190 research outputs found
Collisional Energy Loss of a Fast Muon in a Hot QED Plasma
We calculate the collisional energy loss of a muon of high energy in a
hot QED plasma beyond logarithmic accuracy, i.e., we determine the constant
terms of order O(1) in . Considering first the
-channel contribution to , we show that the terms are
sensitive to the full kinematic region for the momentum exchange in elastic
scattering, including large values . We thus redress a previous
calculation by Braaten and Thoma, which assumed and could not find the
correct constant (in the large limit). The relevance of 'very hard'
momentum transfers then requires, for consistency, that and -channel
contributions from Compton scattering must be included, bringing a second
modification to the Braaten-Thoma result. Most importantly, Compton scattering
yields an additional large logarithm in . Our results might have
implications in the QCD case of parton collisional energy loss in a quark gluon
plasma.Comment: 26 pages, 3 figures, JHEP styl
Asymptotic geometry of negatively curved manifolds of finite volume
We study the asymptotic behaviour of simply connected, Riemannian manifolds
of strictly negative curvature admitting a non-uniform lattice . If
the quotient manifold is asymptotically
-pinched, we prove that is divergent and has finite
Bowen-Margulis measure (which is then ergodic and totally conservative with
respect to the geodesic flow); moreover, we show that, in this case, the volume
growth of balls in is asymptotically equivalent to a purely
exponential function , where is the topological
entropy of the geodesic flow of . \linebreak This generalizes Margulis'
celebrated theorem to negatively curved spaces of finite volume. In contrast,
we exhibit examples of lattices in negatively curved spaces (not
asymptotically -pinched) where, depending on the critical exponent of the
parabolic subgroups and on the finiteness of the Bowen-Margulis measure, the
growth function is exponential, lower-exponential or even upper-exponential.Comment: 25 p. This paper replaces arXiv:1503.03971, withdrawn by the authors
due to the Theorem 1.1 whose statement is far from the main subject of the
paper; for the sake of clearness, this new version concentrates only on the
question of volume growth (theorems 1.2, 1.3 and 1.4). Theorem 1.1 of
arXiv:1503.03971 is now the subject of another paper (Signed only by 2
authors Sambusetti and Peign\'e) focused on this rigidity problem with a much
better presentation of the context and another rigidity resul
The survival probability of a weakly subcritical multitype branching process in iid random environment
We study the asymptotic behavior of the probability of non extinction of a
weakly subcritical multitype branching process in iid random environments.
Under suitable assumptions, the survival probability is of order of for some to specify
Collisional energy loss of a fast heavy quark in a quark-gluon plasma
We discuss the average collisional energy loss dE/dx of a heavy quark
crossing a quark-gluon plasma, in the limit of high quark energy E >> M^2/T,
where M is the quark mass and T >> M is the plasma temperature. In the fixed
coupling approximation, at leading order dE/dx \propto \alpha_s^2, with a
coefficient which is logarithmically enhanced. The soft logarithm arising from
t-channel scattering off thermal partons is well-known, but a collinear
logarithm from u-channel exchange had previously been overlooked. We also
determine the constant beyond those leading logarithms. We then generalize our
calculation of dE/dx to the case of running coupling. We estimate the remaining
theoretical uncertainty of dE/dx, which turns out to be quite large under RHIC
conditions. Finally, we point out an approximate relation between dE/dx and the
QCD Debye mass, from which we derive an upper bound to dE/dx for all quark
energies.Comment: 6 page
Wheat yield and quality as influenced by reduced tillage in organic farming
Organic farmers are interested in soil conservation by reduced tillage, techniques well known in conventional agriculture to protect soil quality and limit labor time and energy costs. However, organic farming and reduced tillage can modify weeds, soil structure, and thus soil nitrogen (N) mineralization which strongly influences wheat yield and quality. The main objectives of this study were to analyze how reduced tillage applied to organic wheat influenced (1) grain yield, protein concentration, and weed infestation; (2) deoxynivalenol (DON) contamination on grain; (3) technological quality parameters such as dry gluten, zeleny index, falling number, and gluten index; (4) protein composition (F1, F2, F3, F4, and F5 fractions, and UPP, gliadin/glutenin ratio); and (5) baking test. For this purpose, we analyzed five site-years of data from winter wheat crops where mouldboard ploughing and reduced tillage were compared in three experimental trials (two in France and one in Switzerland). Main results concern wheat yields: the effect of reduced tillage on wheat yield was influenced by several factors such as weed competition. No significant increase in mycotoxin content (DON) due to reduced tillage was detected. Contamination with DON was always below the European threshold for human consumption. The technological quality parameters were less affected by the tillage treatments than grain yield: protein content, gluten index, zeleny index, and falling number showed on average no significant difference between treatments although the protein composition was slightly different. The main results of this study are that the effect of reduced tillage on grain yield depends very much on soil type, weather conditions, and time after conversion, whereas there is only minor impact on wheat quality. This is in contrast to the hypothesis that reduced tillage under organic farming will cause problems in baking quality
Soft Photon Production Rate in Resummed Perturbation Theory of High Temperature QCD
We calculate the production rate of soft real photons from a hot quark --
gluon plasma using Braaten -- Pisarski's perturbative resummation method. To
leading order in the QCD coupling constant we find a logarithmically
divergent result for photon energies of order , where is the plasma
temperature. This divergent behaviour is due to unscreened mass singularities
in the effective hard thermal loop vertices in the case of a massless external
photon.Comment: 13 pages (2 figures not included), PLAINTEX, LPTHE-Orsay 93/46, BI-TP
93/5
The Asymptotic distribution of circles in the orbits of Kleinian groups
Let P be a locally finite circle packing in the plane invariant under a
non-elementary Kleinian group Gamma and with finitely many Gamma-orbits. When
Gamma is geometrically finite, we construct an explicit Borel measure on the
plane which describes the asymptotic distribution of small circles in P,
assuming that either the critical exponent of Gamma is strictly bigger than 1
or P does not contain an infinite bouquet of tangent circles glued at a
parabolic fixed point of Gamma. Our construction also works for P invariant
under a geometrically infinite group Gamma, provided Gamma admits a finite
Bowen-Margulis-Sullivan measure and the Gamma-skinning size of P is finite.
