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Sökning: WFRF:(Niebuhr O.)

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51.
  • Aktas, A, et al. (författare)
  • Measurement of dijet cross sections in ep interactions with a leading neutron at HERA
  • 2005
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 41:3, s. 273-286
  • Tidskriftsartikel (refereegranskat)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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52.
  • Aktas, A, et al. (författare)
  • Measurement of dijet production at low Q(2) at HERA
  • 2004
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 37:2, s. 141-159
  • Tidskriftsartikel (refereegranskat)abstract
    • Triple differential dijet cross sections in e(+/-)p interactions are presented in the region of photon virtualities 2 < Q(2) < 80 GeV2, inelasticities 0.1 < y < 0.85, jet transverse energies E*(T1) > 7GeV, E*(T2) > 5 GeV, and pseudorapidities - 2.5 < η(1)*, η(2)* < 0. The measurements are made in the gamma*p centre-of-mass frame, using an integrated luminosity of 57 pb(-1). The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q(2) and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E-T jet production.
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53.
  • Aktas, A., et al. (författare)
  • Measurement of event shape variables in deep-inelastic scattering at HERA
  • 2006
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:2, s. 343-356
  • Tidskriftsartikel (refereegranskat)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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54.
  • Aktas, A, et al. (författare)
  • 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
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 40:3, s. 349-359
  • Tidskriftsartikel (refereegranskat)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.
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55.
  • Aktas, A, et al. (författare)
  • 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
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 45:1, s. 23-33
  • Tidskriftsartikel (refereegranskat)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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56.
  • Aktas, A., et al. (författare)
  • Measurement of inclusive jet production in deep-inelastic scattering at high Q(2) and determination of the strong coupling
  • 2007
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 653:2-4, s. 134-144
  • Tidskriftsartikel (refereegranskat)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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57.
  • Aktas, A, et al. (författare)
  • Measurement of prompt photon cross sections in photoproduction at HERA
  • 2005
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 38:4, s. 437-445
  • Tidskriftsartikel (refereegranskat)abstract
    • Results are presented on the photoproduction of isolated prompt photons, inclusively and associated with jets, in the gammap center of mass energy range 142 < W < 266 GeV. The cross sections are measured for the transverse momentum range of the photons 5 < E. T < 10 GeV and for associated jets with E-T(jet) > 4.5 GeV. They are measured differentially as a function of E-T(gamma), E-T(jet), the pseudorapidities eta(gamma) and eta(jet) and estimators of the momentum fractions x(gamma) and x(p) of the incident photon and proton carried by the constituents participating in the hard process. In order to further investigate the underlying dynamics, the angular correlation between the prompt photon and the jet in the transverse plane is studied. Predictions by perturbative QCD calculations in next to leading order are about 30% below the inclusive prompt photon data after corrections for hadronisation and multiple interactions, but are in reasonable agreement with the results for prompt photons associated with jets. Comparisons with the predictions of the event generators PYTHIA and HERWIG are also presented.
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58.
  • Aktas, A, et al. (författare)
  • Measurement of the proton structure function F-2 at low Q(2) in QED Compton scattering at HERA
  • 2004
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 598:3-4, s. 159-171
  • Tidskriftsartikel (refereegranskat)abstract
    • The proton structure function F-2(x, Q(2)) is measured in inelastic QED Compton scattering using data collected with the H1 detector at HERA. QED Compton events are used to access the kinematic range of very low virtualities of the exchanged photon, Q(2), down to 0.5 GeV2, and Bjorken x up to similar to 0.06, a region which has not been covered previously by inclusive measurements at HERA. The results are in agreement with the measurements from fixed target lepton-nucleon scattering experiments. (C) 2004 Published by Elsevier B.V.
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59.
  • Aktas, A, et al. (författare)
  • Multi-electron production at high transverse momenta in ep collisions at HERA
  • 2003
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 31:1, s. 17-29
  • Tidskriftsartikel (refereegranskat)abstract
    • Multi-electron production is studied at high electron transverse momentum in positron- and electron-proton collisions using the H1 detector at HERA. The data correspond to an integrated luminosity of 115 pb(-1). Di-electron and tri-electron event yields are measured. Cross sections are derived in a restricted phase space region dominated by photon-photon collisions. In general good agreement is found with the Standard Model predictions. However, for electron pair invariant masses above 100 GeV, three di-electron events and three tri-electron events are observed, compared to Standard Model expectations of 0.30 +/- 0.04 and 0.23 +/- 0.04, respectively.
