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1.
  • Aktas, A., et al. (author)
  • Inclusive D*(+/-) meson and associated dijet production in deep-inelastic scattering at HERA
  • 2007
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 51:2, s. 271-287
  • Journal article (peer-reviewed)abstract
    • 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 GeV2 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 kT- unintegrated gluon distribution of the proton.
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2.
  • Aktas, A., et al. (author)
  • Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA
  • 2007
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 51:3, s. 549-568
  • Journal article (peer-reviewed)abstract
    • Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q(2) < 0.01 GeV2) and deep-inelastic scattering processes (DIS, 4 < Q2 < 80 GeV2). The event topology is given by ep -> eXY, in which the system X, containing at least two jets, is separated from a leading low-mass baryonic system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5 +/- 0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.
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3.
  • Aktas, A., et al. (author)
  • Diffractive deep-inelastic scattering with a leading proton at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 749-766
  • Journal article (peer-reviewed)abstract
    • 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.
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4.
  • Aktas, A., et al. (author)
  • Inclusive D-*+/- meson cross sections and D-*+/--jet correlations in photoproduction at HERA
  • 2007
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 50:2, s. 251-267
  • Journal article (peer-reviewed)abstract
    • Differential photoproduction cross sections are measured for events containing D-*+/- mesons. The data were taken with the H1 detector at the ep collider HERA and correspond to an integrated luminosity of 51.1 pb(-1). The kinematic region covers small photon virtualities Q(2) < 0.01 GeV2 and photon-proton centre-of-mass energies of 171 < W gamma(Cep) < 256 GeV. The details of the heavy quark production process are further investigated in events with one or two jets in addition to the D-*+/- meson. Differential cross sections for D-* + jet production are determined and the correlations between the D-*+/- meson and the jet(s) are studied. The results are compared with perturbative QCD predictions applying collinear- or k(t)-factorisation.
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5.
  • Aktas, A., et al. (author)
  • Measurement and QCD analysis of the diffractive deep-inelastic scattering cross section at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 715-748
  • Research review (peer-reviewed)abstract
    • A detailed analysis is presented of the diffractive deep-inelastic scattering process ep -> eXY, where Y is a proton or a low mass proton excitation carrying a fraction 1 - x(IP) > 0.95 of the incident proton longitudinal momentum and the squared four-momentum transfer at the proton vertex satisfies |t| < 1 GeV2. Using data taken by the H1 experiment, the cross section is measured for photon virtualities in the range 3.5 <= Q(2) <= 1600 GeV2, triple differentially in x(IP), Q(2) and beta = x/x(P), where x is the Bjorken scaling variable. At low x(IP), the data are consistent with a factorisable x(IP) dependence, which can be described by the exchange of an effective pomeron trajectory with intercept alpha(IP)(0) = 1.118 +/- 0.008(exp.)(-0.010)(+0.029)(model). Diffractive parton distribution functions and their uncertainties are determined from a next-to-leading order DGLAP QCD analysis of the Q(2)and beta dependences of the cross section. The resulting gluon distribution carries an integrated fraction of around 70% of the exchanged momentum in the Q(2) range studied. Total and differential cross sections are also measured for the diffractive charged current process e(+) p -> (v) over bar eXY and are found to be well described by predictions based on the diffractive parton distributions. The ratio of the diffractive to the inclusive neutral current ep cross sections is studied. Over most of the kinematic range, this ratio shows no significant dependence on Q(2) at fixed x(P) and x or on x at fixed Q(2) and beta.
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6.
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7.
  • Aktas, A., et al. (author)
  • Search for a narrow baryonic resonance decaying to K(s)(0)p or K-s(0)(p)over-bar in deep inelastic scattering at HERA
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 639:3-4, s. 202-209
  • Journal article (peer-reviewed)abstract
    • A search for a narrow baryonic resonance decaying to K(s)(0)p or K-s(0)(p) over bar is carried out in deep inelastic ep scattering with the H1 detector at HERA. ss Such a resonance could be a strange pentaquark Theta(+), evidence for which has been reported by several experiments. The K(s)(0)p and K-s(0)(p) over bar invariant ss 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 sigma(ep -> e Theta X+) x BR(Theta(+) -> K(0)p) are obtained at the 95% confidence level.
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8.