Some concrete circle packings to which our result applies include Apollonian
circle packings, Sierpinski curves,
Schottky dances, etc.Comment: 31 pages, 8 figures. Final version. To appear in Inventiones Mat
Améliorer la qualité technologique, nutritionnelle et sanitaire du bléiologique. Principaux leviers agronomiques et technologiques
Communication lors du colloque "DinABio 2013", 13 et 14 novembre 2013 Ă Tours (Fance)The organic bread wheat market has been diversified over time through the emergence of different sale channels. Processors require organic bread wheat of higher quality and safety in order to meet the consumersâ demand. The overall objective of the AGTEC-Org project was to identify agronomical and technological ways to improve the performance of organic wheat and flour. The findings would contribute to enhanced baking quality and nutritional value of organic flour, as well as prevention of mycotoxin contamination. The project involved 9 research centers or universities from 5 European countries for a total budget of about 1.5 million âŹ. More than 400 experimental treatments were analyzed from 23 agronomic trials and 4 lab-experiments on food technology. Choice of cultivar is an efficient way to obtain higher grain quality. Intercropping legumes (grain or forage) improves weed control and N availability for wheat crop or succeeding crop. Green manure can be an effective alternative to farmyard manure. Fertilization with organic fertilizers improves yield and quality when water is available. Reduced tillage affects soil fertility and wheat yield but has little effects on grain quality. Milling process strongly influences flour characteristics. Stone milling improves the nutritive value and flour characteristics remain very stable independently of the milling yield. However, stone milling slightly raises DON levels. Characteristics of flour produced by means of roller milling appear very dependent on milling yield, instead. Increasing milling yield with the aim of enriching nutritional quality has a detrimental effect on either safety (DON) or bread-making quality (bread volume). Debranning before milling has a very positive impact on flour safety by reducing its DON content by 50 %.Les acteurs de la filiĂšre blĂ© biologique panifiable font face Ă divers enjeux complĂ©mentaires voire parfois contradictoires. Les agriculteurs et transformateurs souhaitent augmenter les niveaux de production tout en satisfaisant prioritairement les attentes des consommateurs pour une alimentation de meilleure qualitĂ© nutritionnelle et sanitaire, plus durable et plus respectueuse de lâenvironnement. Le projet europĂ©en AGTEC-Org a eu pour objectif dâidentifier les leviers agronomiques et technologiques permettant dâamĂ©liorer conjointement la productivitĂ© et la qualitĂ© des blĂ©s et des farines biologiques. Il a rĂ©uni agronomes, technologues et Ă©conomistes issus de 9 centres de recherche et universitĂ©s europĂ©ennes pour un budget total de 1,5 MâŹ. Plus de 400 traitements expĂ©rimentaux ont Ă©tĂ© analysĂ©s Ă partir de 23 expĂ©rimentations agronomiques et 4 expĂ©rimentations technologiques. Le choix de la variĂ©tĂ© est un moyen efficace pour obtenir une qualitĂ© de grain supĂ©rieure. Lâinsertion de lĂ©gumineuses (grain ou fourrage) dans le blĂ© amĂ©liore la compĂ©tition avec les adventices et la nutrition azotĂ©e du blĂ© et/ou de la culture de printemps suivante. Lâutilisation rĂ©guliĂšre dâengrais vert peut ĂȘtre une alternative pour compenser lâabsence de fumiers sur lâexploitation. La fertilisation organique amĂ©liore le rendement et la qualitĂ© des blĂ©s lorsque la nutrition hydrique est assurĂ©e. La suppression du labour peut affecter la fertilitĂ© du sol et le rendement du blĂ© alors quâelle nâintervient pas sur la qualitĂ© du grain. Le mode de broyage (meule de pierre vs cylindres) influence fortement les caractĂ©ristiques technologiques, la sĂ©curitĂ© sanitaire et la valeur nutritionnelle de la farine. Le broyage sur meule amĂ©liore la composition nutritionnelle mais augmente aussi la prĂ©sence de certains contaminants. Les caractĂ©ristiques de la farine broyĂ©e sur meule de pierre sont moins influencĂ©es par le rendement en mouture. Au contraire, les caractĂ©ristiques de la farine obtenue par cylindre apparaissent trĂšs dĂ©pendantes du rendement en mouture. Augmenter le rendement en farine dans lâobjectif dâenrichir la qualitĂ© nutritionnelle rĂ©duit aussi bien la sĂ©curitĂ© sanitaire (DON) que la qualitĂ© boulangĂšre (volume du pain). Le dĂ©corticage des grains avant broyage diminue les risques sanitaires en rĂ©duisant le taux de mycotoxines (DON) de 50%
- âŠ