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Search Results: 1 - 10 of 130510 matches for " V. Papavassiliou "
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Gastric carcinoids: Review a propos of two cases
C. Christopoulos, E. Papavassiliou, V. Dimakopoulou, P. Economopoulos
Annals of Gastroenterology , 2007,
Abstract: SUMMARY Gastric carcinoids are neoplasms originating from the endocrine cells of the gastric wall and represent less than 1% of all gastric tumours. Their frequency appears to be rising in parallel with the increasing use of upper gastrointestinal endoscopy and the development of techniques for specific immunohistochemical study of gastric biopsies. The pathophysiological classification of gastric carcinoids into three types is clinically useful. The majority (75-80%) belong to Type I, which is associated with chronic atrophic gastritis type A. They develop as a response to chronic hypergastrinaemia, causing sequentially hyperplasia, dysplasia and neoplasia of the ECL (enterochromaffin- like) cells. They follow a benign clinical course and only exceptionally metastasize. Type II carcinoids (5-13%) are also associated with hypergastrinaemia and develop in patients with the Zollinger-Ellison syndrome as part of multiple endocrine neoplasia type 1 (MEN1). They have a slightly higher malignant potential than type I carcinoids. Type III includes the sporadic gastric carcinoids (14-25%) which are invasive tumours with metastases often present at the time of diagnosis. They are not associated with hypergastrinaemia and require more aggressive surgical treatment than types I and II. We review the current management approaches and report two cases of gastric carcinoids, one demonstrating that classification of these tumours is not always straightforward and the other highlighting the importance of extensive endoscopic sampling in the setting of multiple gastric polypoid lesions. Key words: carcinoid, neuroendocrine tumour, gastric tumour, gastric polyp, atrophic gastritis, enterochromaffinlike cells, gastrin, gastrinoma
Strange Quark Contribution to the Vector and Axial Form Factors of the Nucleon: Combined Analysis of G0, HAPPEx, and Brookhaven E734 Data
S. F. Pate,D. W. McKee,V. Papavassiliou
Physics , 2008, DOI: 10.1103/PhysRevC.78.015207
Abstract: The strange quark contribution to the vector and axial form factors of the nucleon has been determined for momentum transfers in the range $0.45
Domain orbital state in ferromagnetic insulating La$_{0.825}$Ca$_{0.175}$Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$ compound
M. Pissas,G. Papavassiliou,E. Devlin,A. Simopoulos,V. Likodimos
Physics , 2004,
Abstract: Using M\"ossbauer and EPR spectroscopy we have studied the insulating ferromagnetic La$_{1-x}$Ca$_x$Mn$_{0.99}$$^{57}$Fe$_{0.01}$O$_3$ ($x=0.175$) compound prepared in air and reduced atmosphere. The average hyperfine field follows a mean field approximation solution in contrast with the ferromagnetic metallic regime where it significantly deviates from the order parameter deduced from neutron diffraction data or mean field approximation. Although the magnetic measurements show remarkable differences between air prepared and reduced R samples the corresponding M\"ossbauer spectra are almost identical. The strong temperature dependance of the hyperfine field distribution at the $^{57}$Fe nucleus is related with the supertransferred magnetic field between ferric ion and the oxygen bridged six nearest-neighbor manganese ions. The sudden increasing of the width of the hyperfine field distribution above $T_B\approx 100$ K has been attributed to the orbital disordering occurring for $T>T_B$.
Efficient Random Walk Algorithm for Simulating Thermal Transport in Composites With High Conductivity Contrast
K. G. S. H. Gunawardana,Kieran Mullen,Dimitrios V. Papavassiliou
Physics , 2009,
Abstract: In dealing with thermal transport in composite systems, high contrast materials pose a special problem for numerical simulation: the time scale or step size in the high conductivity material must be much smaller than in the low conductivity material. In the limit that the higher conductivity inclusion can be treated as having an infinite conductivity, we show how a standard random walk algorithm can be alterred to improve speed while still preserving the second law of thermodynamics. We demonstrate the principle in a 1D system, and then apply it to 3D composites with spherical inclusions.