  • Aktas, A., et al. (author)
  • Search for doubly-charged Higgs boson production at HERA
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 638:5-6, s. 432-440
  • Journal article (peer-reviewed)abstract
    • A search for the single production of doubly-charged Higgs bosons H-+/-+/- 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) = root 4 pi alpha(em) similar or equal to 0.3, a lower limit of 141 GeV on the H-+/-+/- 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) similar or equal to 0.3, masses below 112 GeV are ruled out.
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9.
  • Aktas, A., et al. (author)
  • Tau lepton production in ep collisions at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 699-714
  • Journal article (peer-reviewed)abstract
    • 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.
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10.
  • Aktas, A., et al. (author)
  • Diffractive photoproduction of rho mesons with large momentum transfer at HERA
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 638:5-6, s. 422-431
  • Journal article (peer-reviewed)abstract
    • The diffractive photoproduction of rho mesons, ep -> e rho Y, with large momentum transfer squared at the proton vertex, vertical bar t vertical bar, is studied with the H1 detector at HERA using an integrated luminosity of 20.1 pb(-1). The photon-proton centre of mass energy spans the range 75 < W < 95 GeV, the photon virtuality is restricted to Q(2) < 0.01 GeV2 and the mass My of the proton remnant is below 5 GeV. The t dependence of the cross section is measured for the range 1.5 < vertical bar t vertical bar < 10.0 GeV2 and is well described by a power law, d sigma/d vertical bar t vertical bar proportional to vertical bar t vertical bar(-n). The spin density matrix elements, which provide information on the helicity structure of the interaction, are extracted using measurements of angular distributions of the rho decay products. The data indicate a violation of s-channel helicity conservation, with contributions from both single and double helicity-flip being observed. The results are compared to the predictions of perturbative QCD models.
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11.
  • Aktas, A., et al. (author)
  • Elastic J/psi production at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:3, s. 585-603
  • Journal article (peer-reviewed)abstract
    • Cross sections for elastic production of J/psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb(-1). Results are presented for photon virtualities Q(2) up to 80 GeV2. The dependence on the photon-proton centre of mass energy W-gamma p is analysed in the range 40 <= W-gamma p <= 305 GeV in photoproduction and 40 <= W-gamma p <= 160 GeV in electroproduction. The W-gamma p dependences of the cross sections do not change significantly with Q(2) and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q(2). Differential cross sections d sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range vertical bar t vertical bar < 1.2 GeV2 as functions of W-gamma p and Q(2). Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q(2) and is found to be described by perturbative QCD based models.
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12.
  • Aktas, A., et al. (author)
  • Forward jet production in deep inelastic scattering at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:1, s. 27-42
  • Journal article (peer-reviewed)abstract
    • The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (xB(j)) and as triple differential cross sections d3 sigma/dx(Bj)dQ(2)dp(t)(,jet)(2), where Q(2) is the four momentum transfer squared and p(t)(,iet)(2) is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.
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13.
  • Aktas, A., et al. (author)
  • Photoproduction of dijets with high transverse momenta at HERA
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 639:1, s. 21-31
  • Journal article (peer-reviewed)abstract
    • Differential dijet cross sections are measured in photoproduction in the region of photon virtualities Q(2) < 1 GeV2 with the H1 detector at the HERA ep collider using an integrated luminosity of 66.6 pb(-1). Jets are defined with the inclusive k perpendicular to algorithm and a minimum transverse momentum of the leading jet of 25 GeV is required. Dijet cross sections are measured indirect and resolved photon enhanced regions separately. Longitudinal proton momentum fractions up to 0.7 are reached. The data compare well with predictions from Monte Carlo event generators based on leading order QCD and parton showers and with next-to-leading order QCD calculations corrected for hadronisation effects.
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14.
  • Aaron, F. D., et al. (author)
  • Measurement of diffractive scattering of photons with large momentum transfer at HERA
  • 2009
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 672:3, s. 219-226
  • Journal article (peer-reviewed)abstract
    • The first measurement of diffractive scattering of quasi-real photons with large momentum transfer gamma p -> gamma Y, where Y is the proton dissociative system, is made using the H1 detector at HERA. The measurement is performed for initial photon virtualities Q(2) < 0.01 GeV2. Single differential cross sections are Measured as a function of W, the incident photon-proton centre of mass energy, and t, the square of the four-momentum transferred at the proton vertex, in the range 175 < W < 247 GeV and 4 < |t| < 36 GeV2. The W dependence is well described by a model based on perturbative QCD using a leading logarithmic approximation of the BFKL evolution. The measured |t| dependence is harder than that predicted by the model and those observed in exclusive vector meson production. (C) 2009 Published by Elsevier B.V.
