A Fabry-Perot cavity polarimeter, installed in 2003 at HERA for the second phase of its operation, is described. The cavity polarimeter was designed to measure the longitudinal polarisation of the HERA electron beam with high precision for each electron bunch spaced with a time interval of 96ns. Within the cavity the laser intensity was routinely enhanced up to a few kW from its original value of 0.7W in a stable and controllable way. By interacting such a high intensity laser beam with the HERA electron beam it is possible to measure its polarisation with a relative statistical precision of 2% per bunch per minute. Detailed systematic studies have also been performed resulting in a systematic uncertainty of 1%.

Three- and four-jet production is measured in deep-inelastic ep scattering at low x and Q 2 with the H1 detector using an integrated luminosity of 44.2 pb -1 . Several phase space regions are selected for the three-jet analysis in order to study the underlying parton dynamics from global topologies to the more restrictive regions of forward jets close to the proton direction. The measurements of cross sections for events with at least three jets are compared to fixed order QCD predictions of 𝒪(α_s^2) and 𝒪(α_s^3) and with Monte Carlo simulation programs where higher order effects are approximated by parton showers. A good overall description is provided by the 𝒪(α_s^3) calculation. Too few events are predicted at the lowest x∼10 -4 , especially for topologies with two forward jets. This hints to large contributions at low x from initial state radiation of gluons close to the proton direction and unordered in transverse momentum. The Monte Carlo program in which gluon radiation is generated by the colour dipole model gives a good description of both the three- and the four-jet data in absolute normalisation and shape.

A search for first generation excited neutrinos is performed using the full e(-) p data sample collected by the HI experiment at HERA at a centre-of-mass energy of 319 GeV, corresponding to a total luminosity of 184 pb(-1). The electroweak decays of excited neutrinos nu* -> nu gamma, v* -> nu Z and nu* -> eW with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited neutrino production is found. Mass dependent exclusion limits on nu* production cross sections and on the ratio of the coupling to the compositeness scale f/boolean AND are derived within gauge mediated models. A limit on f/boolean AND A, independent of the relative couplings to the SU(2) and U(1) gauge bosons, is also determined. These limits extend the excluded region to higher masses than has been possible in previous excited neutrino searches. (c) 2008 Elsevier B.V. All rights reserved.

The production of dijets in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 61 pb-1. The dijet cross section has been measured for virtualities of the exchanged virtual photon, 5 < Q2 < 100 GeV2, and γ*p centre-of-mass energies, 100 < W < 250 GeV. The jets, identified using the inclusive kT algorithm in the γ*p frame, were required to have a transverse energy E* T,jet > 4 GeV and the jet with the highest transverse energy was required to have E* T,jet > 5 GeV. All jets were required to be in the pseudorapidity range -3.5<η* jet<0. The differential cross sections are compared to leading-order predictions and next-to-leading-order QCD calculations based on recent diffractive parton densities extracted from inclusive diffractive deep inelastic scattering data.

Inclusive D* production is measured in deep-inelastic ep scattering at HERA with the H1 detector. In addition, the production of dijets in events with a D* meson is investigated. The analysis covers values of photon virtuality 2< Q^2 <=100 GeV^2 and of inelasticity 0.05<= y <= 0.7. Differential cross sections are measured as a function of Q^2 and x and of various D* meson and jet observables. Within the experimental and theoretical uncertainties all measured cross sections are found to be adequately described by next-to-leading order (NLO) QCD calculations, based on the photon-gluon fusion process and DGLAP evolution, without the need for an additional resolved component of the photon beyond what is included at NLO. A reasonable description of the data is also achieved by a prediction based on the CCFM evolution of partons involving the k_T-unintegrated gluon distribution of the proton.

Measurements are presented of diffractive open charm production at HERA. The event topology is given by ep→eXY where the system X contains at least one charmed hadron and is well separated by a large rapidity gap from a leading low-mass proton remnant system Y. Two analysis techniques are used for the cross section measurements. In the first, the charm quark is tagged by the reconstruction of a D*±(2010) meson. This technique is used in deep-inelastic scattering (DIS) and photoproduction (γp). In the second, a method based on the displacement of tracks from the primary vertex is used to measure the open charm contribution to the inclusive diffractive cross section in DIS. The measurements are compared with next-to-leading order QCD predictions based on diffractive parton density functions previously obtained from a QCD analysis of the inclusive diffractive cross section at H1. A good agreement is observed in the full kinematic regime, which supports the validity of QCD factorization for open charm production in diffractive DIS and γp.