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60.
  • Aktas, A, et al. (författare)
  • Muon pair production in ep collisions at HERA
  • 2004
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 583:1-2, s. 28-40
  • Tidskriftsartikel (refereegranskat)abstract
    • Cross sections for the production of two isolated muons up to high di-muon masses are measured in ep collisions at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 71 pb(-1) at a centre of mass energy of roots = 319 GeV. The results are in good agreement with Standard Model predictions, the dominant process being photon-photon interactions. Additional muons or electrons are searched for in events with two high transverse momentum muons using the full data sample corresponding to 114 pb(-1), where data at roots = 301 GeV and roots = 319 GeV are combined. Both the di-lepton sample and the tri-lepton sample agree Well with the predictions. (C) 2004 Published by Elsevier B.V.
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61.
  • Aktas, A., et al. (författare)
  • Photoproduction of dijets with high transverse momenta at HERA
  • 2006
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 639:1, s. 21-31
  • Tidskriftsartikel (refereegranskat)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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62.
  • Aktas, A., et al. (författare)
  • 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
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 639:3-4, s. 202-209
  • Tidskriftsartikel (refereegranskat)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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63.
  • Aktas, A, et al. (författare)
  • Search for bosonic stop decays in R-parity violating supersymmetry in e(+)p collisions at HERA
  • 2004
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 599:3-4, s. 159-172
  • Tidskriftsartikel (refereegranskat)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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64.
  • Aktas, A., et al. (författare)
  • Search for doubly-charged Higgs boson production at HERA
  • 2006
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 638:5-6, s. 432-440
  • Tidskriftsartikel (refereegranskat)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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65.
  • Aktas, A, et al. (författare)
  • Search for leptoquark bosons in ep collisions at HERA
  • 2005
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 629:1, s. 9-19
  • Tidskriftsartikel (refereegranskat)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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66.
  • Aktas, A, et al. (författare)
  • Search for light gravitinos in events with photons and missing transverse momentum at HERA
  • 2005
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 616:1-2, s. 31-42
  • Tidskriftsartikel (refereegranskat)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.
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67.
  • Aktas, A, et al. (författare)
  • Search for single top quark production in ep collisions at HERA
  • 2004
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 33:1, s. 9-22
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for single top quark production is performed in e(+/-) pcollisions at HERA. The search exploits data corresponding to an integrated luminosity of 118.3 pb(-1). A model for the anomalous production of top quarks in a flavour changing neutral current process involving a tugamma coupling is investigated. Decays of top quarks into a b quark and a W boson are considered in the leptonic and the hadronic decay channels of the W. Both a cut-based analysis and a multivariate likelihood analysis are performed to discriminate anomalous top quark production from Standard Model background processes. In the leptonic channel, 5 events are found while 1.31 +/- 0.22 events are expected from the Standard Model background. In the hadronic channel, no excess above the expectation for Standard Model processes is found. These observations lead to a cross section (ep --> e t X) = 0.29(-0.14)(+0.15) pb at roots = 319 GeV. Alternatively, assuming that the observed events are due to a statistical fluctuation, upper limits of 0.55 pb on the anomalous top production cross section and of 0.27 on the tugamma coupling k(tugamma) are established at the 95% confidence level.
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68.
  • Aktas, A, et al. (författare)
  • Search for squark production in R-parity violating supersymmetry at HERA
  • 2004
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 36:4, s. 425-440
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for squarks in R-parity violating supersymmetry is performed in e(+/-)p collisions at HERA using the H1 detector. 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. The resonant production of squarks via a Yukawa coupling lambda' is considered, taking into account direct and indirect R-parity violating decay modes. No evidence for squark production is found in the multi-lepton and multi-jet final state topologies investigated. Mass dependent limits on lambda' are obtained in the framework of the Minimal Supersymmetric Standard Model. In addition, the results are interpreted in terms of constraints on the parameters of the minimal Supergravity model. At the 95% confidence level squarks of all flavours with masses up to 275 GeV are excluded in a large part of the parameter space for a Yukawa coupling of electromagnetic strength. For a coupling strength 100 times smaller, masses up to 220 GeV can be ruled out.
