Electromagnetic Properties of Baryons

Static observables of bound state systems in field theoretic descriptions are usually extracted from form factors in the limit of vanishing squared four-momentum transfer of the probing exchange particle. On the other hand, static properties in nonrelativistic quantum mechanics can be formulated by means of expectation values involving essentially scalar products of wave functions. The main objective of this work is to show that a synthesis of both approaches is indeed possible - at least if certain restrictions are made to the kind of interactions between the constituents of the bound system - leading to new insights into the structure of static properties. The focus lies especially on the charge radii and magnetic moments of baryons described within a covariant constituent quark model having its field theoretic foundations in the Bethe-Salpeter equation. The results can be generalized to other three-fermion systems in an obvious way. The current matrix element in the Breit frame between the vertex functions is derived, which is the basis of the following calculations. The charge radius and magnetic moment of a bound three-fermion system is then derived by starting from their usual definition from form factors and in case of the charge radius also from the well-known radius of a charge distribution in classical electrodynamics. In both cases the static limit at the photon point is taken analytically and subsequently the integration over the relative energy variables is done. Finally the vertex functions are replaced by Salpeter amplitudes and the expression is symmetrized over the three fermions. The final results express the charge radius and magnetic moment of the three-fermion system as expectation values with respect to Salpeter amplitudes. The resulting operators in both cases show an interesting structure: The center of mass motion of the system is accounted for by the subtraction of the relativistic center of mass. Besides a relativistic weight factor the expressions resemble their nonrelativistic counterparts with the particle masses replaced by the relativistic single-particle energies. In addition, the magnetic moment operator allows for a decomposition of the total magnetic moment into contributions from the fermions intrinsic spins and their orbital angular momenta - a result which is hardly to obtain in a form factor calculation. The numerical implementation of the analytic results is done within a covariant constituent quark model with quark confinement and a residual instanton interaction accounting for the fine structure of the observed mass spectra. The Salpeter amplitudes which where obtained by solving the Salpeter equation are used to compute the expectation values of the charge radius and magnetic moment operators. The charge radii and magnetic moments of the baryon octet are thus obtained The proton charge radius is in good agreement with experiment whereas the neutron radius is much too large. A study of the dependence of these quantities on the particular choices of the instanton force parameters reveals a strong variation of the neutron radius with the instanton range parameter. The dependence of the proton radius in contrast is quite moderate and the spectra are hardly affected by a variation, which allows to propose an adapted parameter set describing both radii equally well. The octet magnetic moments are nicely reproduced with an average accuracy of roughly ten percent. A study on the dependence of the contributions of quark spins and orbital angular momenta to the total magnetic moment on the constituent quark mass reveals that at a quark mass of 330 MeV (which is the usual value in the model) roughly ninety percent are due to spin. Decreasing the quark mass, while keeping the ground state masses fixed at the empirical values lowers the spin contribution to roughly sixty percent at a quark mass of 25 MeV. The calculated values for the hyperon radii and magnetic moments of selected exited states as well as mostly the baryon decuplet are predictions. There are however ongoing experiments to measure the magnetic moments of the Δ+ and the S11(1535) resonance. The method presented in this work to compute static properties of three-fermion systems gives new insight into the nature of such observables. When applied in the context of a covariant constituent quark model it allows for unprecedented studies, not least because of a much reduced computation time as compared to form factor calculations and an improved numerical stability. The quark model that was used to conduct those studies describes baryon charge radii and magnetic moments quite well. As has already been touched in this work, the method may also be applied to other moments such as magnetic radii and electric polarizabilities as well in the future

