Quasielastic Electron Scattering from Nuclei: Random-Phase vs. Ring Approximations

We investigate the extent to which the nuclear transverse response to electron scattering in the quasielastic region, evaluated in the random-phase approximation can be described by ring approximation calculations. Different effective interactions based on a standard model of the type g'+V_pi+V_rho are employed. For each momentum transfer, we have obtained the value of g'_0 permitting the ring response to match the position of the peak and/or the non-energy weighted sum rule provided by the random-phase approach has been obtained. It is found that, in general, it is not possible to reproduce both magnitudes simultaneously for a given g'_0 value.Comment: 7 pages, 4 Postscript figures, to appear in Physical Review

Determination of spin and orbital magnetization in the ferromagnetic superconductor UCoGe

International audienceThe magnetism in the ferromagnetic superconductor UCoGe has been studied using a combination of magnetic Compton scattering, bulk magnetization, X-ray magnetic circular dichroism and electronic structure calculations, in order to determine the spin and orbital moments. The experimentally observed total spin moment, Ms, was found to be-0.24 ± 0.05 µB at 5 T. By comparison with the total moment of 0.16 ± 0.01 µB, the orbital moment, M l , was determined to be 0.40 ± 0.05 µB. The U and Co spin moments were determined to be antiparallel. We find that the U 5f electrons carry a spin moment of Us ≈-0.30 µB and that there is a Co spin moment of Cos ≈ 0.06 µB induced via hybridization. The ratio U l /Us, of −1.3 ± 0.3, shows the U moment to be itinerant. In order to ensure an accurate description of the properties of 5f systems, and to provide a critical test of the theoretical approaches, it is clearly necessary to obtain experimental data for both the spin and orbital moments, rather than just the total magnetic moment. This can be achieved simply by measuring the spin moment with magnetic Compton scattering and comparing this to the total moment from bulk magnetizatio

A two-step learning approach for solving full and almost full cold start problems in dyadic prediction

Dyadic prediction methods operate on pairs of objects (dyads), aiming to infer labels for out-of-sample dyads. We consider the full and almost full cold start problem in dyadic prediction, a setting that occurs when both objects in an out-of-sample dyad have not been observed during training, or if one of them has been observed, but very few times. A popular approach for addressing this problem is to train a model that makes predictions based on a pairwise feature representation of the dyads, or, in case of kernel methods, based on a tensor product pairwise kernel. As an alternative to such a kernel approach, we introduce a novel two-step learning algorithm that borrows ideas from the fields of pairwise learning and spectral filtering. We show theoretically that the two-step method is very closely related to the tensor product kernel approach, and experimentally that it yields a slightly better predictive performance. Moreover, unlike existing tensor product kernel methods, the two-step method allows closed-form solutions for training and parameter selection via cross-validation estimates both in the full and almost full cold start settings, making the approach much more efficient and straightforward to implement

Factorizing the hard and soft spectator scattering contributions for the nucleon form factor F_1 at large Q^2

We investigate the soft spectator scattering contribution for the FF $F_{1}$. We focus our attention on factorization of the hard-collinear scale $\sim Q\Lambda$ corresponding to transition from SCET-I to SCET-II. We compute the leading order jet functions and find that the convolution integrals over the soft fractions are logarithmically divergent. This divergency is the consequence of the boost invariance and does not depend on the model of the soft correlation function describing the soft spectator quarks. Using as example a two-loop diagram we demonstrated that such a divergency corresponds to the overlap of the soft and collinear regions. As a result one obtains large rapidity logarithm which must be included in the correct factorization formalism. We conclude that a consistent description of the factorization for $F_{1}$ implies the end-point collinear divergencies in the hard and soft spectator contributions, i.e. convolution integrals with respect to collinear fractions are not well-defined. Such scenario can only be realized when the twist-3 nucleon distribution amplitude has specific end-point behavior which differs from one expected from the evolution of the nucleon distribution amplitude. Such behavior leads to the violation of the collinear factorization for the hard spectator scattering contribution. We suggest that the soft spectator scattering and chiral symmetry breaking provide the mechanism responsible for the violation of collinear factorization in case of form factor $F_{1}$.Comment: 25 pages, 6 figures, text is improved, few typos corrected, one figure added, statement about end-point behavior of the nucleon DA is formulated more accuratel

Decay of Classical Chaotic Systems - the Case of the Bunimovich Stadium

The escape of an ensemble of particles from the Bunimovich stadium via a small hole has been studied numerically. The decay probability starts out exponentially but has an algebraic tail. The weight of the algebraic decay tends to zero for vanishing hole size. This behaviour is explained by the slow transport of the particles close to the marginally stable bouncing ball orbits. It is contrasted with the decay function of the corresponding quantum system.Comment: 16 pages, RevTex, 3 figures are available upon request from [email protected], to be published in Phys.Rev.