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69.
  • Aktas, A., et al. (författare)
  • Tau lepton production in ep collisions at HERA
  • 2006
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 699-714
  • Tidskriftsartikel (refereegranskat)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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70.
  • Aktas, A., et al. (författare)
  • Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA
  • 2007
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 51:3, s. 549-568
  • Tidskriftsartikel (refereegranskat)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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71.
  • Andreev, V, et al. (författare)
  • Isolated electrons and muons in events with missing transverse momentum at HERA
  • 2003
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 561:3-4, s. 241-257
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for events with a high-energy isolated electron or muon and missing transverse momentum has been performed at the electron-proton collider HERA using an integrated luminosity of 13.6 pb(-1) in e(-) p scattering and 104.7 pb(-1) in e(+) p scattering. Within the Standard Model such events are expected to be mainly due to W boson production with subsequent leptonic decay. In e(-) p interactions one event is observed in the electron channel and none in the muon channel, consistent with the expectation of the Standard Model. In the e(+) p data a total of 18 events are seen in the electron and muon channels compared to an expectation of 12.4 +/- 1.7 dominated by W production (9.4 +/- 1.6). Whilst the overall observed number of events is broadly in agreement with the number predicted by the Standard Model, there is-an excess of events with transverse momentum of the hadronic system greater than 25 GeV with 10 events found compared to 2.9 +/- 0.5 expected. The results are used to determine the cross-section for events with an isolated electron or muon and missing transverse momentum. (C) 2003 Published by Elsevier Science B.V.
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72.
  • Aaron, F. D., et al. (författare)
  • A general search for new phenomena at HERA
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 674:4-5, s. 257-268
  • Tidskriftsartikel (refereegranskat)abstract
    • A model-independent search for deviations from the Standard Model prediction is performed using the full e(+/-) p data sample collected by the H1 experiment at HERA. All event topologies involving isolated electrons, photons, muons, neutrinos and jets with transverse momenta above 20 GeV are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A dedicated algorithm is used to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or the invariant mass of final state particles and to quantify their significance. Variables related to angular distributions and energy sharing between final state particles are also introduced to study the final state topologies. No significant deviation from the Standard Model expectation is observed in the phase space covered by this analysis. (C) 2009 Elsevier B.V. All rights reserved.
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73.
  • Aaron, F. D., et al. (författare)
  • Events with isolated leptons and missing transverse momentum and measurement of W production at HERA
  • 2009
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 64:2, s. 251-271
  • Tidskriftsartikel (refereegranskat)abstract
    • Events with high energy isolated electrons, muons or tau leptons and missing transverse momentum are studied using the full e(+/-)p data sample collected by the H1 experiment at HERA, corresponding to an integrated luminosity of 474 pb(-1). Within the Standard Model, events with isolated leptons and missing transverse momentum mainly originate from the production of single W bosons. The total single W boson production cross section is measured as 1.14 +/- 0.25 (stat.) +/- 0.14 (sys.) pb, in agreement with the Standard Model expectation. The data are also used to establish limits on the WW gamma gauge couplings and for a measurement of the W boson polarisation.
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74.
  • Aaron, F. D., et al. (författare)
  • Inclusive photoproduction of rho(0), K*(0) and phi mesons at HERA H1 Collaboration
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 673:2, s. 119-126
  • Tidskriftsartikel (refereegranskat)abstract
    • Inclusive non-diffractive photoproduction of rho(770)(0), K*(892)(0) and phi(1020) mesons is investigated with the H1 detector in ep collisions at HERA. The corresponding average gamma p centre-of-mass energy is 210 GeV. The mesons are measured in the transverse momentum range 0.5 < p(T) < 7 GeV and the rapidity range vertical bar y(lab)vertical bar < 1. Differential cross sections are presented as a function of transverse momentum and rapidity, and are compared to the predictions of hadroproduction models. (C) 2009 Elsevier B.V. All rights reserved.
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75.
