1,721,099 research outputs found
The pion vector form factor from lattice QCD and NNLO chiral perturbation theory
No full textWe present a comprehensive study of the electromagnetic form factor, the decay constant and the mass of the pion computed in lattice QCD with two degenerate O(a)-improved Wilson quarks at three different lattice spacings in the range 0.05-0.08fm and pion masses between 280 and 630MeV at mπ L ≥ 4. Using partially twisted boundary conditions and stochastic estimators, we obtain a dense set of precise data points for the form factor at very small momentum transfers, allowing for a model-independent extraction of the charge radius. Chiral Perturbation Theory (ChPT) augmented by terms which model lattice artefacts is then compared to the data. At next-to-leading order the effective theory fails to produce a consistent description of the full set of pion observables but describes the data well when only the decay constant and mass are considered. By contrast, using the next-to-next-to-leading order expressions to perform global fits result in a consistent description of all data. We obtain ⟨r2π⟩ = 0.481(33)(13)fm2 as our final result for the charge radius at the physical point. Our calculation also yields estimates for the pion decay constant in the chiral limit, Fπ/F = 1.080(16)(6), the quark condensate, Σ1/3MSbar (2GeV) = 261(13)(1)MeV and several low-energy constants of SU(2) ChPT
Towards a precise lattice determination of the leading hadronic contribution to (g-2)_mu
Full text linkWe report on our computation of the leading hadronic contribution to the anomalous magnetic moment of the muon using two dynamical flavours of non-perturbatively O(a) improved Wilson fermions. The strange quark is introduced in the quenched approximation. Partially twisted boundary conditions are applied to improve the momentum resolution in the relevant integral. Our results, obtained at three different values of the lattice spacing, allow for a preliminary study of discretization effects. We explore a wide range of lattice volumes, namely 2 fm < L < 3 fm, with pion masses from 600 to 280 MeV and discuss different chiral extrapolations to the physical point. We observe a non-trivial dependence of a_mu(HLO) on m_pi especially for small pion masses. The final result, a_mu(HLO)=618(64)*10^(-10), is obtained by considering only the quark connected contribution to the vacuum polarization. We present a detailed analysis of systematic errors and discuss how they can be reduced in future simulations.We report on our computation of the leading hadronic contribution to the anomalous magnetic moment of the muon using two dynamical flavours of non-perturbatively O(a) improved Wilson fermions. The strange quark is introduced in the quenched approximation. Partially twisted boundary conditions are applied to improve the momentum resolution in the relevant integral. Our results, obtained at three different values of the lattice spacing, allow for a preliminary study of discretization effects. We explore a wide range of lattice volumes, namely 2 fm < L < 3 fm, with pion masses from 600 to 280 MeV and discuss different chiral extrapolations to the physical point. We observe a non-trivial dependence of a_mu(HLO) on m_pi especially for small pion masses. The final result, a_mu(HLO)=618(64)*10^(-10), is obtained by considering only the quark connected contribution to the vacuum polarization. We present a detailed analysis of systematic errors and discuss
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The hadronic contribution to the running of the electromagnetic coupling and the electroweak mixing angle
Full text linkThe electromagnetic coupling and the electroweak mixing angle are two quantities used in precision SM tests and probes of new physics. However, their dependence on the energy suffers from uncertainties at low momenta, induced by hadronic effects.
In this work, we study the latter in the space-like energy range 0-10 GeV squared, employing a lattice regularisation of isosymmetric QCD, which allows us to access the confining regime of the strong coupling constant.
One may compute the hadronic contribution to the electromagnetic coupling and the electroweak mixing angle from vacuum polarisation functions. For the first case, we use two electromagnetic currents, while for the second, we require the mixing of an electromagnetic current with the vector component of a weak neutral current. We employ the time-momentum representation to compute both vacuum polarisation functions.
We use a set of seventeen Monte Carlo simulations based on CLS ensembles with 2+1 flavours, order-a-improved Wilson fermion action and tree-level improved Lüscher-Weisz gauge action. The simulations include the charm quark at the quenched level, and we include both quark-connected and quark-disconnected diagrams.
Our simulations possess pion masses that span from 420 MeV to 130 MeV, slightly below the physical pion mass, and include four different lattice spacings, 0.086 fm, 0.076 fm, 0.064 fm and 0.050 fm. This wide set allows us to combine extrapolating to the continuum limit and interpolating the physical pion and kaon masses in a single, correlated fit.
Since improvement and renormalisation mix the various contributions to the vacuum polarisation function to order a, we rearrange the flavour components in a SU(3)-flavour basis.
Autocorrelations in our two-point functions are studied and removed using a combination of binning and Ulli Wolff's Gamma-method, and we employ bootstrap sampling to carry the statistical uncertainty and data correlations through the entire analysis.
The signal-to-noise ratio is improved using a bounding procedure, which we compare with the more straightforward single-exponential fit and find the former more reliable for our ensembles with lighter pion mass.
We remove the lattice infrared cut-off ---the simulation spatial size--- adding the difference between finite and infinite volumes, which we estimate using two different methods, the so-called Hansen-Patella and Meyer-Lellouch-Lüscher procedures. We find both methods to give compatible results. Finally, we cross-check the finite-volume correction by comparing ensembles with the same simulation parameters but different volumes.