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15.
  • Aaron, F. D., et al. (author)
  • Measurement of isolated photon production in deep-inelastic scattering at HERA
  • 2008
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 54:3, s. 371-387
  • Journal article (peer-reviewed)abstract
    • The production of isolated photons in deep-inelastic scattering ep -> e gamma X is measured with the H1 detector at HERA. The measurement is performed in the kinematic range of negative four-momentum transfer squared 4 < Q(2)< 150 GeV2 and a mass of the hadronic system W-X > 50 GeV. The analysis is based on a total integrated luminosity of 227 pb(-1). The production cross section of isolated photons with a transverse energy in the range 3 < E-T(gamma)< 10 GeV and pseudorapidity range -1.2
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16.
  • Aaron, F. D., et al. (author)
  • Measurement of the proton structure function F-L(x, Q(2)) at low x
  • 2008
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 665:4, s. 139-146
  • Journal article (peer-reviewed)abstract
    • A first measurement is reported of the longitudinal proton structure function FL(X, Q(2)) at the ep collider HERA. It is based on inclusive deep inelastic e(+)p scattering cross section measurements with a positron beam energy of 27.5 GeV and proton beam energies of 920, 575 and 460 GeV. Employing the energy dependence of the cross section, FL is measured in a range of squared four-momentum transfers 12 <= Q2 <=, 90 GeV2 and low Bjorken x 0.00024 <= x <= 0.0036. The F-L values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA. (C) 2008 Elsevier B.V. All rights reserved.
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17.
  • Aaron, F. D., et al. (author)
  • Multi-lepton production at high transverse momenta in ep collisions at HERA
  • 2008
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 668:4, s. 268-276
  • Journal article (peer-reviewed)abstract
    • Processes leading to a final state with at least two high transverse momentum leptons (electrons or muons) are studied using the full e(+/-) p data sample collected by the H1 experiment at HERA. The data correspond to an integrated luminosity of 463 pb(-1). Di-lepton and tri-lepton event classes are investigated. Cross sections of the production of e(+)e(-) and mu(+)mu(-) pairs are derived in a restricted phase space dominated by photon-photon collisions. In general, good agreement is found with Standard Model predictions. Events are observed with a total scalar sum of lepton transverse momenta above 100 GeV where the Standard Model expectation is low. In this region, combining di-lepton and tri-lepton classes, five events are observed in e(+) p collisions, compared to a Standard Model expectation of 0.96 +/- 0.12, while no such event is observed in e(-) p data for 0.64 +/- 0.09 expected. (C) 2008 Elsevier B.V. All rights reserved.
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18.
  • Aaron, F. D., et al. (author)
  • Strangeness production at low Q(2) in deep-inelastic ep scattering at HERA
  • 2009
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 61:2, s. 185-205
  • Journal article (peer-reviewed)abstract
    • The production of neutral strange hadrons is investigated using deep-inelastic scattering events measured with the H1 detector at HERA. The measurements are made in the phase space defined by the negative four-momentum transfer squared of the photon 2 < Q(2) < 100 GeV2 and the inelasticity 0.1 < y < 0.6. The K-s(0) s and Lambda((Lambda) over bar) production cross sections and their ratios are determined. K-s(0) s production is compared to the production of charged particles in the same region of phase space. The Lambda-(Lambda) over bar asymmetry is also measured and found to be consistent with zero. Predictions of leading order Monte Carlo programs are compared to the data.
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19.
  • Aaron, F. D., et al. (author)
  • Study of charm fragmentation into D (* +/-) mesons in deep-inelastic scattering at HERA
  • 2009
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 59:3, s. 589-606
  • Journal article (peer-reviewed)abstract
    • The process of charm quark fragmentation is studied using D (* +/-) meson production in deep-inelastic scattering as measured by the H1 detector at HERA. The parameters of fragmentation functions are extracted for QCD models based on leading order matrix elements and DGLAP or CCFM evolution of partons together with string fragmentation and particle decays. Additionally, they are determined for a next-to-leading order QCD calculation in the fixed flavour number scheme using the independent fragmentation of charm quarks to D (* +/-) mesons. Two different regions of phase space are investigated defined by the presence or absence of a jet containing the D (* +/-) meson in the event. The fragmentation parameters extracted for the two phase space regions are found to be different.