  • Aktas, A, et al. (författare)
  • First measurement of charged current cross sections at HERA with longitudinally polarised positrons
  • 2006
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 634:2-3, s. 173-179
  • Tidskriftsartikel (refereegranskat)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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76.
  • Aaron, F. D., et al. (författare)
  • A precision measurement of the inclusive ep scattering cross section at HERA
  • 2009
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 64:4, s. 561-587
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the inclusive deep inelastic neutral current e(+) p scattering cross section is reported in the region of four-momentum transfer squared, 12GeV(2) <= Q(2) <= 150 GeV2, and Bjorken x, 2 x 10(-4) <= x <= 0.1. The results are based on data collected by the H1 Collaboration at the ep collider HERA at positron and proton beam energies of E-e = 27.6 GeV and E-p = 920 GeV, respectively. The data are combined with previously published data, taken at E-p = 820 GeV. The accuracy of the combined measurement is typically in the range of 1.3-2%. A QCD analysis at next-to-leading order is performed to determine the parton distributions in the proton based on H1 data.
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77.
  • Aaron, F. D., et al. (författare)
  • Deeply virtual Compton scattering and its beam charge asymmetry in e(+/-)p collisions at HERA
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 681:5, s. 391-399
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of elastic deeply virtual Compton scattering gamma*p --> gamma p using e(+) p and e(-) p collision data recorded with the HI detector at HERA is presented. The analysed data sample corresponds to an integrated luminosity of 306 pb(-1), almost equally shared between both beam charges. The cross section is measured as a function of the virtuality Q(2) of the exchanged photon and the centre-of-mass energy W of the gamma*p system in the kinematic domain 6.5 < Q(2) < 80 GeV2, 30 < W < 140 GeV and |t| < 1 GeV2, where t denotes the squared momentum transfer at the proton vertex. The cross section is determined differentially in t for different Q(2) and W values and exponential t-slope parameters are derived. Using e(+) p and e(-) p data samples, a beam charge asymmetry is extracted for the first time in the low Biorken x kinematic domain. The observed asymmetry is attributed to the interference between Bethe-Heitler and deeply virtual Compton scattering processes. Experimental results are discussed in the context of two different models, one based on generalised parton distributions and one based on the dipole approach. (C) 2009 Elsevier B.V. All rights reserved.
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78.
  • Aaron, F. D., et al. (författare)
  • Diffractive electroproduction of rho and phi mesons at HERA
  • 2010
  • Ingår i: Journal of High Energy Physics. - 1029-8479. ; 2010:5, s. 1-111
  • Tidskriftsartikel (refereegranskat)abstract
    • Diffractive electroproduction of rho and phi mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb(-1). About 10500 rho and 2000 phi events are analysed in the kinematic range of squared photon virtuality 2.5 <= Q(2) <= 60 GeV2, photon-proton centre of mass energy 35 <= W <= 180 GeV and squared four-momentum transfer to the proton vertical bar t vertical bar <= 3 GeV2. The total, longitudinal and transverse cross sections are measured as a function of Q(2), W and vertical bar t vertical bar. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small q (q) over bar dipoles, for Q(2) larger than 10 to 20 GeV2. They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q(2) + M-V(2))/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.
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79.
  • Aaron, F. D., et al. (författare)
  • Jet production in ep collisions at high Q(2) and determination of alpha(s)
  • 2010
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 65:3-4, s. 363-383
  • Tidskriftsartikel (refereegranskat)abstract
    • The production of jets is studied in deep-inelastic e(+/-) p scattering at large negative four momentum transfer squared 150 < Q(2) < 15000 GeV2 using HERA data taken in 1999-2007, corresponding to an integrated luminosity of 395 pb(-1). Inclusive jet, 2-jet and 3-jet cross sections, normalised to the neutral current deep-inelastic scattering cross sections, are measured as functions of Q(2), jet transverse momentum and proton momentum fraction. The measurements are well described by perturbative QCD calculations at next-to-leading order corrected for hadronisation effects. The strong coupling as determined from these measurements
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80.
