The cross section of the pure QED process e+ e-> "/"/ has been measured using data accumulated during the 1989 and 1990 scans of the Z resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction, Upper limits on the branching ratios of Z -> "f"f, Z -> 1r "f and Z -> rn have been set at lAx I o-4 , 1 Ax 10-'1 and 2.0 x 10-4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be A+> 117 GeV and A_ > 110 GeV. The reaction e+e-> "/"1"1 has also been studied and was found to be consistent with the QED prediction_ An upper limit on the branching ratio of Z _,"/"/"/has been set at 6 .. 6 x 10• All the limits are given at 95% confidence leveL (Submitted to Physics Letters D) The OPAL Collaboration lii.Z. Akrawy 12 , G. Alexander22 , J. Allison 15 , P.P. Allport5 , K.J. Anderson9 , J.C. Armitage6 , G.T.J. Arnison 9, P. Ashton15 , G. Azuelos·d, J.T.l\I. Baines15 , A. H. Ball 16 , J. Banks15 , G.J. Barker , R.J. Barlow 15 , .I.R. Batley5 , G. Beaudoin'\ A. Beck 22 , J. Becker10 , T. Behnkes, K.W. Bell' 9 , G. Bella22 , S. Bethken, 0. Bicbel , U. Binder 10 , I.J. Bloodworth', P. Bockn, II. Breukers, R.l\1. Brown 19 , R. Bruns, A. Buijss, H.J. Burckharts, P. Capiluppi2 , R. K. Carncgie , A.A. Carler12 , J .R. Carter5 , C.Y. Chang16 , D.G. Charltons, J .T.l\1. Chrin 15 , P.E.L. Clarkc , I. Cohen , W.J. Collins , J .E. Conboy14 , i\1. Couch , 1\l. Coupland , 1\l. Cuffiani , S. Dado21 , G.l\1. Dallamlle , S. De Jongs, P. Dcbu20 , 1\l.l\l. Deninno , A. Dieckmann ll, 1\l. Dittmar', 1\l.S. Dixit 1 , E. Duchovni25 , l.P. Duerdoth 15 , D .. J.P. Dumas6 , P.A. Elcombe , P.G. Estabrooks6 , E. Etzion , F. Fabbr?, P. Farthouat20 , H.l\1. Fischer , D.G. Fong , 1\LT. French19 , C. Fukunaga , A. Gaidot20 , 0. GaneF5 , J.W. Garyll, .) . Gascon , N .I. Geddes19 , C.N .P. Gee19 , C. Geich-GimbeP, S. W. Gensler9 , F .X. Gentit20 , G. Giacomcll?, V. Gibson 5 , W.R. Gibson 12 , J.D. Gillies , J. Goldberg21 , i\I.J. Goodrick5 , W. Gorn4 , D. Granite21 , E. Gross25 , J. Grunhaus , H. Hagedorn 10 , J. llagemanns, 1\l.llansrouls, C.K.IIargrove7, I. Harrus21 , J. Hart5 , P.l\1. Hattersley', 1\l.llauschilds, C.l\1. Haw kess, E. Heflin 4 , R.J. llemingway6 , ltD. Heuers, J .C. llill5 , S.J. Hillier1 , D.A.llinshaw 17 , C. Ho4, J.D. Hobbs9 , P.R. Hobson', D. Hochman25 , B. HoUS, R.J. l!omer1 , S.R. Ilou16 , C.P.llowarth1\ R.E.llughcs-Jones , R. Humbert10 , P.lgo-Kemcnesn, H. lhssenll, D.C. lmrie24 , L. Janissen6 , A. Jawahery , P.