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20.
  • Aaron, F. D., et al. (author)
  • Three- and four-jet production at low x at HERA
  • 2008
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 54:3, s. 389-409
  • Journal article (peer-reviewed)abstract
    • 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(alpha(2)(s)) and O(alpha(3)(s)) and with Monte Carlo simulation programs where higher order effects are approximated by parton showers. A good overall description is provided by the O(alpha(3)(s)) calculation. Too few events are predicted at the lowest x similar to 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.
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21.
  • Aktas, A., et al. (author)
  • Measurement of event shape variables in deep-inelastic scattering at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:2, s. 343-356
  • Journal article (peer-reviewed)abstract
    • Deep-inelastic ep scattering data taken with the H1 detector at HERA and corresponding to an integrated luminosity of 106 pb(-1) are used to study the differential distributions of event shape variables. These include thrust, jet broadening, jet mass and the C-parameter. The four-momentum transfer Q is taken to be the relevant energy scale and ranges between 14 GeV and 200 GeV. The event shape distributions are compared with perturbative QCD predictions, which include resummed contributions and analytical power law corrections, the latter accounting for non-perturbative hadronisation effects. The data clearly exhibit the running of the strong coupling alpha(s)(Q) and are consistent with a universal power correction parameter alpha(0) for all event shape variables. A combined QCD fit using all event shape variables yields alpha(s)(m(Z)) = 0.1198 +/- 0.0013 (+0.0056)(-0.0043) and alpha(0) = 0.476 +/- 0.008 (+0.018)(-0.059).
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22.
  • Aaron, F. D., et al. (author)
  • Search for excited electrons in ep collisions at HERA
  • 2008
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 666:2, s. 131-139
  • Journal article (peer-reviewed)abstract
    • A search for excited electrons is performed using the full e(+/-) p data sample collected by the HI experiment at HERA, corresponding to a total luminosity of 475 pb(-1). The electroweak decays of excited electrons e* -> e gamma, e* -> eZ and e* -> vW 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/Lambda 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. (C) 2008 Elsevier B.V. All rights reserved.
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23.
  • Aktas, A., et al. (author)
  • Measurement of inclusive jet production in deep-inelastic scattering at high Q(2) and determination of the strong coupling
  • 2007
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 653:2-4, s. 134-144
  • Journal article (peer-reviewed)abstract
    • Inclusive jet production is studied in neutral current deep-inelastic positron-proton scattering at large four momentum transfer squared Q(2) > 150 GeV2 with the H1 detector at HERA. Single and double differential inclusive jet cross sections are measured as a function of Q(2) and of the transverse energy E-T 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 alpha(s)(M-Z) 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 ces (M-Z) = 0.1193 +/- 0.0014(exp.)(-0.0030)(+0.0047) (th.) +/- 0.0016(pdf). (c) 2007 Elsevier BX All rights reserved.
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24.
  • Aaron, F. D., et al. (author)
  • A search for excited neutrinos in e(-) p collisions at HERA
  • 2008
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 663:5, s. 382-389
  • Journal article (peer-reviewed)abstract
    • 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.
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25.
  • Aktas, A, et al. (author)
  • A direct search for stable magnetic monopoles produced in positron-proton collisions at HERA
  • 2005
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 41, s. 133-141
  • Journal article (peer-reviewed)abstract
    • A direct search has been made for magnetic monopoles produced in e(+)p collisions at a centre of mass energy of 300 GeV at HERA. The beam pipe surrounding the interaction region in 1995-1997 was investigated using a SQUID magnetometer to look for stopped magnetic monopoles. During this time an integrated luminosity of 62 pb(-1) was delivered. No magnetic monopoles were observed and charge and mass dependent upper limits on the e(+)p production cross section are set.
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26.
  • Aktas, A, et al. (author)
  • Search for leptoquark bosons in ep collisions at HERA
  • 2005
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 629:1, s. 9-19
  • Journal article (peer-reviewed)abstract
    • A search for scalar and vector leptoquarks coupling to first generation fermions is performed using the e(+) p and e(-) p scattering data collected by the H1 experiment between 1994 and 2000. The data correspond to a total integrated luminosity of 117 pb(-1). No evidence for the direct or indirect production of such particles is found in data samples with a large transverse momentum final state electron or with large missing transverse momentum. Constraints on leptoquark models are established. For leptoquark couplings of electromagnetic strength, leptoquarks with masses up to 275-325 GeV are ruled out at 95% confidence level. These limits improve and supercede earlier H1 limits based on subsamples of the data used here.
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27.
  • Aktas, A, et al. (author)
  • A determination of electroweak parameters at HERA
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 632:1, s. 35-42
  • Journal article (peer-reviewed)abstract
    • Using the deep inelastic e(+) p and e(-) p charged and neutral current scattering cross sections previously published, a combined electroweak and QCD analysis is performed to determine electroweak parameters accounting for their correlation with parton distributions. The data used have been collected by the HI experiment in 1994-2000 and correspond to an integrated luminosity of 117.2 pb(-1). A measurement is obtained of the W propagator mass in charged current ep scattering. The weak mixing angle sin(2)theta w is determined in the on-mass-shell renormalisation scheme. A first measurement at HERA is made of the light quark weak couplings to the Z(0) boson and a possible contribution of right-handed isospin components to the weak couplings is investigated. (c) 2005 Elsevier B.V. All rights reserved.
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28.
  • Aktas, A, et al. (author)
  • Measurement of Fc(c)over-bar(2) and Fb(b)over-bar(2) at low Q(2) and x using the H1 vertex detector at HERA
  • 2006
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 45:1, s. 23-33
  • Journal article (peer-reviewed)abstract
    • Measurements are presented of inclusive charm and beauty cross sections in e(+)p collisions at HERA for values of photon virtuality 12 <= Q(2) = 60 GeV2 and of the Bjorken scaling variable 0.0002 <= x <= 0.005. The fractions of events containing charm and beauty quarks are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 vertex detector. Values for the structure functions F-2(cc) and F-2(bb) are obtained. This is the first measurement of F-2(bb) in this kinematic range. The results are found to be compatible with the predictions of perturbative quantum chromodynamics and with previous measurements of F-2(cc).
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29.
  • Aktas, A, et al. (author)
  • First measurement of charged current cross sections at HERA with longitudinally polarised positrons
  • 2006
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 634:2-3, s. 173-179
  • Journal article (peer-reviewed)abstract
    • 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 -> nu X, for negative four-momentum transfer squared Q(2) > 400 GeV2 and inelasticity gamma < 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. (c) 2006 Elsevier B.V. All rights reserved.
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30.
  • Aktas, A, et al. (author)
  • Measurement of charm and beauty photoproduction at HERA using D*mu correlations
  • 2005
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 621:1-2, s. 56-71
  • Journal article (peer-reviewed)abstract
    • A measurement of charm and beauty photoproduction at the electron proton collider HERA is presented based on the simultaneous detection of a D*(+/-) meson and a muon. The correlation between the D* meson and the muon serves to separate the charm and beauty contributions and the analysis provides comparable sensitivity to both. The total and differential experimental cross sections are compared to LO and NLO QCD calculations. The measured charm cross section is in good agreement with QCD predictions including higher order effects while the beauty cross section is higher.
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31.
  • Aktas, A, et al. (author)
  • Measurement of deeply virtual Compton scattering at HERA
  • 2005
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 44:1, s. 1-11
  • Journal article (peer-reviewed)abstract
    • A measurement is presented of elastic deeply virtual Compton scattering gamma* p -> gamma p made using e(+) p collision data corresponding to a luminosity of 46.5 pb(-1), taken with the H1 detector at HERA. The cross section is measured as a function of the photon virtuality, Q(2), the invariant mass of the gamma* p system, W, and for the first time, differentially in the squared momentum transfer at the proton vertex, t, in the kinematic range 2 < Q(2) < 80 GeV2, 30 < W < 140 GeV and vertical bar t vertical bar < 1 GeV2. QCD based calculations at next-to-leading order using generalized parton distributions can describe the data, as can colour dipole model predictions.
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32.