Using grounded theory for theory building in operations management research:a study on inter-firm relationship governance

Purpose – Qualitative theory building approaches, such as grounded theory method (GTM), are still not very widespread and rigorously applied in operations management (OM) research. Yet it is agreed that more systematic observation of current industrial phenomena is necessary to help managers deal with their problems. The purpose of this paper is to provide an example to help guide other researchers on using GTM for theory building in OM research. Design/methodology/approach – A GTM study in the German automotive industry consisting of 31 interviews is followed by a validation stage comprising a survey (110 responses) and a focus group. Findings – The result is an example of conducting GTM research in OM, illustrated by the development of the novel collaborative enterprise governance framework for inter-firm relationship governance in the German automotive industry. Research limitations/implications – GTM is appropriate for qualitative theory building research, but the resultant theories need further testing. Research is necessary to identify the transferability of the collaborative enterprise governance concept to other industries than automotive, to other organisational areas than R&D and to product and service settings that are less complex and innovative. Practical implications – The paper helps researchers make more informed use of GTM when engaging in qualitative theory building research in OM. Originality/value – There is a lack of explicit and well-informed use of GTM in OM research because of poor understanding. This paper addresses this deficiency. The collaborative enterprise governance framework is a significant contribution in an area of growing importance within OM

Slow relaxation in weakly open vertex-splitting rational polygons

The problem of splitting effects by vertex angles is discussed for nonintegrable rational polygonal billiards. A statistical analysis of the decay dynamics in weakly open polygons is given through the orbit survival probability. Two distinct channels for the late-time relaxation of type 1/t^delta are established. The primary channel, associated with the universal relaxation of ''regular'' orbits, with delta = 1, is common for both the closed and open, chaotic and nonchaotic billiards. The secondary relaxation channel, with delta > 1, is originated from ''irregular'' orbits and is due to the rationality of vertices.Comment: Key words: Dynamics of systems of particles, control of chaos, channels of relaxation. 21 pages, 4 figure

Larval gryporhynchid tapeworms (Cestoda: Cyclophyllidea) of British freshwater fish, with a description of the pathology caused by Paradilepis scolecina

Larvae of the cyclophyllidean tapeworms Paradilepis scolecina (Rudolphi, 1819), Neogryporhynchus cheilancristrotus (Wedl, 1855) and Valipora campylancristrota (Wedl, 1855), are described from British freshwater fish. The morphometrics of the rostellar hooks, infection characteristics and host ranges of these parasites from fisheries in England and Wales are presented. Difficulties in the detection, handling and identification of these tapeworms are highlighted, and may in part explain the paucity of records from Britain. Tissue digestion was shown to be a useful technique for the examination of these parasites, providing clear and consistent preparations of the rostellar hooks for measurement. The pathological changes caused by P. scolecina to the liver of wild tench, Tinca tinca, are detailed for the first time. Tapeworms located in the hepatic parenchyma and pancreatic tissues caused little pathological damage and invoked only mild inflammatory responses. The small size of these tapeworms and their encapsulation within host tissues appear to limit the severity of pathology, compared with parasites that insert their rostellum during attachment

Seasonal development and pathological changes associated with the parasitic nematode Philometroides sanguineus in wild crucian carp Carassius carassius (L.) in England

Pathological changes associated with the parasitic nematode Philometroides sanguineus (Rudolphi, 1819) are described for the first time from observations of infections in wild crucian carp Carassius carassius (L.) in England. The damage caused by P. sanguineus was influenced strongly by host size, parasite development and the seasonal migrations of female nematodes within host tissues. Male and unfertilized female nematodes located on the swim-bladder wall caused only minor, localized changes. In contrast, the migration of gravid female nematodes into the fins during autumn provoked an acute inflammatory response comprising neutrophils, eosinophils and lymphocytes. This was accompanied by fin distortion, swelling of the dorsal and caudal tissues, degenerative changes and localized oedema. The encapsulation of female nematodes in connective tissue throughout winter limited additional tissue damage. The emergence of gravid nematodes from the fins in spring to facilitate larval dispersal caused necrosis, hyperplasia and loss of skin integrity. This activity was again accompanied by acute inflammatory reactions. Pathological changes were more severe in crucian carp measuring less than 60 mm in length, with no signs of debilitation in larger fish. These observations suggest that any impact of P. sanguineus is strictly seasonal and may be limited to fry. Lesions caused by this parasite, only recently recorded in Britain, may represent an additional pressure upon wild crucian carp populations already threatened by hybridization, competition and habitat loss

Fusion evaporation-residue cross sections for Si28+40Ca at E(28Si)=309, 397, and 452 MeV

Velocity distributions of mass-identified evaporation residues produced in the Si28+40Ca reaction have been measured at bombarding energies of 309, 397, and 452 MeV using time-of-flight techniques. These distributions were used to identify evaporation residues and to separate the complete-fusion and incomplete-fusion components. Angular distributions and upper limits for the total evaporation-residue and complete-fusion evaporation-residue cross sections were extracted at all three bombarding energies. The complete-fusion evaporation-residue cross sections and the deduced critical angular momenta are compared with earlier measurements and the predictions of existing models. The ratios of the complete-fusion evaporation-residue cross section to the total evaporation-residue cross section, along with those measured for the Si28+12C and Si28+28Si systems at the same energies, support the entrance-channel mass-asymmetry dependence of the incomplete-fusion evaporation-residue process reported earlier