Zuletzt publiziert

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    Contribution of Jahn-Teller and charge transfer excitations to the photovoltaic effect of manganite/titanite heterojunctions 

    Ifland, Benedikt; Hoffmann, Joerg; Kressdorf, Birte; Roddatis, Vladimir; Seibt, Michael; Jooss, Christian
    New Journal of Physics 2017; 19(6): Art. 063046
    The effect of correlation effects on photovoltaic energy conversion at manganite/titanite heterojunctions is investigated. As a model system we choose a heterostructure consisting of the small polaron absorber Pr0.66Ca0.34MnO3 (PCMO) epitaxially grown on single-crystalline Nb-doped SrTi0.998Nb0.002O3 (STNO) substrates. The high structural and chemical quality of the interfaces is proved by detailed characterization using high-resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) studies. Spectrally resolved and temperature-dependent photovoltaic measurements show pronounced contributions of both the Jahn-Teller (JT) excitations and the charge transfer (CT) transitions to the photovoltaic effect at different photon energies. A linear temperature dependence of the open-circuit voltage for an excitation in thePCMOmanganite is only observed below the charge-ordering temperature, indicating that the diffusion length of the photocarrier exceeds the size of the space charge region. The photovoltaic response is compared to that of a heterojunction of lightly doped Pr0.05Ca0.95MnO3 (CMO)/STNO, where the JT transition is absent. Here, significant contributions of the CTtransition to the photovoltaic effect set in below the Neel temperature.Weconclude that polaronic correlations and ordering effects are essentials for photovoltaic energy conversion in manganites.
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    Sustainability, collapse and oscillations in a simple World-Earth model 