\V. Jcffreys19 , H. Jeremie , M.Jimacks,.M.Jobes1 , R.W.L.Jones12 , P.Jovanovic1 , D. Karlen6 , K.Kawagoe23 , T. Kawamoto23 , R.G. Kcllogg16 , B.W. Kennedy 14 , C. Klein worts, D.E. Klem1s, G. Knop3 , T. Kobayash? , T.P. Kokott3 , L. Kopkes, R. Kowalewski6 , H. Kreutzmann\ J. Kroll , 1\I. Kuwano , P. Kyberd 12 , G.D. Lalferty'5 , F. Lamarche17 , W.J. Larson', J.G. Layter , P. Le Du20 , P. Leblanc17 , A.M. Lee16 , I\!. H. Lehto14 , D. Lellouchs, P. Lennertn, C. Leroy17 , L. Lessard17 , S. Levegriin3 , L. Levinson25 , S.L. Lloyd , F.K. Loebinger15 , J.l\1. Lorah16 , B. Lora.zo17 , 1\l.J. Losty 7, J. Ludwig10 , J. l\Ia4•b, A.A.l\!acbeth15 , 1\L 1\Iannellis, S. Marcellini2, G.l\Iaringer , A.J.l\Iartin12 , J.P. i\Iartin , T. Mashimo , P.l\Iiittig , U.l\Iaur , T.J. Mcl\!ahon 1 , J.R.l\IcNutt24 , F. Meijerss, D. Mensznern, F.S.l\Ierritt9 , H.l\Ies 7, A.l\Iichelinis, R.P. Middleton 19 , G.l\Iikenberg, J. l\!ildenbcrger6 , D.J. Miller 14 , C.l\Iilstcne22 , l\l.l\Iinowa23 , W.l\lohr10 , C.l\loisan17 , A. Montanari2 , T. Mori23 , 1\l.W. Moss , P.G.l\Iurphy' 5 , W.J.l\lurray , B. Ncllen , lUI. Nguyen , 1\l. Nozaki , A.J.P. O'Dowd15 , S.W. O'Neales,c, B.P. O'Neill', F.G. Oakham', F. Odorici2 , 1\I. Ogg6 , II. Oh 4 , 1\I.J. Oreglia9 , S. Orito23 , J.P. Pansart , G.N. Patrick19 , S.J. Pawley' 5 , P. Pfister10 , J.E. Pilcher9 , J.L. Pinfold25 , D.E. Planes, B. Pol?, A. Pouladdej", E. Prebyss, T.W. Pritchard 12 , II. Przysiezniak17 , G. Quasts, M.W. Redmond 9 , D.L. Rees1 , 1\1. Regimbald11 , K. Riles', C.M. Roach , S.A. Robins 12 , A. Rollnik3 , J.l\1. Roney9 , S. Rossberg 10 , A.M. Rossi2·•, P. Routenburg , K. Runge 10 , 0. Runolfssons, S. Sanghera6 , R.A. Sansum , 1\1. Sasaki23 , B.J. Saunders , A.D. Schaile10 , 0. Scha.ile10 , W. Schappert6 , P. Scha.rlf-Ha.nsens, S. Schreiber3 , J. Schwarz , A. Shapira.25 , B.C. Shen\ P. Sherwood", A. Simon3 , P. Singh 12 , G.P. Siroli2 , A. Skuja16 , A.M. Smiths, T.J. Smiths, G.A. Snow 16 , R.W. Springer , 1\I. Sproston , K. Stephens , H.E. Stier10 , R. Stroehmerll, D. Strom9 , H. Ta.kcda , T. Takeshita. , P. Taras, N.J. Tha.ckra.y', T. Tsukamoto23 , I\!. F. Turner5 , G. Tysarczyk-Niemeyeru, D. Va.n den pla.s , R. Va.n Kootens, G.J. VanDalen', G. Va.sseur20 ,