  • Aaron, F. D., et al. (författare)
  • Jet production in ep collisions at low Q(2) and determination of alpha(s)
  • 2010
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 67:1-2, s. 1-24
  • Tidskriftsartikel (refereegranskat)abstract
    • The production of jets is studied in deep-inelastic e(+) p scattering at low negative four momentum transfer squared 5 < Q(2) < 100 GeV2 and at inelasticity 0.2 < y < 0.7 using data recorded by the H1 detector at HERA in the years 1999 and 2000, corresponding to an integrated luminosity of 43.5 pb(-1). Inclusive jet, 2-jet and 3-jet cross sections as well as the ratio of 3-jet to 2-jet cross sections are measured as a function of Q(2) and jet transverse momentum. The 2-jet cross section is also measured as a function of the proton momentum fraction xi. The measurements are well described by perturbative quantum chromodynamics at next-to-leading order corrected for hadronisation effects and are subsequently used to extract the strong coupling as.
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81.
  • Aaron, F. D., et al. (författare)
  • Measurement of D*(+/-) meson production and determination of F-2(c(c)over-bar) at low Q(2) in deep-inelastic scattering at HERA
  • 2011
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 71:10
  • Tidskriftsartikel (refereegranskat)abstract
    • Inclusive production of D* mesons in deep-inelastic ep scattering at HERA is studied in the range 5 < Q(2) < 100 GeV2 of the photon virtuality and 0.02 < y < 0.7 of the inelasticity of the scattering process. The observed phase space for the D* meson is (pT) (D*) > 1.25 GeV and |eta( D*)| < 1.8. The data sample corresponds to an integrated luminosity of 348 pb(-1) collected with the H1 detector. Single and double differential cross sections are measured and the charm contribution F-2(c (c) over bar) to the proton structure function F-2 is determined. The results are compared to perturbative QCD predictions at next-to-leading order implementing different schemes for the charm mass treatment and with Monte Carlo models based on leading order matrix elements with parton showers.
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82.
  • Aaron, F. D., et al. (författare)
  • Measurement of inclusive and dijet D* meson cross sections in photoproduction at HERA
  • 2012
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 72:5
  • Tidskriftsartikel (refereegranskat)abstract
    • The inclusive photoproduction of D* mesons and of D*-tagged dijets is investigated with the H1 detector at the ep collider HERA. The kinematic region covers small photon virtualities Q(2) < 2 GeV2 and photon-proton centre-of-mass energies of 100< W-gamma p < 285 GeV. Inclusive D* meson differential cross sections are measured for central rapidities vertical bar eta(D*)vertical bar < 1.5 and transverse momenta p(T) (D*) > 1.8 GeV. The heavy quark production process is further investigated in events with at least two jets with transverse momentum p(T) (jet) > 3.5 GeV each, one containing the D* meson. Differential cross sections for D*-tagged dijet production and for correlations between the jets are measured in the range vertical bar eta(D*)vertical bar < 1.5 and p(T) (D*) > 2.1 GeV. The results are compared with predictions from Monte Carlo simulations and next-to-leading order perturbative QCD calculations.
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83.
  • Aaron, F. D., et al. (författare)
  • Measurement of the charm and beauty structure functions using the H1 vertex detector at HERA
  • 2010
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 65:1-2, s. 89-109
  • Tidskriftsartikel (refereegranskat)abstract
    • Inclusive charm and beauty cross sections are measured in e (-) p and e (+) p neutral current collisions at HERA in the kinematic region of photon virtuality 5a parts per thousand currency signQ (2)a parts per thousand currency sign2000 GeV2 and Bjorken scaling variable 0.0002a parts per thousand currency signxa parts per thousand currency sign0.05. The data were collected with the H1 detector in the years 2006 and 2007 corresponding to an integrated luminosity of 189 pb(-1). The numbers of charm and beauty events are determined using variables reconstructed by the H1 vertex detector including the impact parameter of tracks to the primary vertex and the position of the secondary vertex. The measurements are combined with previous data and compared to QCD predictions.
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84.
