4,110 research outputs found
Topology in SU(2) Yang-Mills theory
New results on the topology of the SU(2) Yang-Mills theory are presented. At
zero temperature we obtain the value of the topological susceptibility by using
the recently introduced smeared operators as well as a properly renormalized
geometric definition. Both determinations are in agreement. At non-zero
temperature we study the behaviour of the topological susceptibility across the
confinement transition pointing out some qualitative differences with respect
to the analogous result for the SU(3) gauge theory.Comment: 3 pages, 4 figures, contribution to Lattice-97. Latex file including
espcrc2.st
Renormalization and topological susceptibility on the lattice: SU(2) Yang-Mills theory
The renormalization functions involved in the determination of the
topological susceptibility in the SU(2) lattice gauge theory are extracted by
direct measurements, without relying on perturbation theory. The determination
exploits the phenomenon of critical slowing down to allow the separation of
perturbative and non-perturbative effects. The results are in good agreement
with perturbative computations.Comment: 12 pages + 4 figures (PostScript); report no. IFUP-TH 10/9
Gauge-invariant quark-antiquark nonlocal condensates in lattice QCD
We study, by numerical simulations on a lattice, the behaviour of the
gauge-invariant quark-antiquark nonlocal condensates in the QCD vacuum with
dynamical fermions. A determination is also done in the quenched approximation
and the results are compared with the full-QCD case. The fermionic correlation
length is extracted and compared with the analogous gluonic quantity.Comment: 14 pages, LaTeX file, + 6 PS figure
Antimutagenic and antioxidant activity of a protein fraction from aerial parts of Urtica dioica
Abstract Context: Urtica dioica L. (Urticaceae), stinging nettle, has been employed as a folklore remedy for a wide spectrum of ailments, including urinary disorders, prostatic hyperplasia, and liver diseases. It has been also used traditionally for cancer treatment. Object: To evaluate the potential chemopreventive properties of a protein fraction from the aerial part of Urtica dioica (namely UDHL30). Materials and methods: UDHL30 has been tested for the antimutagenic activity in bacteria (50-800âÎŒg/plate; Ames test by the preincubation method) and for the cytotoxicity on human hepatoma HepG2 cells (0.06-2âmg/mL; 24 and 48âh incubation). Moreover, the antioxidant activity of UDHL30 (0.1-1200âÎŒg/mL; ABTS and superoxide-radical scavenger assays) was evaluated as potential protective mechanisms. Results: UDHL30 was not cytotoxic on HepG2 cells up to 2âmg/mL; conversely, it exhibited a strong antimutagenic activity against the mutagen 2-aminoanthracene (2AA) in all strains tested (maximum inhibition of 56, 78, and 61% in TA98, TA100, and WP2uvrA strains, respectively, at 800âÎŒg/plate). In addition, a remarkable scavenging activity against ABTS radical and superoxide anion (IC50 values of 19.9â±â1.0âÎŒg/mL and 75.3â±â0.9âÎŒg/mL, respectively) was produced. Discussion and conclusions: UDHL30 possesses antimutagenic and radical scavenging properties. Being 2AA a pro-carcinogenic agent, we hypothesize that the antimutagenicity of UDHL30 can be due to the inhibition of CYP450-isoenzymes, involved in the mutagen bioactivation. The radical scavenger ability could contribute to 2AA-antimutagenicity. These data encourage further studies in order to better define the potential usefulness of UDHL30 in chemoprevention
A critical comparison of different definitions of topological charge on the lattice
A detailed comparison is made between the field-theoretic and geometric
definitions of topological charge density on the lattice. Their
renormalizations with respect to continuum are analysed. The definition of the
topological susceptibility, as used in chiral Ward identities, is reviewed.
After performing the subtractions required by it, the different lattice methods
yield results in agreement with each other. The methods based on cooling and on
counting fermionic zero modes are also discussed.Comment: 12 pages (LaTeX file) + 7 (postscript) figures. Revised version.
Submitted to Phys. Rev.