The average charged track multiplicity and the normalised distribution of the scaled momentum, $\xp$, of charged final state hadrons are measured in deep-inelastic $\ep$ scattering at high $Q^2$ in the Breit frame of reference. The analysis covers the range of photon virtuality $100 < Q^2 < 20 000 \GeV^{2}$. Compared with previous results presented by HERA experiments this analysis has a significantly higher statistical precision and extends the phase space to higher $Q^{2}$ and to the full range of $\xp$. The results are compared with $e^+e^-$ annihilation data and with various calculations based on perturbative QCD using different models of the hadronisation process.

The average charged track multiplicity and the normalised distribution of the scaled momentum, xp, of charged final state hadrons are measured in deep-inelastic ep scattering at high Q2 in the Breit frame of reference. The analysis covers the range of photon virtuality 100 < Q2 < 20 000 GeV2. Compared with previous results presented by HERA experiments this analysis has a significantly higher statistical precision and extends the phase space to higher Q2 and to the full range of xp. The results are compared with e+e− annihilation data and with various calculations based on perturbative QCD using different models of the hadronisation process. Submitted to Phys. Lett. B. F.D. Aaron, A. Aktas, 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, O. Behrendt, 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, F.W. Büsser, 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. Deak, Y. de Boer, B. Delcourt, M. Del Degan, J. Delvax, A. De Roeck, E.A. De Wolf, C. Diaconu, V. Dodonov, A. Dubak, G. Eckerlin, V. Efremenko, S. Egli, R. Eichler, 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, S. Ginzburgskaya, A. Glazov, I. Glushkov, L. Goerlich, M. Goettlich, N. Gogitidze, S. Gorbounov, M. Gouzevitch, C. Grab, T. Greenshaw, B.R. Grell, G. Grindhammer, S. Habib, D. Haidt, M. Hansson, G. Heinzelmann, 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. Jönsson, 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, V. Korbel, P. Kostka, M. Kraemer, K. Krastev, J. Kretzschmar, A. Kropivnitskaya, K. Krüger, M.P.J. Landon, W. Lange, G. Laštovička-Medin, P. Laycock, A. Lebedev, G. Leibenguth, V. Lendermann, S. Levonian, G. Li, L. Lindfeld, 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, M. Martisikova, 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, A. Mohamed, F. Moreau, A. Morozov, J.V. Morris, M.U. Mozer, K. Müller, P. Murı́n, K. Nankov, B. Naroska, Th. Naumann, P.R. Newman, C. Niebuhr, A. Nikiforov, G. Nowak, K. Nowak, M. Nozicka, R. Oganezov, B. Olivier, J.E. Olsson, S. Osman, D. Ozerov, V. Palichik, I. Panagoulias, M. Pandurovic, Th. Papadopoulou, C. Pascaud, G.D. Patel, H. Peng, E. Perez, D. Perez-Astudillo, A. Perieanu, A. Petrukhin, I. Picuric, S. Piec, D. Pitzl, R. Plačakytė, R. Polifka, B. Povh, T. Preda, P. Prideaux, V. Radescu, A.J. Rahmat, N. Raicevic, T. Ravdandorj, P. Reimer, C. Risler, E. Rizvi, P. Robmann, B. Roland, R. Roosen, A. Rostovtsev, 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. Schöning, 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, J. Stiewe, 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. Truöl, I. Tsakov, B. Tseepeldorj, G. Tsipolitis, I. Tsurin, J. Turnau, E. Tzamariudaki, K. Urban, D. Utkin, A. Valkárová, C. Vallée, P. Van Mechelen, A. Vargas Trevino, Y. Vazdik, S. Vinokurova, V. Volchinski, G. Weber, R. Weber, D. Wegener,

A search for second and third generation scalar and vector leptoquarks produced in ep collisions via the lepton flavour violating processes ep to mu-X and ep to tau-X is performed by the H1 Collaboration at HERA. The full H1 ep data sample taken at a centre-of-mass energy of 319 GeV is used for the analysis, corresponding to an integrated luminosity of 411 pb^-1. No evidence for the production of such leptoquarks is observed in the H1 data. Leptoquarks produced in ep collisions with a coupling strength of lambda=0.3 and decaying with the same coupling strength to a muon-quark pair or a tau-quark pair are excluded at 95% confidence level up to leptoquark masses of 712 GeV and 479 GeV, respectively.

