Zuletzt publiziert

  • Zeitschriftenartikel

    Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses 

    John, R.; Berritta, M.; Hinzke, D.; Müller, C.; Santos, T.; Ulrichs, H.; Nieves, P.; Walowski, J.; Mondal, R.; Chubykalo-Fesenko, O.; et al.
    McCord, J.Oppeneer, P. M.Nowak, U.Münzenberg, M.
    Scientific Reports 2017; 7: Art. 4114
    Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.
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    Nanotip-based photoelectron microgun for ultrafast LEED 

    Storeck, Gero; Vogelgesang, Simon; Sivis, Murat; Schäfer, Sascha; Ropers, Claus
    Structural Dynamics 2017; 4(4): Art. 044024
    We present the design and fabrication of a micrometer-scale electron gun for the implementation of ultrafast low-energy electron diffraction from surfaces. A multi-step process involving photolithography and focused-ion-beam nanostructuring is used to assemble and electrically contact the photoelectron gun, which consists of a nanotip photocathode in a Schottky geometry and an einzel lens for beam collimation. We characterize the low-energy electron pulses by a transient electric field effect and achieve pulse durations of 1.3 ps at an electron energy of 80 eV. First diffraction images in a backscattering geometry (at 50 eV electron energy) are shown.
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    ESTIMATION OF PARAMETERS IN A PLANAR SEGMENT PROCESS WITH A BIOLOGICAL APPLICATION 

    Beneš, Viktor; Večeřa, Jakub; Eltzner, Benjamin; Wollnik, Carina; Rehfeldt, Florian; Králová, Veronika; Huckemann, Stephan
    Image Analysis & Stereology 2017; 36(1) p.25-33
    The paper deals with modeling of segment systems in a bounded planar set (a cell) by means of random segment processes. Two models with a density with respect to the Poisson process are presented. In model I interactions are given by the number of intersections, model II includes the length distribution and takes into account distances from the centre of the cell. The estimation of parameters of the models is suggested based on Takacz-Fiksel method. The method is tested first using simulated data. Further the real data from fluorescence imaging of stress fibres in mesenchymal human stem cells are evaluated. We apply model II which is inhomogeneous. The degree-of-fit testing of the model using various characteristics yields quite satisfactory results.
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    Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser 

    Savelyev, Evgeny; Boll, Rebecca; Bomme, Cédric; Schirmel, Nora; Redlin, Harald; Erk, Benjamin; Düsterer, Stefan; Müller, Erland; Höppner, Hauke; Toleikis, Sven; et al.
    Müller, JostKristin Czwalinna, MarieTreusch, RolfKierspel, ThomasMullins, TerenceTrippel, SebastianWiese, JossKüpper, JochenBrauβe, FelixKrecinic, FarukRouzée, ArnaudRudawski, PiotrJohnsson, PerAmini, KasraLauer, AlexandraBurt, MichaelBrouard, MarkChristensen, LaugeThøgersen, JanStapelfeldt, HenrikBerrah, NoraMüller, MariaUlmer, AnatoliTechert, SimoneRudenko, ArtemRolles, Daniel
    New Journal of Physics 2017; 19(4) p.1-13: Art. 043009
    In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.
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    Measurement of WW/WZ → νqq production with the hadronically decaying boson reconstructed as one or two jets in pp collisions at √ s = 8 TeV with ATLAS, and constraints on anomalous gauge couplings 