Massless bound-state excitations and the Schwinger mechanism in QCD
A. C. Aguilar,D. Ibá?ez,V. Mathieu,J. Papavassiliou
Physics , 2011, DOI: 10.1103/PhysRevD.85.014018
Abstract: The gauge invariant generation of an effective gluon mass proceeds through the well-known Schwinger mechanism, whose key dynamical ingredient is the nonperturbative formation of longitudinally coupled massless bound-state excitations. These excitations introduce poles in the vertices of the theory, in such a way as to maintain the Slavnov-Taylor identities intact in the presence of massive gluon propagators. In the present work we first focus on the modifications induced to the nonperturbative three-gluon vertex by the inclusion of massless two-gluon bound-states into the kernels appearing in its skeleton-expansion. Certain general relations between the basic building blocks of these bound-states and the gluon mass are then obtained from the Slavnov-Taylor identities and the Schwinger-Dyson equation governing the gluon propagator. The homogeneous Bethe-Salpeter equation determining the wave-function of the aforementioned bound state is then derived, under certain simplifying assumptions. It is then shown, through a detailed analytical and numerical study, that this equation admits non-trivial solutions, indicating that the QCD dynamics support indeed the formation of such massless bound states. These solutions are subsequently used, in conjunction with the aforementioned relations, to determine the momentum-dependence of the dynamical gluon mass. Finally, further possibilities and open questions are briefly discussed.
Allelic Frequency in Human SNPs Predicts the Rate of Non-Synonymous Nucleotide Substitutions between Human and Chimpanzee Genes  [PDF]
Hippokratis Kiaris, Athanasios G. Papavassiliou
Advances in Anthropology (AA) , 2014, DOI: 10.4236/aa.2014.41007

The combination of comparative genomics and population genetics may provide important clues regarding human evolution. We have hypothesized that the divergence between various human and chimpanzee orthologs will be reflected in the variability of single nucleotide polymorphisms (SNPs) that are localized in the vicinity of the corresponding loci in different human populations. Consistently with this notion, more diverged genes between humans and chimpanzees are more likely to be associated with human speciation and are anticipated to contain SNPs with reduced variability between different human populations. In order to test this hypothesis, we have compared the rate of non synonymous nucleotide substitutions (Ka) between 255 chimpanzee and human orthologs with the average deviation in the allelic frequencies of corresponding closely linked SNPs in two distinct human populations: The Yoruba people in Ibadan, Nigeria (YRI) and US residents with ancestry from Northern and Western Europe, collected in 1980 by the Centre d’Etude du Polymorphisme Humain (CEU). We found a significant (p < 0.05) negative association between the Ka and the degree of variation in the corresponding allelic frequencies between the human populations which implies that the most significant genes for human speciation are associated lower variability between the human populations examined. This observation is consistent with the strong selective advantage offered by these nucleotide substitutions during human evolution and predicts that a low polymorphic rate is consistent with the presence of genes with an essential role in human speciation.

Standard Model Higher Order Corrections to the WWγ/WWZ Vertex
Joannis Papavassiliou
Physics , 1995, DOI: 10.1063/1.49293
Abstract: Using the S--matrix pinch technique we obtain to one loop order gauge independent $\gamma W^-W^+$ and $Z W^-W^+$ vertices in the context of the standard model, with all incoming momenta off--shell. We show that the vertices so constructed satisfy simple QED--like Ward identities. These gauge invariant vertices give rise to expressions for the magnetic dipole and electric quadrupole form factors of the $W$ gauge boson, which, unlike previous treatments, satisfy the crucial properties of infrared finiteness and perturbative unitarity.
CP violation in the Weinberg multi-Higgs model
Joannis Papavassiliou
Physics , 1995,
Abstract: We report new results in the study of CP violation in semileptonic top decays, in the context of the Weinberg Model.
Joannis Papavassiliou
Physics , 1995,
Abstract: Some of the most important theoretical and phenomenological aspects of the pinch technique are presented, and several recent developments are briefly reviewed.
Gauge independent transverse and longitudinal self-energies and vertices via the pinch technique
Joannis Papavassiliou
Physics , 1994, DOI: 10.1103/PhysRevD.50.5958
Abstract: In the context of the Standard Model we show how to apply the pinch technique to four-fermion amplitudes with non-conserved external charged currents,in order to construct to one-loop order gauge independent self-energies and vertices. We discuss the technical difficulties arising due to the presence of longitudinal contributions from the W and Goldstone boson ($\phi$) propagators, and derive gauge independent $WW$, $\phi W$, and $\phi\phi$ effective self-energies, and vertices. The quantities so constructed satisfy a set of Ward identities, whose validity enforces the gauge invariance of the physical amplitude considered; their derivation does not require knowledge of the explicit closed form of the gauge independent self-energies and vertices. Use of these Ward identities enables the decomposition of the amplitude in manifestly gauge independent transverse and longitudinal structures with distinct kinematic properties. Explicit one-loop calculations are carried out, and several applications of the results are briefly discussed.}
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