Inclusive D *± production is measured in deep-inelastic ep scattering at HERA with the H1 detector. In addition, the production of dijets in events with a D *± meson is investigated. The analysis covers values of photon virtuality 2 ≤ Q 2 ≤ 100 GeV 2 and of inelasticity 0.05≤y≤0.7. Differential cross sections are measured as a function of Q 2 and x and of various D *± meson and jet observables. Within the experimental and theoretical uncertainties all measured cross sections are found to be adequately described by next-to-leading order (NLO) QCD calculations, based on the photon–gluon fusion process and DGLAP evolution, without the need for an additional resolved component of the photon beyond what is included at NLO. A reasonable description of the data is also achieved by a prediction based on the CCFM evolution of partons involving the k T -unintegrated gluon distribution of the proton.

Inclusive jet production is studied in neutral current deep-inelastic positron–proton scattering at large four momentum transfer squared Q2>150GeV2 with the H1 detector at HERA. Single and double differential inclusive jet cross sections are measured as a function of Q2 and of the transverse energy ET of the jets in the Breit frame. The measurements are found to be well described by calculations at next-to-leading order in perturbative QCD. The running of the strong coupling is demonstrated and the value of αs(MZ) is determined. The ratio of the inclusive jet cross section to the inclusive neutral current cross section is also measured and used to extract a precise value for αs(MZ)=0.1193±0.0014(exp.)−0.0030+0.0047(th.)±0.0016(pdf).

A search for narrow baryonic resonances decaying into (cid:4) (cid:0) (cid:25) (cid:0) or (cid:4) (cid:0) (cid:25) + and their an-tiparticles is carried out with the H1 detector using deep inelastic scattering events at HERA in the range of negative photon four-momentum transfer squared 2 < Q 2 < 100 GeV 2 . No signal for a new baryonic state in the mass range 1600 (cid:0) 2300 MeV is observed in either the doubly charged or the neutral decay channels. The known baryon (cid:4)(1530) 0 is observed through its decay mode into (cid:4) (cid:0) (cid:25) + . Mass/dependent upper limits are given on the ratio of the production rates of new baryonic states, such as the hypothetical pentaquark states (cid:4) (cid:0)(cid:0) 5 q or (cid:4) 05 q , relative to the (cid:4)(1530) 0 baryon state. are no indications of any new baryonic state decaying into (cid:4) (cid:25) in the mass range 1600 (cid:0) 2300MeV. Thus H1 can not con(cid:12)rm the signal reported by the NA49 collaboration. Mass-dependent upper limits at the 95 % C.L. are set on the production ratio of hypothetical states, such as (cid:4) (cid:0)(cid:0) 5 q and (cid:4) 05 q , to the total number of observed (cid:4)(1530) 0 baryons. These limits are comparable to those measured by the ZEUS Collaboration[11].

The production of tau leptons in ep collisions is investigated using data recorded by the H1 detector at HERA in the period 1994–2000. Tau leptons are identified by detecting their decay products, using leptonic and hadronic decay modes. The cross section for the production of tau lepton pairs is measured for the first time at HERA. Furthermore, a search for events with an energetic isolated tau lepton and with large missing transverse momentum is performed. The results are found to be in agreement with the Standard Model predictions.

