946 research outputs found
New results in the theory of dust grain alignment
Two complementary approaches are used in an attempt to propose an appropriate formulation of the solution to the problem of magnetic alignment of grains in the diffuse and/or the more denser clouds, whatever the mechanism of rotational excitation can be. The interest of such a unified formulation is mainly that the same theoretical expression for polarization can be used everywhere, allowing for easier comparisons between regions where the physical conditions are highly different. The first consists in applying a Monte-Carlo method to a limited number of representative cases, for which all the torques acting on the grain are taken into account: impulsive random torques due to direct collisions with gas atoms, to evaporation of atoms from the surface, and to exo-energetic recombinations forming hydrogen molecules, followed by violent ejections from peculiar sites; magnetic torques. Three characteristic times are associated with these torques: the collisional damping time, the time necessary to change completely the actual sites configuration narrowly bound to the correlation time of the suprathermal torque; and the magnetic damping time. The second approach starts from a heuristic point of view. It consists in a generalization of results (Cugnon, 1983; see also Purcell and Spitzer, 1971; Greenberg, 1978) obtained for thermal alignment to the suprathermal case. It appears indeed that in two extreme cases, the thermal formulation may be used after redefinition of involved times and temperatures
Phase transition and thermodynamics of a hot and dense system in a scaled NJL model
The chiral phase transition of a hot and dense system of quarks is studied
within a modified SU(3) NJL lagrangian that implements the QCD scale anomaly.
The u- and s-quark condensates can feel or not the same chiral restoration
depending on the considered region of the 3-dimension space T_c(\mu_uc,\mu_sc).
The temperature behaviour of the pressure and of the energy and entropy
densities of the u- and s-quark system is investigated. At high temperature,
the non-vanishing bare s-quark mass only modifies slightly the usual behaviour
associated with an ideal quark gas.Comment: 23 pages, LaTeX, uses elsart.sty. This version of the paper includes
the figures (ps
Probing the density dependence of the symmetry potential with peripheral heavy-ion collisions
The peripheral heavy-ion collisions of at are studied by means of the Improved Quantum Molecular Dynamics
Model(ImQMD). It is shown that the slope of the average N/Z ratio of emitted
nucleons vs impact parameters for these reactions is very sensitive to the
density dependence of the symmetry energy. Our study also shows that the yields
of and decrease with impact parameters and slope of the yield
of vs impact parameters as well as the ratio of Y()/Y()
depend on the symmetry potential strongly for peripheral heavy-ion collisions.Comment: 10 pages,6 figures, accepted by Phys.Rev.
Integral equation for gauge invariant quark Green's function
We consider gauge invariant quark two-point Green's functions in which the
gluonic phase factor follows a skew-polygonal line. Using a particular
representation for the quark propagator in the presence of an external gluon
field, functional relations between Green's functions with different numbers of
segments of the polygonal lines are established. An integral equation is
obtained for the Green's function having a phase factor along a single straight
line. The related kernels involve Wilson loops with skew-polygonal contours and
with functional derivatives along the sides of the contours.Comment: 7 pages; talk given at the Joint Meeting
Heidelberg-Liege-Paris-Wroclaw, Spa, 6-8 March 2008; to appear in the
Proceedings (AIP
Forward Physics at the LHC: within and beyond the Standard Model
We review the detection capabilities in the forward direction of the various
LHC experiments together with the associated physics programme. A selection of
measurements accessible with near-beam instrumentation in various sectors (and
extensions) of the Standard Model (SM) is outlined, including QCD (diffractive
and elastic scattering, low-x parton dynamics, hadronic Monte Carlos for
cosmic-rays), electroweak processes in gamma-gamma interactions, and Higgs
physics (vector-boson-fusion and central exclusive production).Comment: 9 pages, 18 figs. Lectures given at the LAWHEP'07 School (Sao Miguel
das Missoes, Brazil, 3-7 Dec 2007) to appear in Braz. J. Phys. Also presented
in HLPW08 (Spa, Belgium, 6-8 Mar 2008) AIP Conf. Proceeds, to appear; and in
HANUC European Grad. School (Jyvaskyla, Finland, 25-29 Aug. 2008
The charge conjugation quantum number in multiquark systems
We discuss the charge conjugation quantum number for tetraquarks or
meson-meson molecules, seen as possible interpretations of the newly found
charmonium-like resonances.Comment: 7 pages, 1 figure, based on a talk given at the Joint Meeting
Heidelberg-Liege-Paris-Wroclaw (HLPW08): Three Days of Strong Interactions
and Astrophysics, Spa, March 6-8, 2008, Eqs. (18)-(25) corrected, text
slightly polished, conclusions unchange
Effect of Ground State Correlations on the Imaginary Part of the Optical-Model Potential
peer reviewe
Unified description of fission in fusion and spallation reactions
We present a statistical-model description of fission, in the framework of
compound-nucleus decay, which is found to simultaneously reproduce data from
both heavy-ion-induced fusion reactions and proton-induced spallation reactions
at around 1 GeV. For the spallation reactions, the initial compound-nucleus
population is predicted by the Li\`{e}ge Intranuclear Cascade Model. We are
able to reproduce experimental fission probabilities and fission-fragment mass
distributions in both reactions types with the same parameter sets. However, no
unique parameter set was obtained for the fission probability. The introduction
of fission transients can be offset by an increase of the ratio of
level-density parameters for the saddle-point and ground-state configurations.
Changes to the finite-range fission barriers could be offset by a scaling of
the Bohr-Wheeler decay width as predicted by Kramers. The parameter sets
presented allow accurate prediction of fission probabilities for excitation
energies up to 300 MeV and spins up to 60 \hbar.Comment: 16 pages, 20 figures. Submitted to Phys. Rev.
Core-collapse supernova neutrinos and neutrino properties
Core-collapse supernovae are powerful neutrino sources. The observation of a
future (extra-)galactic supernova explosion or of the relic supernova neutrinos
might provide important information on the supernova dynamics, on the supernova
formation rate and on neutrino properties. One might learn more about unknown
neutrino properties either from indirect effects in the supernova (e.g. on the
explosion or on in the r-process) or from modifications of the neutrino time or
energy distributions in a detector on Earth. Here we will discuss in particular
possible effects of CP violation in the lepton sector. We will also mention the
interest of future neutrino-nucleus interaction measurements for the precise
knowledge of supernova neutrino detector response to electron neutrinos.Comment: 9 pages, 5 figures, Proceedings to "Three days of Strong Interactions
& Astrophysics HLPW08", 6-8 March 2008, SP
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