We present evidence for the existence of the strange b-flavoured meson B~ in a data sample of 470 628 hadronic Z decays recorded with the OPAL detector at LEP. A signal of 18.3±5.2(stat.)±0.9(syst.) D;;-£+ pairs (or charge conju,gate, R=e or R=Jl) is found after background subtraction, where the D, meson is detected in the )'lr and K*°K final states. Interpreting this signal as coming from the semileptonic deeay B~ --+ D; e+ v X and combining the two decay modes of the D., we find J(b--+ B~) · B(B~--+ D;;-R+vX) · B(D;;---+ >7r-)= (3.9±l.l(stat.)±0.8(syst.))x10-\ where J(b--+ B~) is the fraction of b quarks that result in a B~ meson in Z decays. This signal for the B~ is supported by our observation of an excess of 147±48 inclusive D, mesons in the >1r and K*°K modes above the number expected from B0 and B+ decays and from the fragmentation of primary c quarks. In addition, a search is made for the exclusive decay B~--+ JflP>. Based on one candidate event the 90% confidence level upper limit is determined to be f(b-+ B~) · B(B~ -+Jj./J>) < 0.22%. (Submitted to Physics Letters B) The 0 PAL Collaboration P.D. Acton, G. Alexander, J. Allison's, P.P. Allports, K.J. Anderson9 , S. Arcell?, A. Astbury2s, D. Axen29 , G. Azuelos1s,a, G.A. Bahan's, J.T.M. Baines's, A.H. Ball17, J. Banks's, R.J. Barlow's, S. Barnett's, J.R. Batley\ G. Bea.udoin18, A. Beck23, J. Becker10, T. Behnke27, K.W. BelP0, G. Bella, P. Berlich, S. Bethke , 0. Biebel', U. Binder10, l.J.Bloodworth , P.Bock11 , B.Boden3 , H.M.Bosch11 , S.Bougerolle29, H.Breukers, R.M. Brown20, A. Buijss, H.J. Burckharts, C. Burgard, I'. Capilupp?, R.K. Carnegies, A.A. Carter, J.R. CarterS, C.Y. Chang , D.G. Charlton8 , P.E.L. Clarke2S, I. Cohen23 , J.C. Clayton', W.J. Collins , J.E. Conboy , M. Cooper , M. Coupland , M. Cuffiani , S. Dado , G.M. Dallavalle, S. De Jong , L.A. del Pozos, H. Deng17, A. Dieckmann11 , M. Dittmar\ M.S. Dixit7 , E. do Couto e Silva12 , J.E. Duboscq , E. Duchovnis, G. Duckeck11 , l.P. Duerdoth1s, D.J.P. Dumass, P.A. ElcombeS, P.G. Estabrooks6 , E. Etzion23 , H. G. Evans9 , F. Fabbri2, M. Fincke-Keeler28, H.M. Fischer3, D.G. Fong17 , M. Foucher17, A. Gaidot 21 , 0. GaneFs, J.W. Gary\ J. Gascon , R.F. McGowan's, N.I. Geddes20, C. Geich-Gimbel3, S.W. Gensler , F.X. Gentit 21 , G. Giacomelli 2, V. Gibsons, W.R. Gibson13, J.D. Gillies20, J.Goldberg22 , M.J.Goodrick5 , W.Gorn\ C.Grandi , F.C.Grant5 , J.Hagemann27, G.G. Hanson , M. Hansroul8 , C.K. Hargrove7, P.F. Harrison1\ J. Hart8 , P.M. Hattersley', M. Hauschild , C.M. Haw kess, E. Heflin\ R.J. Hemingways, R.D. Heuer , J.C. Hill, S.J. Hillier', T. Hilse, D.A. Hinshaw, J.D. Hobbs , P.R. Hobson2s, D. Hochman26, R.J. Homer', A.K. Honma28·•, C.P. Howarth15 , R.E. Hughes-Jones16 , R. Humbert10, P. lgo-Kemenes11 , H. Ihssen11 , D.C. Imrie, A. C. Janissens, A. Jawahery, P.W. Jeffreys20 , H. Jeremie, M. Jimack2 , M. Jobes', R.W.L. Jones13, P. Jovanovic', C. Jui4 , D. Karlen6 , K. Kawagoe24, T. Kawamoto4, R.K. Keeler, R.G. Kellogg, B.W. Kennedy, S. Kluth', T. Kobayashi; T.P. Kokott3 , S. Komamiya24, L. Kopke8 , J.F. Krals, R. Kowalewski6, J. von Krogh11 , J. Kroll , M. Kuwano4, P. Kyberd, G.D. Lafferty's, F. Lamarche, J.G. Layter\ P. Le Du2', P. Leblanc1s, A.M. Lee17, M.H. Lehto15, D. Lellouch2s, P. Lennert11 , C. Leroy18, J. Letts\ S. Levegriin3 , L. Levinsons, S.L. Lloyd'', F.K. Loebinger16, J.M. Lorah17 , B. Lorazo , M.J. Losty7, X. C. Lou12 , J. Ludwig10 , M. Mannellis, S. Marcellini 2, G. Maringer3 , C. Markus3 , A.J. Martin, J.P. Martin, T. Mashimo<, P. Mattig , U. Maur', J. McKenna2s, T.J. McMahon', J.R. McNutt2s, F. Meijerss, D. Menszner11 , F.S. Merritt9 , H. Mes , A. Michelinis, R.P. Middleton, G. Mikenberg, J. Mildenberger, D.J. Miller, R. Mir, W. Mohr, C. Moisan18 , A. Montanari2 , T. Mori24, M. Mo:rii24, T. Mouthuy12 •b, B. Nellen3, H.H. Nguyen9 , M. Nozaki24, S.W. O'Neale•', F.G. Oakham, F. Odorici, H.O. Ogren12 , C.J. Oram•", M.J. Oreglia9 , S. Orito24, J.P. Pansart2', B. Panzer-Steindel8 , P. Paschievici2s, G.N. Patrick, N. Paz-Jaoshvili" , P. Pfister10 , J.E. Pilcher9 , D. Pitman28, D.E. Plane8 , P. Poffenberger28 , B. Poli2 , A. Pouladdejs, E. Prebys8 , T.W. Pritchard13, H. Przysiezniak1s, G. Quast, M.W. Redmond , D.L. Rees', G.E. RichardE