    Nitzbon, Jan; Heitzig, Jobst; Parlitz, Ulrich
    Environmental Research Letters 2017; 12(7): Art. 074020
    The Anthropocene is characterized by close interdependencies between the natural Earth system and the global human society, posing novel challenges to model development. Here we present a conceptual model describing the long-term co-evolution of natural and socio-economic subsystems of Earth. While the climate is represented via a global carbon cycle, we use economic concepts to model socio-metabolic flows of biomass and fossil fuels between nature and society. A well-being-dependent parametrization of fertility and mortality governs human population dynamics. Our analysis focuses on assessing possible asymptotic states of the Earth system for a qualitative understanding of its complex dynamics rather than quantitative predictions. Low dimension and simple equations enable a parameter-space analysis allowing us to identify preconditions of several asymptotic states and hence fates of humanity and planet. These include a sustainable co-evolution of nature and society, a global collapse and everlasting oscillations. We consider different scenarios corresponding to different socio-cultural stages of human history. The necessity of accounting for the ‘human factor’ in Earth system models is highlighted by the finding that carbon stocks during the past centuries evolved opposing to what would ‘naturally’ be expected on a planet without humans. The intensity of biomass use and the contribution of ecosystem services to human well-being are found to be crucial determinants of the asymptotic state in a (pre-industrial) biomass-only scenario without capital accumulation. The capitalistic, fossil-based scenario reveals that trajectories with fundamentally different asymptotic states might still be almost indistinguishable during even a centuries-long transient phase. Given current human population levels, our study also supports the claim that besides reducing the global demand for energy, only the extensive use of renewable energies may pave the way into a sustainable future.
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    Measurement of the cross section for inclusive isolated-photon production in ppcollisions at √s=13TeVusing the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Physics Letters B p.473-493
    Inclusive isolated-photon production in ppcollisions at a centre-of-mass energy of 13TeVis studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2fb−1. The cross section is measured as a function of the photon transverse energy above 125GeVin different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.
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    Measurements of top quark spin observables in tt¯ events using dilepton final states in s√=8 TeV pp collisions with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Journal of High Energy Physics 2017; 2017: Art. 113
    Measurements of top quark spin observables in tt¯ events are presented based on 20.2 fb−1 of s√=8 TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of tt¯ production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels. The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.
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    Measurements of top-quark pair differential cross-sections in the eμ channel in pp collisions at √ s = 13 TeV using the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 292
    This article presents measurements of t ¯t differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb−1 of proton–proton data at a centre-of-mass energy of √s = 13 TeV recorded by the ATLAS experiment at the LHC in 2015. Differential crosssections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the t ¯t system. The t ¯t events are selected by requiring one electron and onemuonof opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box + Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the t ¯t system.
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    Measurement of the prompt J/ψ pair production cross-section in pp collisions at √ s = 8 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 76
    The production of two prompt J/ψ mesons, each with transverse momenta pT > 8.5 GeV and rapidity |y| < 2√.1, is studied using a sample of proton-proton collisions at s = 8 TeV, corresponding to an integrated luminosity of 11.4 fb−1 collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised J/ψ production, is measured as a function of the transverse momentum of the lower-pT J/ψ meson, di-J/ψ pT and mass, the difference in rapidity between the two J/ψ mesons, and the azimuthal angle between the two J/ψ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/ψ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be σeff = 6.3 ± 1.6(stat) ± 1.0(syst) mb.
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    Precision measurement and interpretation of inclusive W+, W− and Z/γ ∗ production cross sections with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 367
    High-precision measurements by the ATLAS Collaboration are presented of inclusive W+ → + ν, W− → −¯ν and Z/γ ∗ → ( = e,μ) Drell–Yan production cross sections at the LHC. The data were collected in proton–proton collisions at √ s = 7 TeV with an integrated luminosity of 4.6 fb−1. Differential W+ and W− cross sections are measured in a lepton pseudorapidity range |η | < 2.5. Differential Z/γ ∗ cross sections are measured as a function of the absolute dilepton rapidity, for |y | < 3.6, for three intervals of dilepton mass, m , extending from 46 to 150 GeV. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive e± p scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element |Vcs | is also provided.
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    Measurement of the W boson polarisation in t ¯t events from pp collisions at √ s = 8 TeV in the lepton + jets channel with ATLAS 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 264
    This paper presents a measurement of the polarisation of W bosons from t ¯t decays, reconstructed in events with one high-pT lepton and at least four jets. Data from pp collisions at the LHC were collected at √s = 8 TeV and correspond to an integrated luminosity of 20.2 fb−1. The angle θ ∗ between the b-quark from the top quark decay and a direct W boson decay product in theW boson rest frame is sensitive to theW boson polarisation. Two differentW decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos θ ∗ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F0 = 0.709 ± 0.019, FL = 0.299 ± 0.015 and FR = −0.008 ± 0.014, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.
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    Electron efficiency measurements with the ATLAS detector using 2012 LHC proton–proton collision data 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 195
    This paper describes the algorithms for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC). These algorithms were used for all ATLAS results with electrons in the final state that are based on the 2012 pp collision data produced by the LHC at s√ = 8 TeV. The efficiency of these algorithms, together with the charge misidentification rate, is measured in data and evaluated in simulated samples using electrons from Z→ee, Z→eeγ and J/ψ→ee decays. For these efficiency measurements, the full recorded data set, corresponding to an integrated luminosity of 20.3 fb−1, is used. Based on a new reconstruction algorithm used in 2012, the electron reconstruction efficiency is 97% for electrons with ET=15 GeV and 99% at ET=50 GeV. Combining this with the efficiency of additional selection criteria to reject electrons from background processes or misidentified hadrons, the efficiency to reconstruct and identify electrons at the ATLAS experiment varies from 65 to 95%, depending on the transverse momentum of the electron and background rejection.
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    Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 332
    This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of √s = 8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30 μm is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20 μm and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.
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    Performance of the ATLAS trigger system in 2015 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 317
    During 2015 the ATLAS experiment recorded 3.8 fb−1 of proton–proton collision data at a centre-of-mass energy of 13 TeV. The ATLAS trigger system is a crucial component of the experiment, responsible for selecting events of interest at a recording rate of approximately 1 kHz from up to 40 MHz of collisions. This paper presents a short overview of the changes to the trigger and data acquisition systems during the first long shutdown of the LHCand shows the performance of the trigger system and its components based on the 2015 proton–proton collision data.
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    Measurement of jet activity produced in top-quark events with an electron, a muon and two b-tagged jets in the final state 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. 220
    Measurements of jet activity in top-quark pair events produced in proton–proton collisions are presented, using 3.2 fb−1 of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an oppositecharge eμ pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, pT. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the pT threshold for additional jets, and is also presented for different invariant mass regions of the eμb¯b system.All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.
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    Search for supersymmetry in a final state containing two photons and missing transverse momentum in √ s = 13 TeV pp collisions at the LHC using the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 76: Art. 517
    A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of 3.2 fb−1 of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be 0.27+0.22 −0.10 events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.
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    Measurement of forward-backward multiplicity correlations in lead-lead, proton-lead, and proton-proton collisions with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    PHYSICAL REVIEW C 2017; 95(6): Art. 064914
    Two-particle pseudorapidity correlations are measured in √sNN=2.76TeVPb+Pb, √sNN=5.02TeVp+Pb, and √s=13TeVpp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7μb−1, 28 nb−1, and 65 nb−1, respectively. The correlation function CN(η1,η2) is measured as a function of event multiplicity using charged particles in the pseudorapidity range |η|<2.4. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by 1+⟨a21⟩1/2η1η2 in all collision systems over the full multiplicity range. The values of ⟨a21⟩1/2 are consistent for the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of ⟨a21⟩1/2 and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given η than in pp collisions with comparable multiplicity.
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    Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1 

    Aad, G. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77 p.1-73: Art. 490
    The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a wellperforming calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.
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    Working Memory Requires a Combination of Transient and Attractor-Dominated Dynamics to Process Unreliably Timed Inputs. 

