2,347 research outputs found
Local and Global relations between the number of contacts and density in monodisperse sphere packs
The topological structure resulting from the network of contacts between
grains (\emph{contact network}) is studied for large samples of monosized
spheres with densities (fraction of volume occupied by the spheres) ranging
from 0.59 to 0.64. We retrieve the coordinates of each bead in the pack and we
calculate the average coordination number by using three different methods. We
show that, in the range of density investigated, the coordination number is
larger than 4 and it increases with the packing fraction. At local level we
also observe a positive correlation between local packing fraction and number
of neighbors. We discover a dependence between the local densities of
configurations with few neighbors in contact and the global sample-denities.
This might indicate that local configurations with small number of neighbors
are able to deform plastically when the sample is compactifying.
PACS: 45.70.-n, Granular Systems; 45.70.Cc, Static sandpiles; Granular
Compaction.Comment: 10 pages, 6 figure
Comment on `A simple explanation of the non-appearance of physical gluons and quarks'
In a recent paper by Johan Hansson [hep-ph/0208137] it is claimed that the
non-appearance of quarks and gluons as physical particles is an automatic
result of the nonabelian nature of the color interaction in quantum
chromodynamics. It is shown that the arguments given by Hansson are
insufficient to support his claim by giving simple counter arguments.Comment: 3 pages, LATE
Dynamical partitions of space in any dimension
Topologically stable cellular partitions of D dimensional spaces are studied.
A complete statistical description of the average structural properties of such
partition is given in term of a sequence of D/2-1 (or (D-1)/2) variables for D
even (or odd). These variables are the average coordination numbers of the
2k-dimensional polytopes (2k < D) which make the cellular structure. A
procedure to built D dimensional space partitions trough cell-division and
cell-coalescence transformations is presented. Classes of structures which are
invariant under these transformations are found and the average properties of
such structures are illustrated. Homogeneous partitions are constructed and
compared with the known structures obtained by Voronoi partitions and sphere
packings in high dimensions.Comment: LaTeX 5 eps figures, submetted to J. Phys.
Aspects of the derivative coupling model in four dimensions
A concise discussion of a 3+1-dimensional derivative coupling model, in which
a massive Dirac field couples to the four-gradient of a massless scalar field,
is given in order to elucidate the role of different concepts in quantum field
theory like the regularization of quantum fields as operator valued
distributions, correlation distributions, locality, causality, and field
operator gauge transformations.Comment: 17 pages, LaTeX, corresponding to the published versio
Bound-free pair production cross section in heavy-ion colliders from the equivalent photon approach
Exact calculations of the electron-positron pair production by a single
photon in the Coulomb field of a nucleus with simultaneous capture of the
electron into the K-shell are discussed for different nuclear charges. Using
the equivalent photon method of Weizsaecker and Williams, a simple expression
for the bound-free production of electron-positron pairs by colliding
very-high-energy fully stripped heavy ions is derived for nuclei of arbitrary
charge.Comment: 4 pages, 2 figures, style file include
Scalar models of formally interacting non-standard quantum fields in Minkowski space-time
For decades, a lot of work has been devoted to the problem of constructing a
non-trivial quantum field theory in four-dimensional space time. This letter
addresses the attempts to construct an algebraic quantum field theory in the
framework of non-standard theories like hyperfunction or ultra-hyperfunction
quantum field theory. For this purpose model theories of formally interacting
neutral scalar fields are constructed and some of their characteristic
properties like two-point functions are discussed. The formal self-couplings
are obtained from local normally-ordered analytic redefinitions of the free
scalar quantum field, mimicking a non-trivial structure of the resulting
Lagrangians and equations of motion.Comment: 4 pages, 6 references and a conclusion section added, abstract
slightly modifie
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