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 .

Events with no hadronic energy ow in a large interval of pseudo-rapidity in the proton direction are observed in photon-proton interactions at an average centre of mass energy < p s p > of 200 GeV. These events are interpreted as photon diirac-tive dissociation. Evidence for hard scattering in photon diiractive dissociation is demonstrated using inclusive single particle spectra, thrust as a function of transverse energy, and the observation of jet production. The data can be described by a Monte Carlo calculation including hard photon-Pomeron scattering.

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.

An investigation of the hadronic nal state in diiractive and non{diiractive deep{inelastic electron{proton scattering at HERA is presented, where diiractive data are selected experimentally by demanding a large gap in pseudo{rapidity around the proton remnant direction. The transverse energy ow in the hadronic nal state is evaluated using a set of estimators which quantify topological properties. Using available Monte Carlo QCD calculations, it is demonstrated that the nal state in diiractive DIS exhibits the features expected if the interaction is interpreted as the scattering of an electron oo a current quark with associated eeects of perturbative QCD. A model in which deep{inelastic diiraction is taken to be the exchange of a pomeron with partonic structure is found to reproduce the measurements well. Models for deep{inelastic ep scattering, in which a sizeable diiractive contribution is present because of non{perturbative eeects in the production of the hadronic nal state, reproduce the general tendencies of the data but in all give a worse description.

An analysis is presented of scaling violations of the proton structure function F 2 (x; Q 2) measured with the H1 detector at HERA in the range of Bjorken x values between x = 310 ?4 and 10 ?2 for four-momentum transfers Q 2 larger than 8.7 GeV 2. The structure function F 2 (x; Q 2) is observed to rise linearly with lnQ 2. Under the assumption that the observed scaling violations at small x 0:01 are described correctly by perturbative QCD, an estimate is obtained of the gluon distribution function G(x; Q 2 o) at Q 2 o = 20 GeV 2 .

Evidence is presented using data taken with the H1 detector at HERA for a class of deep inelastic electron{proton scattering (DIS) events (5 < Q 2 < 120 GeV 2) at low Bjorken-x (10 ?4 < x < 10 ?2) which have almost no hadronic energy ow in a large interval of pseudo-rapidity around the proton remnant direction and which cannot be attributed to our present understanding of DIS and uctuations in-nal state hadronic fragmentation. From an integrated luminosity of 273 nb ?1 , 734 events, that is about 5% of the total DIS sample, have no energy deposition greater than 400MeV forward of laboratory pseudo-rapidity max = 1:8 up to the largest measurable pseudo-rapidity of about 3:65. Evidence that about 10% of observed rapidity gap events are exclusive vector meson electroproduction is presented. Good descriptions of the data are obtained using models based either on a vector meson dominance like picture, which includes a large fraction of inelastic virtual photon dissociation, or on deep inelastic electron{pomeron scattering in which the partonic sub{structure of the latter is resolved.

