1,721,083 research outputs found
Few-Body Systems In Nuclear And Hadronic Physics - Results and perspectives of the research initiative della I.S. PD32
No full textThe pion--NNN--NNN problem: Connectedness, transition amplitudes, and quasiparticle approximation
No full textIn this paper we review the present status of the πNNN-NNN problem. In particular, we reconsider the chain-labeled approach recently proposed by us, and identify a class of graphs, previously overlooked, which prevents the kernel of the corresponding πNNN-NNN equations from being connected. We propose some approximate schemes, yielding connected-kernel equations. A generalization of the residue method allows us to relate the transition amplitudes for the coupled πNNN-NNN system to the chain-labeled formalism. The quasiparticle approach is extended to the present situation, where emission/absorption of particles is allowed. The open problems for the πNNN-NNN system in light of the present and of previous approaches are finally discussed
N--body theory revisited and its extension to the piNNN--NNN problem
No full textIn order to approach the pion-multinucleon problem, we have found it convenient to reformulate the general N-body theory starting from the fully unclusterized (i.e.,NN) amplitude. If we rewrite such an amplitude in terms of new unknowns, which can be later identified as the amplitudes for all the (N–1)(N–1) cluster processes, and repeat recursively the procedure up to the treatment of the 2-2 cluster processes, we obtain very naturally the hierarchy of equations which ranges from the N-body fully-disconnected Lippmann-Schwinger equation to the N-body connected-kernel Yakubovskii-Grassberger-Sandhas one. This revisitation turns out to be very useful when considering the modifications required in case one of the bodies is a pion and the remaining are nucleons, with the pion being allowed to disappear and reappear through the action of a pion-nucleon vertex. In fact, we obtain a new set of coupled pion-multinucleon equations, which allow a consistent and simultaneous treatment of pion scattering and absorption. For the pion-NNN system, the kernel of these coupled equations is shown to be connected after three iterations
Theory of coupled pi--trinucleon systems
No full textWe derive the dynamical equations which couple the four-body (πNNN) system to the underlying three-nucleon system. Our treatment can be considered the proper generalization of the Afnan-Blankleider equations for the coupled NN-πNN system. The resulting connected-kernel equations resemble in structure the Yakubovskiǐ-Grassberger-Sandhas equations for the standard four-body problem, but involve 24 chain-labeled components (rather than the usual 18 ones) and, within the scheme where the Hilbert space is truncated to states with at most one pion, allow for a consistent evaluation of reaction amplitudes involving π absorption/production
Pion absorption on 3He. II. Antisymmetrization and angular decomposition of the Faddeev--based amplitude
No full textIn order to develop a procedure for calculation of pion absorption on 3He, we perform the decomposition of the absorption amplitude in three-nucleon partial waves, with full inclusion of antisymmetrization effects among the nucleons. Within this formalism, the matrix elements of the Δ-rescattering term, believed to be the dominant elementary absorption mechanism, are evaluated and embedded in the full (Faddeev-based) nuclear dynamics. This implies the treatment of the 3←3 and 2←3 processes in the final state. Since no attempt is made to include distortions due to the four-body (πNNN) dynamics, the trinucleon bound-state wave function is the only requirement in the initial state
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
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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