    Nachstedt, Timo; Tetzlaff, Christian
    Scientific reports 2017-05-30; 7(1): Art. 2473
    Working memory stores and processes information received as a stream of continuously incoming stimuli. This requires accurate sequencing and it remains puzzling how this can be reliably achieved by the neuronal system as our perceptual inputs show a high degree of temporal variability. One hypothesis is that accurate timing is achieved by purely transient neuronal dynamics; by contrast a second hypothesis states that the underlying network dynamics are dominated by attractor states. In this study, we resolve this contradiction by theoretically investigating the performance of the system using stimuli with differently accurate timing. Interestingly, only the combination of attractor and transient dynamics enables the network to perform with a low error rate. Further analysis reveals that the transient dynamics of the system are used to process information, while the attractor states store it. The interaction between both types of dynamics yields experimentally testable predictions and we show that this way the system can reliably interact with a timing-unreliable Hebbian-network representing long-term memory. Thus, this study provides a potential solution to the long-standing problem of the basic neuronal dynamics underlying working memory.
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    Anisotropic x-ray scattering and orientation fields in cardiac tissue cells 

    Bernhardt, M.; Nicolas, J.-D.; Eckermann, M.; Eltzner, B.; Rehfeldt, F.; Salditt, T.
    New Journal of Physics 2017; 19(1): Art. 013012
    X-ray diffraction from biomolecular assemblies is a powerful technique which can provide structural information about complex architectures such as the locomotor systems underlying muscle contraction. However, in its conventional form, macromolecular diffraction averages over large ensembles. Progress in x-ray optics has now enabled to probe structures on sub-cellular scales, with the beam confined to a distinct organelle. Here, we use scanning small angle x-ray scattering (scanning SAXS) to probe the diffraction from cytoskeleton networks in cardiac tissue cells. In particular, we focus on actin-myosin composites, which we identify as the dominating contribution to the anisotropic diffraction patterns, by correlation with optical fluorescence microscopy. To this end, we use a principal component analysis approach to quantify direction, degree of orientation, nematic order, and the second moment of the scattering distribution in each scan point.Wecompare the fiber orientation from micrographs of fluorescently labeled actin fibers to the structure orientation of the x-ray dataset and thus correlate signals of two different measurements: the native electron density distribution of the local probing area versus specifically labeled constituents of the sample. Further, we develop a robust and automated fitting approach based on a power law expansion, in order to describe the local structure factor in each scan point over a broad range of the momentum transfer qr. Finally, we demonstrate how the methodology shown for freeze dried cells in the first part of the paper can be translated to alive cell recordings.
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    Search for triboson W±W±W∓ production in pp collisions at s√=8 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 141
    This paper reports a search for triboson W±W±W∓ production in two decay channels (W±W±W∓→ℓ±νℓ±νℓ∓ν and W±W±W∓→ℓ±νℓ±νjj with ℓ=e,μ) in proton-proton collision data corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of 8 TeV with the ATLAS detector at the Large Hadron Collider. Events with exactly three charged leptons, or two leptons with the same electric charge in association with two jets, are selected. The total number of events observed in data is consistent with the Standard Model (SM) predictions. The observed 95% confidence level upper limit on the SM W±W±W∓ production cross section is found to be 730 fb with an expected limit of 560 fb in the absence of SM W±W±W∓ production. Limits are also set on WWWW anomalous quartic gauge couplings.
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    Performance of algorithms that reconstruct missing transverse momentum in s√ = 8 TeV proton–proton collisions in the ATLAS detector 

    Aad, G. et al. (ATLAS Collaboration)
    The European Physical Journal C 2017; 77: Art. 241
    The reconstruction and calibration algorithms used to calculate missing transverse momentum (Emiss T ) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton–proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the Emiss T reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton–proton collisions at a centre-of-mass energy of 8 TeV during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3 fb−1. The simulation and modelling of Emiss T in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for Emiss T , and estimates of the systematic uncertainties in the Emiss T measurements are presented.
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    Measurement of the inclusive cross-sections of single top-quark and top-antiquark t-channel production in pp collisions at s√=13 TeV with the ATLAS detector 

    Aaboud, M. et al. (ATLAS Collaboration)
    Journal of High Energy Physics: Art. 86
    A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be σ(tq) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and σ(t¯q)=91±4 (stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be Rt=σ(tq)/σ(t¯q)=1.72±0.09 (stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.
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