ISSN 0418-9833 August 1994 A Search for Heavy Leptons at HERA H1 Collaboration Abstract: A search for direct production of new leptons in the mass range from 10 GeV up to 225 GeV is presented by the H1 experiment at HERA. The data were obtained during 1993 and correspond to an integrated luminosity of 528 nb 1. The search includes heavy lepton decays to nal states e ( ) and e ( )W , e ( )Z with the subsequent decay of the W and Z bosons into jets or lepton pairs. No evidence was found for the production of new massive electrons or neutrinos in any of the decay channels. Rejection limits for excited electrons and neutrinos are derived.

In a calorimeter with silicon readout, the use of a combination of low-Z (Fe) and high-Z (Pb) materials as absorbers enables the transformation of the electron energy distribution of the incident showers in two media with different critical energies via the filtering effect. As a result, the electromagnetic energy and consequently the e/MIP ratio value can be substantially reduced as a function of the thickness of high-Z (Pb) material present in the absorber, making it possible to tune the e/ pi ratio and making possible the achievement of the compensation condition e/ pi =1.<>

The cross section for the elastic photoproduction of ρ mesons (γp → ρp) has been measured with the H1 detector at HERA for two average photon-proton centreof-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the ρ decay giving a cross section of 9.1±0.9 (stat.)±2.5 (syst.) μb. The logarithmic slope parameter of the differential cross section, dσ/dt, is found to be 10.9 ± 2.4 (stat.)± 1.1 (syst.) GeV. The ρ decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6 ± 0.8 (stat.) ± 2.4 (syst.) μb. S. Aid, V. Andreev, B. Andrieu, R.-D. Appuhn, M. Arpagaus, A. Babaev, J. Bähr, J. Bán, Y. Ban, P. Baranov, E. Barrelet, R. Barschke, W. Bartel, M. Barth, U. Bassler, H.P. Beck, H.-J. Behrend, A. Belousov, Ch. Berger, G. Bernardi, R. Bernet, G. Bertrand-Coremans, M. Besançon, R. Beyer, P. Biddulph, P. Bispham, J.C. Bizot, V. Blobel, K. Borras, F. Botterweck, V. Boudry, A. Braemer, W. Braunschweig, V. Brisson, D. Bruncko, C. Brune, R. Buchholz, L. Büngener, J. Bürger, F.W. Büsser, A. Buniatian, S. Burke, M.J. Burton, G. Buschhorn, A.J. Campbell, T. Carli, F. Charles, M. Charlet, D. Clarke, A.B. Clegg, B. Clerbaux, S. Cocks, J.G. Contreras, C. Cormack, J.A. Coughlan, A. Courau, M.-C. Cousinou, Ch. Coutures, G. Cozzika, L. Criegee, D.G. Cussans, J. Cvach, S. Dagoret, J.B. Dainton, W.D. Dau, K. Daum, M. David, C.L. Davis, B. Delcourt, A. De Roeck, E.A. De Wolf, M. Dirkmann, P. Dixon, P. Di Nezza, W. Dlugosz, C. Dollfus, J.D. Dowell, H.B. Dreis, A. Droutskoi, D. Düllmann, O. Dünger, H. Duhm, J. Ebert, T.R. Ebert, G. Eckerlin, V. Efremenko, S. Egli, R. Eichler, F. Eisele, E. Eisenhandler, R.J. Ellison, E. Elsen, M. Erdmann, W. Erdmann, E. Evrard, A.B. Fahr, L. Favart, A. Fedotov, D. Feeken, R. Felst, J. Feltesse, J. Ferencei, F. Ferrarotto, K. Flamm, M. Fleischer, M. Flieser, G. Flügge, A. Fomenko, B. Fominykh, M. Forbush, J. Formánek, J.M. Foster, G. Franke, E. Fretwurst, E. Gabathuler, K. Gabathuler, F. Gaede, J. Garvey, J. Gayler, M. Gebauer, A. Gellrich, H. Genzel, R. Gerhards, A. Glazov, U. Goerlach, L. Goerlich, N. Gogitidze, M. Goldberg, D. Goldner, K. Golec-Biernat, B. Gonzalez-Pineiro, I. Gorelov, C. Grab, H. Grässler, R. Grässler, T. Greenshaw, R. Griffiths, G. Grindhammer, A. Gruber, C. Gruber, J. Haack, D. Haidt, L. Hajduk, M. Hampel, M. Hapke, W.J. Haynes, G. Heinzelmann, R.C.W. Henderson, H. Henschel, I. Herynek, M.F. Hess, W. Hildesheim, K.H. Hiller, C.D. Hilton, J. Hladký, K.C. Hoeger, M. Höppner, D. Hoffmann, T. Holtom, R. Horisberger, V.L. Hudgson, M. Hütte, H. Hufnagel, M. Ibbotson, H. Itterbeck, M.-A. Jabiol, A. Jacholkowska, C. Jacobsson, M. Jaffre, J. Janoth, T. Jansen, L. Jönsson, K. Johannsen, D.P. Johnson, L. Johnson, H. Jung, P.I.P. Kalmus, M. Kander, D. Kant, R. Kaschowitz, U. Kathage, J. Katzy, H.H. Kaufmann, S. Kazarian, I.R. Kenyon, S. Kermiche, C. Keuker, C. Kiesling, M. Klein, C. Kleinwort, G. Knies, W. Ko, T. Köhler, J.H. Köhne, H. Kolanoski, F. Kole, S.D. Kolya, V. Korbel, M. Korn, P. Kostka, S.K. Kotelnikov, T. Krämerkämper, M.W. Krasny, H. Krehbiel, D. Krücker, U. Krüger, U. Krüner-Marquis, H. Küster, M. Kuhlen, T. Kurča, J. Kurzhöfer, D. Lacour, B. Laforge, F. Lamarche, R. Lander, M.P.J. Landon, W. Lange, U. Langenegger, P. Lanius, J.-F. Laporte, A. Lebedev, F. Lehner, C. Leverenz, S. Levonian, Ch. Ley, G. Lindström, M. Lindstroem, J. Link, F. Linsel, J. Lipinski, B. List, G. Lobo, P. Loch, H. Lohmander, J.W. Lomas, G.C. Lopez, V. Lubimov, D. Lüke, N. Magnussen, E. Malinovski, S. Mani, R. Maraček, P. Marage, J. Marks, R. Marshall, J. Martens, G. Martin, R. Martin, H.-U. Martyn, J. Martyniak, S. Masson, T. Mavroidis, S.J. Maxfield, S.J. McMahon, A. Mehta, K. Meier, T. Merz, A. Meyer, A. Meyer, H. Meyer, J. Meyer, P.-O. Meyer, A. Migliori, S. Mikocki, D. Milstead, J. Moeck, F. Moreau, J.V. Morris,