A search for a narrow baryonic resonance decaying to K0 s p or K 0 s p̄ is carried out in deep inelasticep scattering with the H1 detector at HERA. Such a resonance cou ld be a strange pentaquark Θ+, evidence for which has been reported by several experiment s. TheK0 s p andK 0 s p̄ invariant mass distributions presented here do not show any significant peak in the mass range from threshold up to 1 :7 GeV. Mass dependent upper limits on σ(ep! eΘ+X) BR(Θ+! K0p) are obtained at the 95% confidence level. Submitted toPhys. Lett.B A. Aktas9, V. Andreev25, T. Anthonis3, B. Antunovic26, S. Aplin9, A. Asmone33, A. Astvatsatourov 3, A. Babaev24;†, S. Backovic30, A. Baghdasaryan 37, P. Baranov 25, E. Barrelet 29, W. Bartel9, S. Baudrand 27, S. Baumgartner 39, J. Becker 40, M. Beckingham9, O. Behnke12, O. Behrendt 6, A. Belousov25, N. Berger39, J.C. Bizot 27, M.-O. Boenig6, V. Boudry28, J. Bracinik26, G. Brandt 12, V. Brisson27, D. Bruncko15, F.W. Büsser 10, A. Bunyatyan11;37, G. Buschhorn 26, L. Bystritskaya24, A.J. Campbell 9, F. Cassol-Brunner 21, K. Cerny32, V. Cerny15;46, V. Chekelian26, J.G. Contreras 22, J.A. Coughlan4, B.E. Cox20, G. Cozzika8, J. Cvach31, J.B. Dainton17, W.D. Dau14, K. Daum36;42, Y. de Boer24, B. Delcourt27, M. Del Degan39, A. De Roeck9;44, E.A. De Wolf3, C. Diaconu21, V. Dodonov11, A. Dubak30;45, G. Eckerlin9, V. Efremenko24, S. Egli35, R. Eichler35, F. Eisele12, A. Eliseev25, E. Elsen9, S. Essenov 24, A. Falkewicz5, P.J.W. Faulkner 2, L. Favart3, A. Fedotov24, R. Felst 9, J. Feltesse 8, J. Ferencei 15, L. Finke10, M. Fleischer 9, G. Flucke33, A. Fomenko25, G. Franke9, T. Frisson28, E. Gabathuler 17, E. Garutti 9, J. Gayler 9, C. Gerlich12, S. Ghazaryan 37, S. Ginzburgskaya 24, A. Glazov9, I. Glushkov38, L. Goerlich5, M. Goettlich9, N. Gogitidze25, S. Gorbounov 38, C. Grab39, T. Greenshaw17, M. Gregori18, B.R. Grell9, G. Grindhammer 26, C. Gwilliam20, D. Haidt9, L. Hajduk5, M. Hansson19, G. Heinzelmann 10, R.C.W. Henderson 16, H. Henschel 38, G. Herrera23, M. Hildebrandt 35, K.H. Hiller38, D. Hoffmann21, R. Horisberger 35, A. Hovhannisyan 37, T. Hreus3;43, S. Hussain18, M. Ibbotson20, M. Ismail20, M. Jacquet 27, L. Janauschek 26, X. Janssen 3, V. Jemanov 10, L. Jönsson19, D.P. Johnson 3, A.W. Jung13, H. Jung19;9, M. Kapichine7, J. Katzy9, I.R. Kenyon2, C. Kiesling26, M. Klein38, C. Kleinwort9, T. Klimkovich9, T. Kluge9, G. Knies9, A. Knutsson19, V. Korbel9, P. Kostka38, K. Krastev9, J. Kretzschmar 38, A. Kropivnitskaya24, K. Krüger13, M.P.J. Landon 18, W. Lange38, G. Laštovička-Medin 30, P. Laycock17, A. Lebedev25, G. Leibenguth39, V. Lendermann 13, S. Levonian9, L. Lindfeld40, K. Lipka38, A. Liptaj26, B. List39, J. List10, E. Lobodzinska 38;5, N. Loktionova25, R. Lopez-Fernandez 23, V. Lubimov24, A.-I. Lucaci-Timoce9, H. Lueders10, D. Lüke6;9, T. Lux10, L. Lytkin11, A. Makankine7, N. Malden20, E. Malinovski25, S. Mangano 39, P. Marage3, R. Marshall 20, L. Marti9, M. Martisikova9, H.-U. Martyn1, S.J. Maxfield17, A. Mehta17, K. Meier13, A.B. Meyer9, H. Meyer36, J. Meyer9, V. Michels9, S. Mikocki5, I. Milcewicz-Mika5, D. Milstead17, D. Mladenov34, A. Mohamed17, F. Moreau28, A. Morozov7, J.V. Morris4, M.U. Mozer12, K. Müller40, P. Murı́n15;43, K. Nankov34, B. Naroska10, Th. Naumann 38, P.R. Newman2, C. Niebuhr9, A. Nikiforov26, G. Nowak5, K. Nowak40, M. Nozicka32, R. Oganezov 37, B. Olivier26, J.E. Olsson 9, S. Osman19, D. Ozerov24, V. Palichik7, I. Panagoulias 9, T. Papadopoulou 9, C. Pascaud 27, G.D. Patel 17, H. Peng9, E. Perez 8, D. Perez-Astudillo22, A. Perieanu9, A. Petrukhin24, D. Pitzl9, R. Plačakytė 26, B. Portheault 27, B. Povh11, P. Prideaux 17, A.J. Rahmat 17, N. Raicevic30, P. Reimer 31, A. Rimmer17, C. Risler9, E. Rizvi18, P. Robmann 40, B. Roland3, R. Roosen3, A. Rostovtsev 24, Z. Rurikova26, S. Rusakov 25, F. Salvaire10, D.P.C. Sankey 4, E. Sauvan 21, S. Schätzel 9, S. Schmidt 9, S. Schmitt 9, C. Schmitz40, L. Schoeffel 8, A. Schöning39, H.-C. Schultz-Coulon 13, F. Sefkow9, R.N. Shaw-West 2, I. Sheviakov25, L.N. Shtarkov25, T. Sloan16, P. Smirnov25, Y. Soloviev25, D. South9, V. Spaskov7, A. Specka28, M. Steder 9, B. Stella33, J. Stiewe13, A. Stoilov34, U. Straumann 40, D. Sunar 3, V. Tchoulakov7, G. Thompson 18, P.D. Thompson 2, T. Toll9, F. Tomasz15, D. Traynor18, P. Truöl40, I. Tsakov34, G. Tsipolitis9;41, I. Tsurin9, J. Turnau5, E. Tzamariudaki 26, K. Urban13, M. Urban40, A. Usik25, D. Utkin24, A. Valkárová32, C. Vallée21, P. Van Mechelen 3, A. Vargas Trevino6, Y. Vazdik25, C. Veelken17, S. Vinokurova9, V. Volchinski37, K. Wacker6, G. Weber 10, R. Weber 39, D. Wegener 6, C. Werner 12,