    Aaboud, M. et al. (ATLAS COllaboration)
    The European Physical Journal C 2017; 77: Art. 563
    This paper presents a study of the production of WW orWZ boson pairs, with oneW boson decaying to eν or μν and one W or Z boson decaying hadronically. The analysis uses 20.2 fb−1 of √ s = 8TeV pp collision data, collected by the ATLAS detector at the Large Hadron Collider. Crosssections for WW/WZ production are measured in high-pT fiducial regions defined close to the experimental event selection. The cross-section is measured for the case where the hadronically decaying boson is reconstructed as two resolved jets, and the case where it is reconstructed as a single jet. The transverse momentum distribution of the hadronically decaying boson is used to search for new physics. Observations are consistent with the Standard Model predictions, and 95% confidence intervals are calculated for parameters describing anomalous triple gauge-boson couplings.
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    Fiducial, total and differential cross-section measurements of t-channel single top-quark production in pp collisions at 8TeV using data collected by the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 531
    Detailed measurements of t-channel single topquark production are presented. They use 20.2 fb−1 of data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial crosssection is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be σtot(tq) = 56.7+4.3 −3.8 pb for top-quark production and σtot(¯tq) = 32.9+3.0 −2.7 pb for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be Rt = 1.72 ± 0.09. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential crosssections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.
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    Measurements of electroweak W j j production and constraints on anomalous gauge couplings with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77(474)
    Measurements of the electroweak production of a W boson in association with two jets at high dijet invariant mass are performed using √ s = 7 and 8 TeV proton–proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb−1 of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of a W boson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.
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    Jet reconstruction and performance using particle flow with the ATLAS Detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 466
    This paper describes the implementation and performance of a particle flow algorithm applied to 20.2 fb−1 of ATLAS data from 8 TeV proton–proton collisions in Run 1 of the LHC. The algorithm removes calorimeter energy deposits due to charged hadrons from consideration during jet reconstruction, instead using measurements of their momenta from the inner tracker. This improves the accuracy of the charged-hadron measurement, while retaining the calorimeter measurements of neutral-particle energies. The paper places emphasis on how this is achieved, while minimising double-counting of charged-hadron signals between the inner tracker and calorimeter. The performance of particle flow jets, formed from the ensemble of signals from the calorimeter and the inner tracker, is compared to that of jets reconstructed from calorimeter energy deposits alone, demonstrating improvements in resolution and pile-up stability.
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    Search for dark matter at √ s = 13 TeV in final states containing an energetic photon and large missing transverse momentum with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 393
    Results of a search for physics beyond the Standard Model in events containing an energetic photon and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. As the number of events observed in data, corresponding to an integrated luminosity of 36.1 fb−1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, is in agreement with the Standard Model expectations, model-independent limits are set on the fiducial cross section for the production of events in this final state. Exclusion limits are also placed in models where dark-matter candidates are pair-produced. For darkmatter production via an axial-vector or a vector mediator in the s-channel, this search excludes mediator masses below 750–1200 GeV for dark-matter candidate masses below230– 480 GeV at 95% confidence level, depending on the couplings. In an effective theory of dark-matter production, the limits restrict the value of the suppression scale M∗ to be above 790 GeV at 95% confidence level. A limit is also reported on the production of a high-mass scalar resonance by processes beyond the Standard Model, in which the resonance decays to Zγ and the Z boson subsequently decays into neutrinos.
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  • Zeitschriftenartikel

    Measurement of multi-particle azimuthal correlations in pp, p + Pb and low-multiplicity Pb + Pb collisions with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 428
    Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in pp collisions at √ s = 5.02 and 13 TeVand in p + Pb collisions at √ sNN = 5.02 TeV, and compared to the results obtained for low-multiplicity Pb + Pb collisions at √ sNN = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in p + Pb and low-multiplicity Pb + Pb collisions. On the other hand, the pp results for fourparticle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from nonflow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in Pb + Pb, smaller in p + Pb and smallest in pp collisions. The pp results show no dependence on the collision energy, nor on the multiplicity.
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    Measurements of the production cross section of a Z boson in association with jets in pp collisions at √ s = 13 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 361
    Measurements of the production cross section of a Z boson in association with jets in proton–proton collisions at s√=13 TeV are presented, using data corresponding to an integrated luminosity of 3.16 fb−1 collected by the ATLAS experiment at the CERN Large Hadron Collider in 2015. Inclusive and differential cross sections are measured for events containing a Z boson decaying to electrons or muons and produced in association with up to seven jets with pT>30 GeV and |y|<2.5. Predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements for up to two additional partons interfaced with parton shower and fixed-order predictions at next-to-leading order and next-to-next-to-leading order are compared with the measured cross sections. Good agreement within the uncertainties is observed for most of the modelled quantities, in particular with the generators which use next-to-leading-order matrix elements and the more recent next-to-next-to-leading-order fixed-order predictions.
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    Measurement of jet fragmentation in Pb+Pb and pp collisions at √ sNN = 2.76 TeV with the ATLAS detector at the LHC 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 379
    The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb−1 of Pb+Pb data and 4.0 pb−1 of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet pT and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.
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    Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in s√=13 TeV pp collisions with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Journal of High Energy Physics 2017; 8: Art. 6
    A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at s√=13 TeV collected with the ATLAS detector at the LHC in 2015–2016, corresponding to an integrated luminosity of 36.1 fb−1. No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks (t~1) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs (t~2) decaying via t~2→Zt~1 or t~2→ht~1. The results exclude at 95% confidence level t~2 and t~1 masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.
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    Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at s√=8 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Journal of High Energy Physics 2017; 4: Art. 124
    To probe the W tb vertex structure, top-quark and W -boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb−1, recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling gR are also set from model-independent measurements.
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    Measurement of charged-particle distributions sensitive to the underlying event in s√=13 TeV proton-proton collisions with the ATLAS detector at the LHC 

    Aaboud, M. et al. (ATLAS Collaboration)
    Journal of High Energy Physics 2017; 3: Art. 157
    We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb−1. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.
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    Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media 