We perform a correlated extrapolation to the physical point of the various components for a dense set of momenta and establish that below 7 GeV squared our fit functions describe the lattice artefacts correctly.
We give the running with the energy of the electromagnetic coupling and the electroweak mixing angle using Padé approximants.xi, 176, 2 Seiten, Diagramm
Electromagnetic form factors and radii of the nucleon from Lattice QCD and the proton radius puzzles
No full textIn this thesis, we investigate the electromagnetic form factors of the proton and neutron in the framework of Quantum Chromodynamics (QCD). To perform a calculation of such low-energy quantities based on first principles, we employ the lattice regularization of QCD. In Lattice-QCD simulations of nucleon-structure observables, systematic errors are inherent due to the finite lattice spacing and volume as well as due to contamination by excited states. These can be controlled and removed in a systematic fashion: for the removal of excited-state contributions, a variety of dedicated methods exists, while discretization and finite-volume effects also need to be taken into account by performing a continuum and infinite-volume extrapolation. However, all previous lattice studies of the electromagnetic form factors of the nucleon have either neglected the numerically challenging quark-disconnected contributions or were not extrapolated to the continuum and infinite-volume limit.
We present results for the electromagnetic form factors of the proton and neutron computed on the (2 + 1)-flavor Coordinated Lattice Simulations (CLS) ensembles including both quark-connected and -disconnected contributions while, at the same time, controlling all sources of systematic uncertainties. For the excited-state analysis, we explore three complementary methods based on two-state fits to the effective form factors and on two different truncations of the summation method, respectively. The Q^2-, pion-mass, lattice-spacing and finite-volume dependence of our form factor data is fitted simultaneously to the expressions resulting from covariant baryon chiral perturbation theory including vector mesons amended by models for lattice artefacts. From these fits, we determine the electric, magnetic, Zemach and Friar radii as well as the magnetic moments of the proton and neutron. To assess the influence of systematic effects, we average over various cuts in the pion mass and the momentum transfer, as well as over different models for the lattice-spacing and finite-volume dependence, using weights derived from the Akaike Information Criterion (AIC).
Our ab-initio QCD results for the electromagnetic radii of the proton are of particular relevance in light of the so-called proton radius puzzle, i.e., the observation of a large tension between different experimental measurements of the proton's electric radius, which is after more than a decade of vigorous research still not completely explained. Also for the magnetic radius, analyses based on different data sets find discrepant results. In this situation, a firm theoretical prediction of the proton radii can contribute towards the clarification of the origins of the discrepancies.
Our results for the magnetic moments of the proton and neutron are in good agreement with the experimentally very precisely known values. For the radii of the proton, we achieve, including all systematic errors, a precision which enables a meaningful comparison to the various experiments and data-driven evaluations. On the one hand, our result for the electric radius of the proton clearly points towards a small value, as favored by muonic hydrogen spectroscopy, the recent ep-scattering experiment by PRad and data-driven dispersive analyses. Our estimate for the magnetic radius, on the other hand, is well compatible with that inferred from the A1 ep-scattering experiment by a z-expansion analysis and in tension with z-expansion on the remaining world data as well as with dispersive approaches.In dieser Doktorarbeit werden die elektromagnetischen Formfaktoren des Protons und Neutrons im Rahmen der Quantenchromodynamik (QCD) untersucht. Um solche Niederenergiegrößen basierend auf ersten Prinzipien zu berechnen, wird die Gitterregularisierung der QCD verwendet. In Gitter-QCD-Simulationen von Nukleonstruktur-Observablen sind systematische Fehler inhärent aufgrund des endlichen Gitterabstands und -volumens sowie aufgrund der Kontamination durch angeregte Zustände. Diese können kontrolliert und auf systematische Weise entfernt werden: Für die Entfernung der Beiträge angeregter Zustände existieren eine Reihe dedizierter Methoden, während Diskretisierungs- und Endlich-Volumen-Effekte ebenfalls berücksichtigt werden müssen, indem man eine Extrapolation ins Kontinuum und zu unendlichem Volumen durchführt. Jedoch haben alle bisherigen Gitterstudien der elektromagnetischen Formfaktoren des Nukleons entweder die numerisch herausfordernden quark-unverbundenen Beiträge vernachlässigt oder waren nicht zum Kontinuums- und Unendlich-Volumen-Limes extrapoliert.