  • Aktas, A, et al. (author)
  • Search for bosonic stop decays in R-parity violating supersymmetry in e(+)p collisions at HERA
  • 2004
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 599:3-4, s. 159-172
  • Journal article (peer-reviewed)abstract
    • A search for scalar top quarks in R-parity violating supersymmetry is performed in e(+)p collisions at HERA using the H1 detector. The data, taken at roots = 319 GeV and 301 GeV, correspond to an integrated luminosity of 106 pb(-1). The resonant production of scalar top quarks t in positron quark fusion via an R-parity violating Yukawa coupling lambda' is considered with the subsequent bosonic stop decay t --> BW. The R-parity violating decay of the sbottom quark b --> dv(e) and leptonic and hadronic W decays are considered. No evidence for stop production is found in the search for bosonic stop decays nor in a search for the direct R-parity violating decay t --> eq. Mass dependent limits on lambda' are obtained in the framework of the minimal supersymmetric Standard Model. Stop quarks with masses up to 275 GeV can be excluded at the 95% confidence level for a Yukawa coupling of electromagnetic strength. (C) 2004 Elsevier B.V. All rights reserved.
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33.
  • Aktas, A, et al. (author)
  • A general search for new phenomena in ep scattering at HERA
  • 2004
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 602:1-2, s. 14-30
  • Journal article (peer-reviewed)abstract
    • A model-independent search for deviations from the Standard Model prediction is performed in e(+) p and e(-) p collisions at HERA using H1 data corresponding to an integrated luminosity of 117 pb(-1). For the first time all event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A statistical algorithm is developed to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or invariant mass of final state particles and to quantify their significance. A good agreement with the Standard Model prediction is observed in most of the event classes. The most significant deviation is found for a topology containing an isolated muon, missing transverse momentum and a jet, consistent with a previously reported observation.
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34.
  • Aktas, A, et al. (author)
  • Measurement of dijet cross sections in ep interactions with a leading neutron at HERA
  • 2005
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 41:3, s. 273-286
  • Journal article (peer-reviewed)abstract
    • Measurements are reported of the production of dijet events with a leading neutron in ep interactions at HERA. Differential cross sections for photoproduction and deep inelastic scattering are presented as a function of several kinematic variables. Leading order QCD simulation programs are compared with the measurements. Models in which the real or virtual photon interacts with a parton of an exchanged pion are able to describe the data. Next-to-leading order perturbative QCD calculations based on pion exchange are found to be in good agreement with the measured cross sections. The fraction of leading neutron dijet events with respect to all dijet events is also determined. The dijet events with a leading neutron have a lower fraction of resolved photon processes than do the inclusive dijet data.
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35.
  • Aktas, A, et al. (author)
  • Measurement of F-2(c(c)over-bar) and F-2(b(b)over-bar) at high Q(2) using the H1 vertex detector at HERA
  • 2005
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 40:3, s. 349-359
  • Journal article (peer-reviewed)abstract
    • Measurements are presented of inclusive charm and beauty cross sections in e(+)p collisions at HERA for values of photon virtuality Q(2) > 150 GeV2 and of inelasticity 0.1 < y < 0.7. The charm and beauty fractions are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 vertex detector. The data are divided into four regions in Q(2) and Bjorken x, and values for the structure functions F-2(c (c) over bar) and F-2(b (b) over bar) are obtained. The results are found to be compatible with the predictions of perturbative quantum chromodynamics.
  •  
36.
  • Abt, I, et al. (author)
  • Inclusive V-0 production cross sections from 920 GeV fixed target proton-nucleus collisions
  • 2003
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 29:2, s. 181-190
  • Journal article (peer-reviewed)abstract
    • Inclusive differential cross sections dsigma(pA)/dx(F) and dsigma(pA)/dp(t)(2) for the production of K-S(0), Lambda, and (&ULambda;) over bar particles are measured at HERA in proton-induced reactions on C, Al, Ti, and W targets. The incident beam energy is 920 GeV, corresponding to roots = 41.6 GeV in the proton-nucleon system. The ratios of differential cross sections dsigma(pA)(K-S(0))/dsigma(pA)(Lambda) and dsigma(pA)((&ULambda;) over bar)/dsigma(pA) (Lambda) are measured to be 6.2 +/- 0.5 and 0.66 +/- 0.07, respectively, for x(F) approximate to -0.06. No significant dependence upon the target material is observed. Within errors, the slopes of the transverse momentum distributions da,Ald t also show no significant dependence upon the target material. The dependence of the extrapolated total cross sections sigma(pA) on the atomic mass A of the target material is discussed, and the deduced cross sections per nucleon sigma(pN) are compared with results obtained at other energies.
