, on the shape of the ρ+ρt - mass spectrum, on the slope of the diffraction peak and on the production and decay angular distributions of the ρ0(770). The cross section for diffractive production of ρ0 by virtual photons on protons depends mainly on Q2. At fixed Q2, no significant dependence on x or ? is observed. The extrapolation to Q2 = 0 yields a photoproduction cross section of (10.30 ± 0.33) μb. The slope of the t′ distribution has a value of (7.0 ± 0.2) GeV−2, with a tendency to decrease as Q2 increases. The production and decay angular distributions of the ρ0 depend strongly on Q2 and are consistent with s-channel helicity conservation. The ratio R = σ l /σ t deduced from the decay angular distributions rises strongly with Q2, passing the value of 1 at Q2 ≈ 2 GeV2.

We present a search for excited electrons, neutrinos and quarks using the H1 detector at the ep collider HERA, based on data taken in 1994 with an integrated luminosity of 2.75 pb(-1). Radiative decays of excited quarks and neutrinos have been investigated as well as decays of excited electrons into all possible electroweak gauge bosons. No evidence for new particle production is found and exclusion limits are derived.

Technical aspects of the three major components of the H1 detector at the electron-proton storage ring HERA are described. This paper covers the detector status up to the end of 1994 when a major upgrading of some of its elements was undertaken. A description of the other elements of the detector and some performance figures from luminosity runs at HERA during 1993 and 1994 are given in a paper previously published in this journal.

A study is presented of the process γp → XY, where there is a large rapidity gap between the systems X and Y. Measurements are made of the differential cross section as a function of the invariant mass M X of the system produced at the photon vertex. Results are presented at centre of mass energies of 〈W〉 = 187 GeV and 〈W〉 =231 GeV, both where the proton dominantly remains intact and, for the first time, where it dissociates. Both the centre of mass energy and the \(M_X^2\) dependence of HERA data and those from a fixed target experiment may simultaneously be described in a triple-Regge model. The low mass photon dissociation process is found to be dominated by diffraction, though a sizable subleading contribution is present at larger masses. The pomeron intercept is extracted and found to be αP(0) = 1.068 ± 0.016 (stat.) ± 0.022 (syst.) ± 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process γp → Xp with \(M_X^2/{W^2} < 0.05\) is measured to be 22.2 ± 0.6 (stat.) ± 2.6 (syst.) ± 1.7 (model)% of the total γp cross section at 〈W〉 = 187 GeV.

Transverse momentum spectra of charged particles produced in deep inelastic scattering are measured as a function of the kinematic variables x and Q(2) using the H1 detector at the ep collider HERA. The data are compared to different parton emission models, either with or without ordering of the emissions in transverse momentum, The data provide evidence for a relatively large amount of parton radiation between the current and the remnant systems.