Edge Partitions of Optimal -plane and -plane Graphs
A topological graph is a graph drawn in the plane. A topological graph is
-plane, , if each edge is crossed at most times. We study the
problem of partitioning the edges of a -plane graph such that each partite
set forms a graph with a simpler structure. While this problem has been studied
for , we focus on optimal -plane and -plane graphs, which are
-plane and -plane graphs with maximum density. We prove the following
results. (i) It is not possible to partition the edges of a simple optimal
-plane graph into a -plane graph and a forest, while (ii) an edge
partition formed by a -plane graph and two plane forests always exists and
can be computed in linear time. (iii) We describe efficient algorithms to
partition the edges of a simple optimal -plane graph into a -plane graph
and a plane graph with maximum vertex degree , or with maximum vertex
degree if the optimal -plane graph is such that its crossing-free edges
form a graph with no separating triangles. (iv) We exhibit an infinite family
of simple optimal -plane graphs such that in any edge partition composed of
a -plane graph and a plane graph, the plane graph has maximum vertex degree
at least and the -plane graph has maximum vertex degree at least .
(v) We show that every optimal -plane graph whose crossing-free edges form a
biconnected graph can be decomposed, in linear time, into a -plane graph and
two plane forests
Improved lattice operators: the case of the topological charge density
We analyze the properties of a class of improved lattice topological charge
density operators, constructed by a smearing-like procedure. By optimizing the
choice of the parameters introduced in their definition, we find operators
having (i) a better statistical behavior as estimators of the topological
charge density on the lattice, i.e. less noisy; (ii) a multiplicative
renormalization much closer to one; (iii) a large suppression of the
perturbative tail (and other unphysical mixings) in the corresponding lattice
topological susceptibility.Comment: 11 pages (REVTEX) + 4 (uuencoded) figure
Regulation of CREB activation by p38 mitogen activated protein kinase during human primary erythroblast differentiation.
Among the molecular events underlying erythroid differentiation, we analyzed the signalling pathway leading to cAMP response element binding (CREB) nuclear transcription factor activation. Normal donor blood light density cells differentiated to pro-erythroblasts during the proliferative phase (10 days) of the Human Erithroblast Massive Amplification (HEMA) culture, and to orthochromatic erythroblasts, during the differentiative phase (4 additional days) of the culture. Since erythropoietin was present all over the culture, also pro-erythroblasts left in proliferative medium for 14 days continued their maturation without reaching the final steps of differentiation. p38 Mitogen Activated Protein Kinase (p38 MAPK) and CREB maximal activation occurred upon 4 days of differentiation induction, whereas a lower activation was detectable in the cells maintained in parallel in proliferative medium (14 days). Interestingly, when SB203580, a specific p38 MAPK inhibitor, was added to the culture the percentage of differentiated cells decreased along with p38 MAPK and CREB phosphorylation. All in all, our results evidence a role for p38 MAPK in activating CREB metabolic pathway in the events leading to erythroid differentiation
High energy parton-parton amplitudes from lattice QCD and the stochastic vacuum model
Making use of the gluon gauge-invariant two-point correlation function,
recently determined by numerical simulation on the lattice in the quenched
approximation and the stochastic vacuum model, we calculate the elementary
(parton-parton) amplitudes in both impact-parameter and momentum transfer
spaces. The results are compared with those obtained from the Kr\"{a}mer and
Dosch ansatz for the correlators. Our main conclusion is that the divergences
in the correlations functions suggested by the lattice calculations do not
affect substantially the elementary amplitudes. Phenomenological and
semiempirical information presently available on elementary amplitudes is also
referred to and is critically discussed in connection with some theoretical
issues.Comment: Text with 11 pages in LaTeX (twocolumn form), 10 figures in
PostScript (psfig.tex used). Replaced with changes, Fig.1 modified, two
references added, some points clarified, various typos corrected. Version to
appear in Phys. Rev.
Perfect topological charge for asymptotically free theories
The classical equations of motion of the perfect lattice action in
asymptotically free spin and gauge models possess scale invariant
instanton solutions. This property allows the definition of a topological
charge on the lattice which is perfect in the sense that no topological defects
exist. The basic construction is illustrated in the O(3) non--linear
--model and the topological susceptibility is measured to high
precision in the range of correlation lengths . Our results
strongly suggest that the topological susceptibility is not a physical quantity
in this model.Comment: Contribution to Lattice'94, 3 pages PostScript, uuencoded compresse
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