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37.
  • Abt, I, et al. (author)
  • Measurement of the b(b)over-bar production cross section in 920 GeV fixed-target proton-nucleus collisions
  • 2003
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 26:3, s. 345-355
  • Journal article (peer-reviewed)abstract
    • Using the HERA-B detector, the b (b) over bar production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The b (b) over bar production was tagged via inclusive bottom quark decays into J/psi by exploiting the longitudinal separation of J/psi --> l(+)l(-) decay vertices from the primary proton-nucleus interaction. Both e(+)e(-) and mu(+)mu(-) channels have been reconstructed and the combined analysis yields the cross section sigma(b (b) over bar) = 32(-12)(+14)(stat) (+6)(-7)(sys) nb/nucleon.
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38.
  • Aktas, A, et al. (author)
  • Search for light gravitinos in events with photons and missing transverse momentum at HERA
  • 2005
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 616:1-2, s. 31-42
  • Journal article (peer-reviewed)abstract
    • A search for gravitinos produced in e(+/-)p collisions is performed using the H1 detector at HERA. The data were taken at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 64.3 pb(-1) for e(+)p collisions and 13.5 pb(-1) for e(-)p collisions. If R-parity is not conserved, the t-channel exchange of a selectron can produce a neutralino, which, in models where the gravitino is the lightest supersymmetric particle, subsequently decays into a photon and a light gravitino. The resulting event signature, which involves an isolated photon, a jet and missing transverse energy, is analysed for the first time at HERA. No deviation from the Standard Model is found. Exclusion limits on the cross section and on R-parity-violating Yukawa couplings are derived in a Gauge Mediated Supersymmetry Breaking scenario. The results are independent of the squark sector. Neutralinos and supersymmetric partners of the left-handed electron with masses up to 112 GeV and 164 GeV, respectively, can be ruled out at the 95% confidence level for R-parity-violating couplings lambda' equal to 1, in some parts of the parameter space of the considered model.
  •  
39.
  • Aktas, A, et al. (author)
  • Measurement of beauty production at HERA using events with muons and jets
  • 2005
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 41:4, s. 453-467
  • Journal article (peer-reviewed)abstract
    • A measurement of the beauty production cross section in ep collisions at a centre-of-mass energy of 319 GeV is presented. The data were collected with the H1 detector at the HERA collider in the years 1999-2000. Events are selected by requiring the presence of jets and muons in the final state. Both the long lifetime and the large mass of b-flavoured hadrons are exploited to identify events containing beauty quarks. Differential cross sections are measured in photoproduction, with photon virtualities Q(2) < 1 GeV2, and in deep inelastic scattering, where 2 < Q(2) < 100 GeV2. The results are compared with perturbative QCD calculations to leading and next-to-leading order. The predictions are found to be somewhat lower than the data.
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40.
  • Bordag, L. A., et al. (author)
  • Optimal allocation-consumption problem for a portfolio with an illiquid asset
  • 2016
  • In: International Journal of Computer Mathematics. - : Informa UK Limited. - 0020-7160 .- 1029-0265. ; 93:5, s. 749-760
  • Journal article (peer-reviewed)abstract
    • During financial crises investors manage portfolios with low liquidity, where the paper-value of an asset differs from the price proposed by the buyer. We consider an optimization problem for a portfolio with an illiquid, a risky and a risk-free asset. We work in the Merton's optimal consumption framework with continuous time. The liquid part of the investment is described by a standard Black-Scholes market. The illiquid asset is sold at a random moment with prescribed distribution and generates additional liquid wealth dependent on its paper-value. The investor has a hyperbolic absolute risk aversion also denoted as HARA-type utility function, in particular, the logarithmic utility function as a limit case. We study two different distributions of the liquidation time of the illiquid asset - a classical exponential distribution and a more practically relevant Weibull distribution. Under certain conditions we show the smoothness of the viscosity solution and obtain closed formulae relevant for numerics.
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