1,720,996 research outputs found
The lifetime of unstable particles in electromagnetic fields
No full textWe show that the electromagnetic moments of unstable particles (resonances) have an absorptive contribution which quantifies the change of the particle's lifetime in an external electromagnetic field. To give an example we compute here the imaginary part of the magnetic moment for the cases of the muon and the neutron at leading order in the electroweak coupling. We also consider an analogous effect for the strongly decaying (1232) resonance. The result for the muon is Im μ = eG2Fm3/768π3, with e being the charge and m the mass of the muon, GF the Fermi constant, which in an external magnetic field of B Tesla give rise to the relative change in the muon lifetime of 3 × 10−15B. For the neutron the effect is of a similar magnitude. We speculate on the observable implications of this effect
Lower bound on the proton charge radius from electron scattering data
Get PDFThe proton charge-radius determinations from the electromagnetic form-factor measurements in electron-proton (ep) scattering require an extrapolation to zero momentum transfer Q² = 0 which is prone to model-dependent assumptions. We show that the data at finite momentum transfer can be used to establish a rigorous lower bound on the proton charge radius, while bypassing the model-dependent assumptions that go into the fitting and extrapolation of the ep data. The near-future precise ep experiments at very low Q², such as PRad, are expected to set a stringent lower bound on the proton radius
Pseudoscalar-Meson Contributions to g−2 via Schwinger’s Sum Rule
Get PDFThe Schwinger sum rule is presented as a new promising tool to study the hadronic contributions to the muon anomalous magnetic moment. In particular, we show preliminary results for the light-by-light scattering contribution of pseudoscalar mesons
Dissecting the Hadronic Contributions to (g − 2)μ by Schwinger’s Sum Rule
Get PDFThe theoretical uncertainty of (g − 2)μ is currently dominated by hadronic contributions. In order to express those in terms of directly measurable quantities, we consider a sum rule relating g − 2 to an integral of a photoabsorption cross section. The sum rule, attributed to Schwinger, can be viewed as a combination of two older sum rules: Gerasimov-Drell-Hearn and Burkhardt-Cottingham. The Schwinger sum rule has an important feature, distinguishing it from the other two: the relation between the anomalous magnetic moment and the integral of a photoabsorption cross section is linear, rather than quadratic. The linear property makes it suitable for a straightforward assessment of the hadronic contributions to (g − 2)μ. From the sum rule, we rederive the Schwinger α/2π correction, as well as the formula for the hadronic vacuumpolarization contribution. As an example of the light-by-light contribution, we consider the single-meson exchange
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
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