A search for the single production of doubly-charged Higgs bosons H^{\pm \pm} in ep collisions is presented. The signal is searched for via the Higgs decays into a high mass pair of same charge leptons, one of them being an electron. The analysis uses up to 118 pb^{-1} of ep data collected by the H1 experiment at HERA. No evidence for doubly-charged Higgs production is observed and mass dependent upper limits are derived on the Yukawa couplings h_{el} of the Higgs boson to an electron-lepton pair. Assuming that the doubly-charged Higgs only decays into an electron and a muon via a coupling of electromagnetic strength h_{e \mu} = \sqrt{4 \pi \alpha_{em}} = 0.3, a lower limit of 141 GeV on the H^{\pm\pm} mass is obtained at the 95% confidence level. For a doubly-charged Higgs decaying only into an electron and a tau and a coupling h_{e\tau} = 0.3, masses below 112 GeV are ruled out.

Data taken with positrons of different longitudinal polarisation states in collision with unpolarised protons at HERA are used to measure the total cross sections of the charged current process, e^+ p \to \bar{\nu}X, for negative four-momentum transfer squared Q^2 > 400 GeV^2 and inelasticity y<0.9. Together with the corresponding cross section obtained from the previously published unpolarised data, the polarisation dependence of the charged current cross section is measured for the first time at high Q^2 and found to be in agreement with the Standard Model prediction.

The cross section for the diffractive deep-inelastic scattering process ep -> eX(P) is measured, with the leading final state proton detected in the H1 Forward Proton Spectrometer. The data analysed cover the range x(P) < 0.1 in fractional proton longitudinal momentum loss, 0.08 < vertical bar t vertical bar < 0.5 GeV-2 in squared four-momentum transfer at the proton vertex, 2 < Q(2) < 50 GeV2 in photon virtuality and 0.004 < beta = x/x(P) < 1, where x is the Bjorken scaling variable. For x(P) less than or similar to 10(-2), the differential cross section has a dependence of approximately d sigma/dt proportional to e(6t), independently of x(P), beta and Q(2) within uncertainties. The cross section is also measured triple differentially in x(P), beta and Q(2). The x(P) dependence is interpreted in terms of an effective pomeron trajectory with intercept alpha(P) (0) = 1.114 +/- 0.018(stat.) +/- 0.012(syst.)(-0.020)(+0.040) (model) and a subleading exchange. The data are in good agreement with an H1 measurement for which the event selection is based on a large gap in the rapidity distribution of the final state hadrons, after accounting for proton dissociation contributions in the latter. Within uncertainties, the dependence of the cross section on x and Q(2) can thus be factorised from the dependences on all studied variables which characterise the proton vertex, for both the pomeron and the sub-leading exchange.

A search for a narrow baryonic resonance decaying to Ks0p or Ks0p¯ is carried out in deep inelastic ep scattering with the H1 detector at HERA. Such a resonance could be a strange pentaquark Θ+, evidence for which has been reported by several experiments. The Ks0p and Ks0p¯ invariant mass distributions presented here do not show any significant peak in the mass range from threshold up to 1.7 GeV. Mass dependent upper limits on σ(ep→eΘ+X)×BR(Θ+→K0p) are obtained at the 95% confidence level.