The electronic system developed for the SpaCal lead/scintillating-fibre calorimeters of the H1 detector in operation at the HERA ep collider is described in detail and the performance achieved during H1 data-taking is presented. The 10 MHz bunch crossing rate of HERA puts severe constraints on the requirements of the electronics. The energy and time readout are performed respectively with a 14-bit dynamic range and with a resolution of about 0.4 ns. The trigger branch consists of a nanosecond-resolution calorimetric time-of-flight for background rejection and an electron trigger based on analog `sliding windows'. The on-line background rejection currently achieved is o(10**6). The electron trigger allows a low energy trigger threshold to be set at about 0.50 +/- 0.08 (RMS) GeV with an efficiency >99.9%. The energy and time performance of the readout and trigger electronics is based on a newly-developed low noise (sigma_noise ca. 0.4 MeV) wideband (f < 200 MHz) preamplifier located at the output of the photomultipliers which are used for the fibre light readout in the ca. 1 Tesla magnetic field of H1.

The new lead/scintillating-fibre calorimeter (“SpaCal”) for the backward region of the H1 experiment at HERA (DESY) is equipped with fine mesh phototubes which operate in a magnetic field close to 1 T. A large sample of these tubes of the types Hamamatsu R5505 and R5506, and Hamamatsu R2490-05, have been tested in fields of up to 1.2T. We have investigated the cathode homogeneity with and without magnetic field, the gain loss under the influence of the magnetic field, and stability with time. For a subsample of tubes, we have performed additional studies on stability with respect to temperature changes, variation of gain as a function of the magnetic field, high voltage discharges, single photo-electron response, and linearity. We finally summarize the experience with these tubes after one year of operation in the experiment.

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.