Wir präsentieren Resultate für die elektromagnetischen Formfaktoren des Protons und Neutrons, die auf den (2 + 1)-flavour Coordinated Lattice Simulations (CLS) Ensembles berechnet wurden. Sie berücksichtigen sowohl quark-verbundene als auch -unverbundene Beiträge und kontrollieren gleichzeitig alle Quellen systematischer Unsicherheiten. Für die Analyse angeregter Zustände werden drei komplementäre Methoden erforscht, die auf Zweizustandsfits an die effektiven Formfaktoren bzw. auf zwei verschiedenen Trunkierungen der Summationsmethode basieren. Die Q^2-, Pionmassen-, Gitterabstands- und Volumen-Abhängigkeit unserer Formfaktor-Daten wird simultan an die Ausdrücke gefittet, die aus kovarianter baryonischer chiraler Störungstheorie inklusive Vektormesonen resultieren, und die um Modelle für Gitterartefakte ergänzt werden. Mithilfe dieser Fits werden die elektrischen, magnetischen, Zemach- und Friar-Radien sowie die magnetischen Momente des Protons und Neutrons bestimmt. Um die Einflüsse systematischer Effekte zu beurteilen, wird über diverse Cuts in der Pionmasse und im Impulsübertrag sowie über verschiedene Modelle für die Gitterabstands- und Volumen-Abhängigkeit gemittelt. Hierzu werden Gewichte verwendet, die aus dem Akaike-Informationskriterium (AIC) hergeleitet sind.
Unsere ab-initio-Resultate für die elektromagnetischen Radien des Protons sind von besonderer Bedeutung im Lichte des sogenannten Proton-Radius-Puzzles, d.h. der großen Spannung zwischen verschiedenen experimentellen Messungen des elektrischen Radius des Protons, die nach mehr als einer Dekade intensiver Forschung immer noch nicht vollständig erklärt ist. Auch für den magnetischen Radius finden Analysen basierend auf verschiedenen Datensätzen diskrepante Ergebnisse. In dieser Situation kann eine solide theoretische Vorhersage der Protonradien dazu beitragen, die Ursprünge der Diskrepanzen zu klären.
Unsere Ergebnisse für die magnetischen Momente des Protons und Neutrons befinden sich in guter Übereinstimmung mit den experimentell sehr genau bekannten Werten. Für die Radien des Protons erreichen wir inklusive aller systematischer Fehler eine Präzision, die einen aussagekräftigen Vergleich mit den diversen Experimenten und datengetriebenen Bestimmungen erlaubt. Einerseits deutet unser Resultat für den elektrischen Radius des Protons klar auf einen kleinen Wert hin, wie er von der Spektroskopie myonischen Wasserstoffs, dem kürzlich erfolgten ep-Streuexperiment von PRad und datengetriebenen dispersiven Analysen bevorzugt wird. Andererseits ist unser Ergebnis für den magnetischen Radius gut kompatibel mit dem mithilfe einer z-expansion-Analyse aus dem A1 ep-Streuexperiment abgeleiteten Wert und in Spannung mit z-expansion auf den verbleibenden Welt-Daten sowie mit dispersiven Ansätzen.viii, 177, 2 Seiten ; Illustrationen, Diagramm
Isospin breaking effects in hadronic matrix elements on the lattice
No full textIn this thesis, we investigate the consequences of isospin symmetry breaking on hadronic matrix elements based on combined calculations in Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED). We employ the lattice regularisation for quantum field theories, which enables the access of both the perturbative and the non-perturbative regime of a coupling constant. The inclusion of isospin-breaking corrections in lattice QCD calculations is required to improve the theoretical prediction of hadronic contributions to high-precision observables, such as the muon anomalous magnetic moment and the running of the fine-structure constant , which are determined from correlation functions of electromagnetic currents. We relate QCD+QED to isosymmetric QCD via Monte Carlo reweighting and formulate a perturbative expansion of correlation functions around isosymmetric QCD. Expansion parameters are the differences of the bare quark masses and their isosymmetric counterparts, a shift of the inverse strong coupling and the electromagnetic coupling. Expectation values with respect to isosymmetric QCD are evaluated by means of Monte Carlo simulations based on gauge ensembles generated with a -improved Wilson fermion action and tree-level improved L{\"u}scher-Weisz gauge action. The handling of the manifest infrared divergence of non-compact lattice QED is addressed in this thesis. We regularise the divergence using the QED prescription. We derive the lattice photon propagator for open and periodic temporal boundary conditions in Feynman and Coulomb gauge. The perturbative expansion is truncated at leading order, so that the electromagnetic coupling does not renormalise. Further expansion parameters are fixed via a hadronic renormalisation scheme based on masses of pseudo-scalar mesons. In our calculation, we only consider isospin breaking effects in the valence quark sector and focus on quark-connected contributions. We derive the axial and vector Ward identity for continuum QCD+QED. A conserved vector current is deduced from the vector Ward identity for the particular lattice regularisation of QCD+QED considered in this thesis. We determine the masses of pseudo-scalar octet mesons including leading isospin breaking corrections and describe the application of the procedure to baryon masses. We provide a detailed discussion of the renormalisation of the local vector current in QCD+QED taking operator mixing into account. We further determine the leading isospin breaking correction for the renormalised HVP function, which is related to the hadronic contribution of the running of the of the fine-structure constant and the anomalous magnetic moment of leptons. The presented formalism is finally used to compute the hadronic vacuum polarisation contribution to the anomalous magnetic moment of the muon including leading isospin breaking corrections. The knowledge of a precise Standard Model prediction for the latter is of particular interest, as it offers a probe for new physics. The calculations are performed on three Monte Carlo ensembles with pion masses of , and MeV and lattice spacings of , and fm
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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