  • Aaron, F. D., et al. (författare)
  • Measurement of the D*(+/-) meson production cross section and F-2(c(c)over-bar) at high Q(2) in ep scattering at HERA
  • 2010
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 686:2-3, s. 91-100
  • Tidskriftsartikel (refereegranskat)abstract
    • The inclusive production of D*(+/-)(2010) mesons in deep-inelastic e(+/-)p scattering is measured in the kinematic region of photon virtuality 100 < Q(2) < 1000 GeV2 and inelasticity 002 < y < 0 7 Single and double differential cross sections for inclusive D* meson production are measured in the visible range defined by vertical bar eta(D*)vertical bar < 1 5 and p(gamma)(D*) > 1 5 GeV The data were collected by the H1 experiment during the period from 2004 to 2007 and correspond to an integrated luminosity of 351 pb(-1) The charm contribution. F-2(c (c) over bar), to the proton structure function F-2 is determined. The measurements are compared with QCD predictions. (C) 2010 Elsevier B V All rights reserved
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85.
  • Aaron, F. D., et al. (författare)
  • Measurement of the inclusive e(+/-) p scattering cross section at high inelasticity y and of the structure function F-L
  • 2011
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 71:3
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement is presented of the inclusive neutral current e(+/-) p scattering cross section using data collected by the H1 experiment at HERA during the years 2003 to 2007 with proton beam energies E-p of 920, 575, and 460 GeV. The kinematic range of the measurement covers low absolute four-momentum transfers squared, 1.5 GeV2 < Q(2) < 120 GeV2, small values of Bjorken x, 2.9 . 10(-5) < x < 0.01, and extends to high inelasticity up to y = 0.85. The structure function F-L is measured by combining the new results with previously published H1 data at E-p = 920 GeV and E-p = 820 GeV. The new measurements are used to test several phenomenological and QCD models applicable in this low Q(2) and low x kinematic domain.
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86.
  • Aaron, F. D., et al. (författare)
  • Observation of the hadronic final state charge asymmetry in high Q(2) deep-inelastic scattering at HERA
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 681:2, s. 125-133
  • Tidskriftsartikel (refereegranskat)abstract
    • A first measurement is presented of the charge asymmetry in the hadronic final state from the hard interaction in deep-inelastic ep neutral current scattering at HERA. The measurement is performed in the range of negative squared four momentum transfer 100 < Q(2) < 8000 GeV2. The difference between the event normalised distributions of the scaled momentum, x(p), for positively and negatively charged particles, measured in the current region of the Breit frame, is studied together with its evolution as a function of Q. The results are compared to Monte Carlo models at the hadron and parton levels. (C) 2009 Elsevier B.V. All rights reserved.
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87.
  • Aaron, F. D., et al. (författare)
  • Prompt photons in photoproduction at HERA
  • 2010
  • Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 66:1-2, s. 17-33
  • Tidskriftsartikel (refereegranskat)abstract
    • The production of prompt photons is measured in the photoproduction regime of electron-proton scattering at HERA. The analysis is based on a data sample corresponding to a total integrated luminosity of 340 pb(-1) collected by the H1 experiment. Cross sections are measured for photons with transverse momentum and pseudorapidity in the range 6< E-T(gamma) < 15 GeV and -1.0< eta(gamma) < 2.4, respectively. Cross sections for events with an additional jet are measured as a function of the transverse energy and pseudorapidity of the jet, and as a function of the fractional momenta x(gamma) and x(p) carried by the partons entering the hard scattering process. The correlation between the photon and the jet is also studied. The results are compared with QCD predictions based on the collinear and on the k(T) factorization approaches.
  •  
88.
  • Aaron, F. D., et al. (författare)
  • Search for excited quarks in ep collisions at HERA
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 678:4, s. 335-343
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for excited quarks is performed using the full e(+/-)p data sample collected by the HI experiment at HERA, corresponding to a total integrated luminosity of 475 pb(-1). The electroweak decays of excited quarks q* -> q gamma. q* -> qZ and q* - qW with Subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for first generation excited quark production is found. Mass dependent exclusion limits on q* 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 quark searches. (C) 2009 Elsevier B.V. All rights reserved.
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89.
  • Aaron, F. D., et al. (författare)
  • Search for single top quark production at HERA
  • 2009
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 678:5, s. 450-458
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for single top quark production is performed in the full e(+/-)p data sample collected by the H1 experiment at HERA, corresponding to an integrated luminosity of 474 pb(-1). Decays of top quarks into a b quark and a W boson with Subsequent leptonic or hadronic decay of the W are investigated. A multivariate analysis is performed to discriminate top quark production from Standard Model background processes. An upper limit on the top quark production cross section via flavour changing neutral current processes sigma(ep -> et X) < 0.25 pb is established at 95% CL. Limits on the anomalous coupling K-tu gamma are derived. (C) 2009 Elsevier B.V. All rights reserved.