The properties of final modules of a high resolution lead/scintillating-fibre calorimeter to upgrade the backward region of the H1 detector were studied with electrons in the energy range from 2–60 GeV. The electromagnetic calorimeter consists of scintillating fibres with a diameter of 0.5 mm embedded in a lead matrix. This small fibre radius, in combination with a lead-to-fibre ratio of 2.27:1, ensures excellent energy resolution which has been measured to be δ/E=7.1%/ E/GeV ⊕ 1.0%. The spatial resolution as a function of energy for impact points at the center of a cell is given by 4.4 mm/ E/GeV + 1.0 mm. The time resolution was found to be better than 0.4 ns.

We report on the production ofe ± μ∓ pairs in 450 GeV/c pBe collisions at the CERN SPS. Theeμ signal, which has average missing energy of 21 GeV, is shown to be consistent with expectations from charm decay, and implies a σ ×B for\(c\bar c\) production in p-nucleon collisions of 0.63 ± 0.35μb. Alternatively, using an estimate of charm production from other experiments, the data imply a 95% confidence level upper limit of 1.16μb on any new physics process which producese ± μ ∓.

The cross section for the elastic photoproduction of ϱ0 mesons (γp → ϱ0p) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the ϱ0 decay giving a cross section of 9.1 ± 0.9 (stat.) ± 2.5 (syst.) μb. The logarithmic slope parameter of the differential cross section, dσ/dt, is found to be 10.9 ± 2.4 (stat.) ± 1.1 (syst.) GeV−2. The ϱ0 decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6 ± 0.8 (stat.) ± 2.4 (syst.) μb.

Hadronic response and electron identification performance of the new H1 lead-scintillating fibre calorimeter are investigated in the 1 to 7 GeV energy range using data taken at the CERN Proton Synchrotron. The energy response to minimum ionizing particles and interacting pions are studied and compared to Monte Carlo simulations. The measured energy of pions interacting either in the electromagnetic or in the hadronic section is found to scale linearly with the incident energy, providing an energy resolution σE ∼ 38% within a depth of one interaction length and σE ∼ 29% for a total depth of two interaction lengths. Several electron identification estimators are studied and combined as a function of energy and impact point. The probability for pions to be misidentified as electrons of any measured energy above 1 GeV ranges from 5% (for 2 GeV incident pions) to 0.4% (at 7 GeV) for an electron detection efficiency of 90%. The probability for pions of a given energy to be misidentified as electrons of the same energy falls to 0.25% at 7 GeV.

A search for leptoquarks at HERA was performed in H1 using 1994 e(+)p data corresponding to an integrated luminosity of about 3 pb(-1). Single leptoquarks were searched for in direct positron-quark fusion processes taking into account possible decays into lepton-quark pairs of either the first, the second, or the third generation. No significant deviation from the Standard Model predictions is found in the various final states studied and mass dependent exclusion limits are derived on the Yukawa couplings of the leptoquarks. Compared with earlier results from an analysis of e(-)p data, exclusion limits are considerably improved for leptoquarks which could be produced via e(+)-valence quark fusion. For leptoquarks with lepton flavour conserving couplings, masses up to 275 GeV (depending on the leptoquark type) are excluded for coupling values larger than root 4 pi alpha(cm). For leptoquarks with lepton flavour violating couplings, masses up to 225 GeV are excluded for couplings with leptons of the second or third generation larger than root 4 pi alpha(em). Fourteen possible combinations of couplings are studied and stringent exclusion limits comparable or better than any existing direct or indirect limits are obtained for each leptoquark type.

Fluctuations due to binding energy losses were determined experimentally for sampling hadron calorimeters with layers of silicon mosaics as active medium and with either Fe layers (Si/Fe) or combinations of Pb and Fe layers (Si/Fe + Pb) as absorbers. The calorimeters were exposed to hadrons (π) at incoming energies of 10, 15, 20, 30, 40, 50 and 70 GeV at the CERN-SPS. The contributions (σbind) of the binding energy losses fluctuations to the energy resolution of these hadron calorimeters were found to be σbind = (19.8±2.4)% and (49.9±14.1)% for Fe and Pb, respectively. σbind = (25.9±2.1)% has been found for a compensating Si/Fe + Pb calorimeter. These results are found in good agreement with previous measurements made at lower energies. As expected, an overall nonlinearity of about 3.5%, in the 10–70 GeV energy range, has been observed in the Si/Fe noncompensating (e/π∼ 1.11 at 10 GeV) calorimeter energy response.