Soft Probes of the Quark-Gluon Plasma with ALICE at LHC

The Large Hadron Collider (LHC) should start its activity of data taking by the end of summer 2009, and will provide beams of p-p and Pb-Pb at colliding energies up to 14 TeV and 5.5 ATeV respectively. The Pb-Pb heavy-ion program aims at reaching the necessary conditions to create a deconfined state of partons, the Quark-Gluon Plasma (QGP), whose study is one of the most exciting physics topics to be explored thanks to the possibilites offered by this new-generation accelerator. In particular, the "soft" observables related to low and intermediate pT processes, will shed light on many fundamental properties of the system, such as thermodynamic parameters, chemical composition, expansion velocity etc. The p-p collisions will be of great interest as well, since they will serve as an essential reference for heavy ions. ALICE (A Large Ion Collider Experiment) is the LHC experiment dedicated to the study of the QGP. Its large acceptance and low magnetic field make it particularly suited for the study of soft phenomena. After having given an overview of this detector, I will present the main motivations and prospects for soft physics in both p-p and Pb-Pb collisions.Comment: 4 pages, 1 figure. Proceedings of the Lake Louise Winter Institute 200

Nanodust detection near 1 AU from spectral analysis of Cassini/RPWS radio data

Nanodust grains of a few nanometer in size are produced near the Sun by collisional break-up of larger grains and picked-up by the magnetized solar wind. They have so far been detected at 1 AU by only the two STEREO spacecraft. Here we analyze the spectra measured by the radio and plasma wave instrument onboard Cassini during the cruise phase close to Earth orbit; they exhibit bursty signatures similar to those observed by the same instrument in association to nanodust stream impacts on Cassini near Jupiter. The observed wave level and spectral shape reveal impacts of nanoparticles at about 300 km/s, with an average flux compatible with that observed by the radio and plasma wave instrument onboard STEREO and with the interplanetary flux models

On the charge of nanograins in cold environments and Enceladus dust

In very-low energy plasmas, the size of nanograins is comparable to the distance (the so-called Landau length) at which the interaction energy of two electrons equals their thermal energy. In that case, the grain's polarization induced by approaching charged particles increases their fluxes and reduces the charging time scales. Furthermore, for grains of radius smaller than the Landau length, the electric charge no longer decreases linearly with size, but has a most probable equilibrium value close to one electron charge. We give analytical results that can be used for nanograins in cold dense planetary environments of the outer solar system. Application to the nanodust observed in the plume of Saturn's moon Enceladus shows that most grains of radius about 1 nm should carry one electron, whereas an appreciable fraction of them are positively charged by ion impacts. The corresponding electrostatic stresses should destroy smaller grains, which anyway may not exist as crystals since their number of molecules is close to the minimum required for crystallization.Comment: Revised version of paper submitted to Icaru

Efficient semiparametric estimation and model selection for multidimensional mixtures

In this paper, we consider nonparametric multidimensional finite mixture models and we are interested in the semiparametric estimation of the population weights. Here, the i.i.d. observations are assumed to have at least three components which are independent given the population. We approximate the semiparametric model by projecting the conditional distributions on step functions associated to some partition. Our first main result is that if we refine the partition slowly enough, the associated sequence of maximum likelihood estimators of the weights is asymptotically efficient, and the posterior distribution of the weights, when using a Bayesian procedure, satisfies a semiparametric Bernstein von Mises theorem. We then propose a cross-validation like procedure to select the partition in a finite horizon. Our second main result is that the proposed procedure satisfies an oracle inequality. Numerical experiments on simulated data illustrate our theoretical results

Radio pulses from cosmic ray air showers - Boosted Coulomb and Cherenkov fields

High-energy cosmic rays passing through the Earth's atmosphere produce extensive showers whose charges emit radio frequency pulses. Despite the low density of the Earth's atmosphere, this emission should be affected by the air refractive index because the bulk of the shower particles move roughly at the speed of radio waves, so that the retarded altitude of emission, the relativistic boost and the emission pattern are modified. We consider in this paper the contribution of the boosted Coulomb and the Cherenkov fields and calculate analytically the spectrum using a very simplified model in order to highlight the main properties. We find that typically the lower half of the shower charge energy distribution produces a boosted Coulomb field, of amplitude comparable to the levels measured and to those calculated previously for synchrotron emission. Higher energy particles produce instead a Cherenkov-like field, whose amplitude may be smaller because both the negative charge excess and the separation between charges of opposite signs are small at these energies.Comment: 10 figures - Accepted by Astronomy & Astrophysic

An Anomalous UV Extension in NGC6251

Deep U-band FOC images of the nuclear region of NGC6251 have revealed a region of extended emission which is most probably radiation scattered from a continuum source in the nucleus. This radiation lies interior to a dust ring, is nearly perpendicular to the radio jet axis, and is seen primarily in the FOC U and b filters. The extension has a low observed polarization($\le 10$), and is unlikely to arise from line emission. We know of no other examples similar to what we have found in NGC 6251, and we offer some tentative explanations. The nuclear morphology shows clear similarities to that seen in the nucleus of NGC 4261 except for the extended U-band radiation.Comment: 14 pages AAStex format + 4 figures; accepted for publication in ApJ Letter

The Solar Wind Energy Flux

The solar-wind energy flux measured near the ecliptic is known to be independent of the solar-wind speed. Using plasma data from Helios, Ulysses, and Wind covering a large range of latitudes and time, we show that the solar-wind energy flux is independent of the solar-wind speed and latitude within 10%, and that this quantity varies weakly over the solar cycle. In other words the energy flux appears as a global solar constant. We also show that the very high speed solar-wind (VSW > 700 km/s) has the same mean energy flux as the slower wind (VSW < 700 km/s), but with a different histogram. We use this result to deduce a relation between the solar-wind speed and density, which formalizes the anti-correlation between these quantities.Comment: 12 pages, 5 figure

Vulcanism and Prehistory in the massif central of France : Future Prospects for Sustainable Development of Rural Highlands

If recent huge projects in Auvergne, as Vulcania near Clermont-Ferrand for example, try to use vulcanism as a matter of sustainable local development, attracting either seasonal touristic flow or all-year academic activities, the remote highlands of Velay begin to discover that archaeology and vulcanism make an original combination to increase interest of populations and can be a key for sustainable rural development.volcanisme;archéologie;développement local;développement durable;Massif central;Haute-Loire