Occupations and Inequality: Theoretical Perspectives and Mechanisms

People’s occupations are strongly related to multiple dimensions of inequality, such as inequalities in wages, health, autonomy, or risk of temporary employment. The theories and mechanisms linking occupations to these inequalities are subject to debate. We review the recent evidence on the relationship between occupations and inequality and discuss the following four overarching theoretical perspectives: occupations and skills, occupations and tasks, occupations and institutions, and occupations and culture. We show that each perspective has strong implications for how scholars conceptualize occupations and which occupational characteristics are seen as relevant when explaining inequalities. Building on this, we review and critically examine the relevant theories related to and the mechanisms of the relationship between occupation and wage inequality, as an example. We conclude that there is sound empirical knowledge available on the relationships between occupations and inequality; however, some of the mechanisms are still unclear

Charakterisierung der amyloidspezifischen Antikörperdomäne B10

Die Bildung von Amyloidfibrillen ist eine intrinsische Eigenschaft von Polypeptidketten. Das Auftreten von Amyloidfibrillen ist mit einer Reihe schwerwiegender Krankheiten verbunden, wie beispielsweise der Alzheimer-Krankheit oder Diabetes mellitus Typ 2. Die klinische Diagnose von Amyloidosen und die Entwicklung von therapeutischen Strategien beruhen auf der Grundlage der Erkennung von Amyloidfibrillen oder der Inhibition der Bildung von Amyloidfibrillen durch spezifische Moleküle. Für die Entwicklung und Anwendung von Immunotherapien spielen insbesondere Antikörper eine wichtige Rolle. Allerdings sind die molekularen Grundlagen der spezifischen Erkennung von Amyloidfibrillen durch Antikörper bislang kaum untersucht. Die vorliegende Arbeit trägt zur Klärung dieser Frage am Beispiel der kameliden Antikörperdomäne B10 bei. B10 unterscheidet Amyloidfibrillen des Alzheimer Aβ-Peptides von löslichen Aβ-Oligomeren und disaggregiertem Aβ-Peptid. Die konformationelle Spezifität von B10 beruht auf elektrostatischen Wechselwirkungen und lässt sich nicht durch strukturelle Charakteristika erklären, die in globulär gefalteten oder intrinsisch ungefalteten Polypeptidketten auftreten. B10 interagiert neben Aβ-Fibrillen mit einer ganzen Reihe weiterer Amyloidfibrillen, sowie den polyanionischen Biopolymeren Heparin und DNS. Zusätzlich weist B10 enge funktionelle Gemeinsamkeiten zu natürlichen Mustererkennungsrezeptoren auf. Diese Zusammenhänge führen zur Formulierung einer Hypothese, die die Bindung von B10 und möglicherweise von anderen konformationssensitiven Antikörpern an Amyloidfibrillen erklärt. Demzufolge erkennt B10 Amyloidfibrillen aufgrund eines hoch geordneten und regelmäßigen anionischen Musters auf der Oberfläche. Dieses Muster findet sich bei vielen, aber nicht bei allen Amyloidfibrillen und reflektiert die zugrunde liegende regelmäßige und hoch geordnete Struktur dieser Biopolymere. Die Antigenspezifität von B10 beruht also auf einem Mustererkennungs-Mechanismus

Realization of an Economical Polymer Optical Fiber Demultiplexer

Polymer Optical Fiber (POF) can be and are being used in various fields of applications. Two of the main fields are the automotive and the home entertainment sector. The POF can be applied in several different optical communication systems as automotive multi-media busses or in-house Ethernet systems. The requirements of bandwidth are increasing very fast in these sectors and therefore solutions that satisfy these demands are of high actuality. One solution is to use the wavelength division multiplexing (WDM) technique. Here, several different wavelengths can carry information over one POF fiber. All wavelengths that are transmitted over the fiber, must be separated at the receiver to regain and redirect the information channels. These separators are so-called Demultiplexers. There are several systems available on the market, which are all afflicted with certain disadvantages. But all these solutions have one main disadvantage, they are all too expensive for most of the applications mentioned above. So the goal of this study is to develop an economical Demultiplexer for WDM transmission over POF. The main idea is to separate the chromatic light in its monochromatic components with the help of a prism with low reciprocal dispersive power. The prism and the other assemblies, which are needed to adjust the optical path, should be manufactured in injection molding technique. This manufacturing technique is a very simple and economical way to produce a mass production applicable Demultiplexer for POF.Comment: 2006 International Students and Young Scientists Workshop "Photonics and Microsystems

Bridging the “Shadow Line”. Action Research of the 1970s between the Poles of Theory and Practice

Starting from the discussion on the theory- practice relation in educational science, this article focuses on the tradition of action research, particularly in the course of the 1970s. Using the example of Laborschule Bielefeld, a state-run experimental school, it examines the challenges that arise in the attempt to fundamentally question the boundaries between theory and practice in the mode of action research. Not only does it become apparent that the tension between theory and practice cannot be completely resolved – even if an attempt is made to resolve it constructively in a group of “teacher researchers” who act as autonomously as possible – but the fundamental difficulties of participatory research in the field of pedagogy are revealed

Berufe und soziale Ungleichheit – Thematische Einführung und Inhalte des Sonderhefts

Die Untersuchung der Bedeutung des Berufs für die soziale Ungleichheit hat eine lange Tradition. In diesem einleitenden Beitrag werden die im hier vorliegenden Sonderheft der Kölner Zeitschrift für Soziologie und Sozialpsychologie gesammelten Aufsätze in den Kontext der bisherigen Berufsforschung gestellt. Der Anspruch des Sonderhefts besteht darin, Berufe nicht wie andernorts als „Blackbox“ zu betrachten, sondern explizit danach zu fragen, welche sozialen Mechanismen den Berufen ihre ungleichheitsrelevante Wirkung verleihen. Der erste Themenblock des Sonderhefts verschafft den Leserinnen und Lesern einen Zugang zu theoretischen Grundlagen und Messkonzepten im Forschungsfeld „Berufe und soziale Ungleichheit“. In den anschließenden Themenblöcken wird anhand konkreter Forschungsfragen aufgezeigt, welches Potenzial in der hier vorgestellten Perspektive auf Mechanismen im Themenfeld Berufe und soziale Ungleichheit steckt. Die Themenblöcke befassen sich mit Fragen von Berufswahl und beruflichen Karrieren, mit dem Zusammenhang von Berufen und Geschlechterungleichheit, mit der Bedeutung von Berufen im Kontext der Debatten um Migration und Zuwanderung sowie mit dem Thema Berufe im Zeitalter der Digitalisierung und Globalisierung. Dabei handelt es sich explizit um theoretisch fundierte, empirische Beiträge, die gleichzeitig die Vielfalt der für diese Art von Analysen verwendbaren Datengrundlagen und methodischen Zugänge aufzeigt

Wie radikal ist noch normal? Zum Verhältnis von etablierter Erziehungswissenschaft und pädagogischer Radikalität

Die Autor*innen diskutieren die Beziehung der etablierten Erziehungswissenschaft zur pädagogischen Radikalität. Am Beispiel der Forderung nach einer Entschulung der Gesellschaft, der Antipädagogik und der Bewegung der Freien Demokratischen Schulen sowie ihrer differenten Rezeptionsgeschichten plädieren sie für eine weitere Untersuchung radikaler Ideen und die darin vollzogenen Grenzverschiebungen. (DIPF/Orig.

Probing relaxation times in graphene quantum dots

Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dot devices have been extensively investigated with respect to their excitation spectrum, spin-filling sequence, and electron-hole crossover. However their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular regarding the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here, we report on pulsed-gate transient spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design, allowing to tune the tunnelling barriers individually down to the low MHz regime and to monitor their asymmetry with integrated charge sensors. Measuring the transient currents through electronic excited states, we estimate lower limit of charge relaxation times on the order of 60-100 ns.Comment: To be published in Nature Communications. The first two authors contributed equally to this work. Main article: 10 pages, 4 figures. Supplementary information: 4 pages, 4 figure