6,757 research outputs found
Electroproduction of vector mesons - factorization of end-point contributions
The end-point contributions in the quark longitudinal momentum fraction of
the virtual photon () to vector meson () impact factor to the
diffractive electroproduction amplitude can be factorized in terms of a
generalized parton evolution of the target parton distribution. The result is
used to model the helicity amplitudes in terms of small
generalized parton distributions.Comment: 24 pages LaTex, 4 eps-figure
The onset of grasses in the Amazon drainage basin, evidence from the fossil record
Poaceae (the grass family) originated in the Cretaceous, but first dominate the palynological records of the Amazon drainage basin (ADB) in the Neogene (23 to 2.5 million years ago (Ma)). However, the ecological role of grasses in the landscape during this time remains to be resolved. In this paper, we summarise the global significance of grasses and the relevance of the fossil record, and evaluate the history of the grasses in the ADB. We present a 3-stage model of the changing role of grasses, which we based on a revision of Neogene depositional environments, the palynological record, and modern grass distribution in the Neotropics. Our model comprises the following hypotheses: (H1) assumes that from c. 23 to 9 Ma western Amazonia was dominated by a megawetland (the ‘Pebas system’) that harboured large amounts of (aquatic?) grasses. In (H2) we propose that from c. 9 Ma Andean uplift prompted megafans (extremely large alluvial fans) that extended from the Andes into the lowlands. Meanwhile, the ‘Pebas’ megawetland gradually transformed into a fluvial system. In this scenario, grasses would have had a competitive advantage and were able to colonise the newly formed megafan and fluvial landscapes. Finally, in (H3) we suggest that landscape dynamics and climatic change intensified from c. 3.5 Ma, allowing for a renewed expansion of the grasses. In addition, both the fossil and molecular records suggest that from c. 5 Ma grasses were firmly established in the tropical alpine vegetation (páramo), the tropical lowland floodplains (várzeas), and savannas (cerrado). Although further study will have to confirm the precise nature of the ADB grass history, we anticipate that abiotic processes during the Neogene and Quaternary left a strong imprint in the grass phytogeography of northern South America
Impeller blade design method for centrifugal compressors
The design of a centrifugal impeller with blades that are aerodynamically efficient, easy to manufacture, and mechanically sound is discussed. The blade design method described here satisfies the first two criteria and with a judicious choice of certain variables will also satisfy stress considerations. The blade shape is generated by specifying surface velocity distributions and consists of straight-line elements that connect points at hub and shroud. The method may be used to design radially elemented and backward-swept blades. The background, a brief account of the theory, and a sample design are described
Orthogonal and symplectic Yangians and Yang-Baxter R-operators
Yang-Baxter R operators symmetric with respect to the orthogonal and
symplectic algebras are considered in an uniform way. Explicit forms for the
spinorial and metaplectic R operators are obtained. L operators, obeying the
RLL relation with the orthogonal or symplectic fundamental R matrix, are
considered in the interesting cases, where their expansion in inverse powers of
the spectral parameter is truncated. Unlike the case of special linear algebra
symmetry the truncation results in additional conditions on the Lie algebra
generators of which the L operators is built and which can be fulfilled in
distinguished representations only. Further, generalised L operators, obeying
the modified RLL relation with the fundamental R matrix replaced by the
spinorial or metaplectic one, are considered in the particular case of linear
dependence on the spectral parameter. It is shown how by fusion with respect to
the spinorial or metaplectic representation these first order spinorial L
operators reproduce the ordinary L operators with second order truncation.Comment: 24 page
High Energy Quark-Antiquark Elastic scattering with Mesonic Exchange
We studies the high energy elastic scattering of quark anti-quark with an
exchange of a mesonic state in the channel with .
Both the normalization factor and the Regge trajectory can be calculated in
PQCD in cases of fixed (non-running) and running coupling constant. The
dependence of the Regge trajectory on the coupling constant is highly
non-linear and the trajectory is of order of in the interesting physical
range.Comment: 29 page
Symmetries of BFKL Equation
We discuss the algebraic structure of the spin chains related to high energy
scattering in QCD. We study the sl(2) Yangian symmetry and possible
generalizations to nonzero spin and anisotropy parameter.Comment: 16 pages, Late
Individual and group-based learning from complex cognitive tasks: Effects on retention and transfer efficiency
Kirschner, F., Paas, F., & Kirschner, P. (2009). Individual and group-based learning from complex cognitive tasks: Effects on retention and transfer efficiency. Computers in Human Behavior, 25, 306-314.The effects of individual versus group learning (in triads) on efficiency of retention and transfer test performance in the domain of biology (heredity) among 70 high-school students were investigated. Applying cognitive load theory, the limitations of the working memory capacity at the individual level were considered an important reason to assign complex learning tasks to groups rather than to individuals. It was hypothesized that groups will have more processing capacity available for relating the information elements to each other and by doing so for constructing higher quality cognitive schemata than individuals if the high cognitive load imposed by complex learning tasks could be shared among group members. In contrast, it was expected that individuals who learn from carrying out the same complex tasks would need all available processing capacity for remembering the interrelated information elements, and, consequently, would not be able to allocate resources to working with them. This interaction hypothesis was confirmed by the data on efficiency of retention and transfer test performance; there was a favorable relationship between mental effort and retention test performance for the individual learners as opposed to a favorable relationship between transfer test performance and mental effort for the students who learned in groups
Superiority of collaborative learning with complex tasks: A research note on an alternative affective explanation
Kirschner, F., Paas, F., & Kirschner, P. A. (2011). Superiority of collaborative learning with complex tasks: A research note on an alternative affective explanation. Computers in Human Behavior, 27(1), 53-57. doi:10.1016/j.chb.2010.05.012Kirschner, Paas, and Kirschner (2009c) used the theoretical framework of cognitive load to explain why the learning of a group of collaborating individuals was more efficient than that of individuals learning alone with high-complexity tasks but not with low-complexity tasks. The authors argued that collaboration circumvented the limitations of an individual’s working memory by creating an expanded cognitive capacity and by allowing for the distribution of
cognitive load among group members. Inspired by research on efficacy, this study explored an alternative affective explanation of the results. By measuring the amount of mental effort learners expected to invest in working on a learning task before actually carrying out the task, this study showed that learners who had to collaboratively solve a high-complexity problem expected to invest less mental effort than learners who had to solve the problem alone. When confronted with low-complexity tasks, the expected amount of mental effort did not differ
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