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90.
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91.
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92.
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93.
  • Huang, C, et al. (författare)
  • The inhomogeneous structure of water at ambient conditions
  • 2009
  • Ingår i: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 106:36, s. 15214-15218
  • Tidskriftsartikel (refereegranskat)abstract
    • Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of approximate to 1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.
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94.
  • Niebuhr, B. B., et al. (författare)
  • Estimating the cumulative impact and zone of influence of anthropogenic features on biodiversity
  • 2023
  • Ingår i: Methods in Ecology and Evolution. - : British Ecological Society. - 2041-210X. ; 14, s. 2362-2375
  • Tidskriftsartikel (refereegranskat)abstract
    • The concept of cumulative impacts is widespread in policy documents, regulations and ecological studies, but quantification methods are still evolving. Infrastructure development usually takes place in landscapes with preexisting anthropogenic features. Typically, their impact is determined by computing the distance to the nearest feature only, thus ignoring the potential cumulative impacts of multiple features. We propose the cumulative ZOI approach to assess whether and to what extent anthropogenic features lead to cumulative impacts. The approach estimates both effect size and zone of influence (ZOI) of anthropogenic features and allows for estimation of cumulative effects of multiple features distributed in the landscape. First, we use simulations and an empirical study to understand under which circumstances cumulative impacts arise. Second, we demonstrate the approach by estimating the cumulative impacts of tourist infrastructure in Norway on the habitat of wild reindeer (Rangifer t. tarandus), a near-threatened species highly sensitive to anthropogenic disturbance. In the simulations, we showed that analyses based on the nearest feature and our cumulative approach are indistinguishable in two extreme cases: when features are few and scattered and their ZOI is small, and when features are clustered and their ZOI is large. The empirical analyses revealed cumulative impacts of private cabins and tourist resorts on reindeer, extending up to 10 and 20 km, with different decaying functions. Although the impact of an isolated private cabin was negligible, the cumulative impact of ‘cabin villages’ could be much larger than that of a single large tourist resort. Focusing on the nearest feature only underestimates the impact of ‘cabin villages’ on reindeer. The suggested approach allows us to quantify the magnitude and spatial extent of cumulative impacts of point, linear, and polygon features in a computationally efficient and flexible way and is implemented in the oneimpact R package. The formal framework offers the possibility to avoid widespread underestimations of anthropogenic impacts in ecological and impact assessment studies and can be applied to a wide range of spatial response variables, including habitat selection, population abundance, species richness and diversity, community dynamics and other ecological processes. © 2023 The Authors. Methods in Ecology and Evolution published by John Wiley & Sons Ltd on behalf of British Ecological Society.
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95.
  • Niebuhr, O., et al. (författare)
  • What makes a charismatic speaker? A computer-based acoustic-prosodic analysis of Steve Jobs tone of voice
  • 2016
  • Ingår i: Computers in Human Behavior. - : Elsevier BV. - 0747-5632. ; 64, s. 366-382
  • Tidskriftsartikel (refereegranskat)abstract
    • The former Apple CEO Steve Jobs was one of the most charismatic speakers of the past decades. However, there is, as yet, no detailed quantitative profile of his way of speaking. We used state-of-the-art computer techniques to acoustically analyze his speech behavior and relate it to reference samples. Our paper provides the first-ever acoustic profile of Steve Jobs, based on about 4000 syllables and 12,000 individual speech sounds from his two most outstanding and well-known product presentations: the introductions of the iPhone 4 and the iPad 2. Our results show that Steve Jobs stands out against our reference samples in almost all key features of charisma, including melody, loudness, tempo and fluency, and he produced significant quantitative differences when addressing customers and investors in his speeches. Against this background, we provide specific advice on how to improve a speaker's charismatic impact. We conclude with describing how further technological advances will move computers and behavioral characteristics like human speech even closer together. © 2016 Elsevier Ltd
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