    Risch, Marcel
    Catalysts 2017; 7(5): Art. 154
    Oxygen reduction is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in fuel cells and metal-air batteries. In this review, the prospect of perovskite oxides for the oxygen reduction reaction (ORR) in alkaline media is reviewed with respect to fundamental insight into activity and possible mechanisms. For gaining these insights, special emphasis is placed on highly crystalline perovskite films that have only recently become available for electrochemical interrogation. The prospects for applications are evaluated based on recent progress in the synthesis of perovskite nanoparticles. The review concludes with the current understanding of oxygen reduction on perovskite oxides and a perspective on opportunities for future fundamental and applied research.
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    Measurement of W±W± vector-boson scattering and limits on anomalous quartic gauge couplings with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Physical Review D
    This paper presents the extended results of measurements of W±W±jj production and limits on anomalous quartic gauge couplings using 20.3  fb−1 of proton–proton collision data at √s=8  TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two leptons (e or μ) with the same electric charge and at least two jets are analyzed. Production cross sections are determined in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. An additional fiducial region, particularly sensitive to anomalous quartic gauge coupling parameters α4 and α5, is introduced, which allows more stringent limits on these parameters compared to the previous ATLAS measurement.
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    Measurements of integrated and differential cross sections for isolated photon pair production in pp collisions at √s=8  TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Physical Review D 2017; 95: Art. 112005
    A measurement of the production cross section for two isolated photons in proton-proton collisions at a center-of-mass energy of √s=8  TeV is presented. The results are based on an integrated luminosity of 20.2  fb−1 recorded by the ATLAS detector at the Large Hadron Collider. The measurement considers photons with pseudorapidities satisfying |ηγ|<1.37 or 1.56<|ηγ|<2.37 and transverse energies of respectively EγT,1>40  GeV and EγT,2>30  GeV for the two leading photons ordered in transverse energy produced in the interaction. The background due to hadronic jets and electrons is subtracted using data-driven techniques. The fiducial cross sections are corrected for detector effects and measured differentially as a function of six kinematic observables. The measured cross section integrated within the fiducial volume is 16.8±0.8  pb. The data are compared to fixed-order QCD calculations at next-to-leading-order and next-to-next-to-leading-order accuracy as well as next-to-leading-order computations including resummation of initial-state gluon radiation at next-to-next-to-leading logarithm or matched to a parton shower, with relative uncertainties varying from 5% to 20%.
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    Measurements of long-range azimuthal anisotropies and associated Fourier coefficients for pp collisions at √s=5.02 and 13 TeV and p+Pb collisions at √sNN=5.02 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Physical Review C 2017; 96: Art. 024908
    ATLAS measurements of two-particle correlations are presented for √s=5.02 and 13 TeV pp collisions and for √sNN=5.02 TeV p+Pb collisions at the LHC. The correlation functions are measured as a function of relative azimuthal angle Δϕ, and pseudorapidity separation Δη, using charged particles detected within the pseudorapidity interval |η|<2.5. Azimuthal modulation in the long-range component of the correlation function, with |Δη|>2, is studied using a template fitting procedure to remove a “back-to-back” contribution to the correlation function that primarily arises from hard-scattering processes. In addition to the elliptic, cos(2Δϕ), modulation observed in a previous measurement, the pp correlation functions exhibit significant cos(3Δϕ) and cos(4Δϕ) modulation. The Fourier coefficients vn,n associated with the cos(nΔϕ) modulation of the correlation functions for n=2–4 are measured as a function of charged-particle multiplicity and charged-particle transverse momentum. The Fourier coefficients are observed to be compatible with cos(nϕ) modulation of per-event single-particle azimuthal angle distributions. The single-particle Fourier coefficients vn are measured as a function of charged-particle multiplicity, and charged-particle transverse momentum for n=2–4. The integrated luminosities used in this analysis are, 64 nb−1 for the √s=13 TeV pp data, 170 nb−1 for the √s=5.02 TeV pp data, and 28 nb−1 for the √sNN=5.02 TeV p+Pb data.
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    Search for the Dimuon Decay of the Higgs Boson in pp Collisions at √s=13  TeV with the ATLAS Detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Physical Review Letters 2017; 119: Art. 051802
    A search for the dimuon decay of the Higgs boson was performed using data corresponding to an integrated luminosity of 36.1  fb−1 collected with the ATLAS detector in pp collisions at √s=13  TeV at the Large Hadron Collider. No significant excess is observed above the expected background. The observed (expected) upper limit on the cross section times branching ratio is 3.0 (3.1) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125 GeV. When combined with the pp collision data at √s=7  TeV and √s=8  TeV, the observed (expected) upper limit is 2.8 (2.9) times the Standard Model prediction.
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