Les rhyolites à topaze de la région de San Luis Potosi (Mexique) : caractéristiques des laves et conditions de croissance des topazes

Au Mexique, les rhyolites à topaze se rencontrent principalement dans la région de San Luis Potosi (Etats de San Luis Potosi et de Guanajuato). Ces rhyolites se présentent sous la forme de dômes mis en place lors d’une tectonique en extension d’âge tertiaire. Trois dômes du champv olcanique de San Luis Potosi ont été retenus pour cette étude en fonction de la couleur, de la forme et de la taille des topazes qu’ils contiennent afin de déterminer les caractéristiques des laves et les conditions de croissance des topazes. Ces rhyolites, riches en silice et alcalins, sont méta-alumineuses à légèrement hyper-alumineuses. Elles sont enrichies en fluor et en éléments incompatibles comme Rb, Cs, Ta, U et Th et appauvries en Ba, Sr, Ca, Mg, Ti et Ni. De ce fait elles apparaissent très comparables aux rhyolites à topaze de l’ouest des Etats-Unis. Le Cerro El Gato contient des topazes soit de couleur ambre, soit incolores qui ont cristallisé dans des géodes et des fractures. En prenant en compte le milieu de croissance, la morphologie, la composition chimique et les caractéristiques RPE de ces différents cristaux, il est possible de proposer un modèle de cristallisation et d’expliquer les différences de couleur observées. Ainsi, les topazes incolores du Cerro El Gato ont cristallisé à une température supérieure à 500oC (absence de centres colorés) et à partir de fluides enrichis en éléments lessivés de la lave, alors que les topazes de couleur ambre ont cristallisé à une température inférieure à 500oC (présence de centres colorés) et à partir d’un fluide plus riche en éléments volatils (As). Les topazes du Cerro el Lobo ont des caractéristiques intermédiaires entre celles des topazes incolores et des topazes ambres du Cerro El Gato

Integrated control of Fascioliasis in Latin America: Results of an International Atomic Energy Agency Project

Even though Fascioliasis has been known for centuries, it is now emerging in many parts of the world . Human cases are rising, two decades ago the estimates were of a few thousand , current estimates reach many millions. The human endemic areas are highest in Latin American countries and animal fascioliasis greatly affects livestock all through the region, often of poor subsistence farmers, thus decreasing even more their meager earnings and contributing to the cycle of disease and poverty . The enourmous geographical diversity of the regions affected by fascioliasis in Latin America ranges from the cold, snowcovered Patagonia to the simmering tropics of Central America

Demagnetization via Nucleation of the Nonequilibrium Metastable Phase in a Model of Disorder

We study both analytically and numerically metastability and nucleation in a two-dimensional nonequilibrium Ising ferromagnet. Canonical equilibrium is dynamically impeded by a weak random perturbation which models homogeneous disorder of undetermined source. We present a simple theoretical description, in perfect agreement with Monte Carlo simulations, assuming that the decay of the nonequilibrium metastable state is due, as in equilibrium, to the competition between the surface and the bulk. This suggests one to accept a nonequilibrium "free-energy" at a mesoscopic/cluster level, and it ensues a nonequilibrium "surface tension" with some peculiar low-T behavior. We illustrate the occurrence of intriguing nonequilibrium phenomena, including: (i) Noise-enhanced stabilization of nonequilibrium metastable states; (ii) reentrance of the limit of metastability under strong nonequilibrium conditions; and (iii) resonant propagation of domain walls. The cooperative behavior of our system may also be understood in terms of a Langevin equation with additive and multiplicative noises. We also studied metastability in the case of open boundaries as it may correspond to a magnetic nanoparticle. We then observe burst-like relaxation at low T, triggered by the additional surface randomness, with scale-free avalanches which closely resemble the type of relaxation reported for many complex systems. We show that this results from the superposition of many demagnetization events, each with a well- defined scale which is determined by the curvature of the domain wall at which it originates. This is an example of (apparent) scale invariance in a nonequilibrium setting which is not to be associated with any familiar kind of criticality.Comment: 26 pages, 22 figure

Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio