820,593 research outputs found
Extended percolation analysis of the cosmic web
Aims. We develop an extended percolation method to allow the comparison of
geometrical properties of the real cosmic web with the simulated dark matter
web for an ensemble of over- and under-density systems. Methods. We scan
density fields of dark matter (DM) model and SDSS observational samples, and
find connected over- and underdensity regions in a large range of threshold
densities. Lengths, filling factors and numbers of largest clusters and voids
as functions of the threshold density are used as percolation functions.
Results. We find that percolation functions of DM models of different box sizes
are very similar to each other. This stability suggests that properties of the
cosmic web, as found in the present paper, can be applied to the cosmic web as
a whole. Percolation functions depend strongly on the smoothing length. At
smoothing length 1 Mpc the percolation threshold density for clusters
is , and for voids is , very different from percolation thresholds for random samples, . Conclusions. The extended percolation analysis is a
versatile method to study various geometrical properties of the cosmic web in a
wide range of parameters. Percolation functions of the SDSS sample are very
different from percolation functions of DM model samples. The SDSS sample has
only one large percolating void which fills almost the whole volume. The SDSS
sample contains numerous small isolated clusters at low threshold densities,
instead of one single percolating DM cluster. These differences are due to the
tenuous dark matter web, present in model samples, but absent in real
observational samples.Comment: 15 pages, 10 figures, Astronomy & Astrophysics (accepted
Electromagnetic structure of A=2 and 3 nuclei and the nuclear current operator
Different models for conserved two- and three-body electromagnetic currents
are constructed from two- and three-nucleon interactions, using either
meson-exchange mechanisms or minimal substitution in the momentum dependence of
these interactions. The connection between these two different schemes is
elucidated. A number of low-energy electronuclear observables, including (i)
radiative capture at thermal neutron energies and deuteron
photodisintegration at low energies, (ii) and radiative capture
reactions, and (iii) isoscalar and isovector magnetic form factors of H and
He, are calculated in order to make a comparative study of these models for
the current operator. The realistic Argonne two-nucleon and Urbana IX
or Tucson-Melbourne three-nucleon interactions are taken as a case study. For
=3 processes, the bound and continuum wave functions, both below and above
deuteron breakup threshold, are obtained with the correlated
hyperspherical-harmonics method. Three-body currents give small but significant
contributions to some of the polarization observables in the
H()He process and the H()H cross section at
thermal neutron energies. It is shown that the use of a current which did not
exactly satisfy current conservation with the two- and three-nucleon
interactions in the Hamiltonian was responsible for some of the discrepancies
reported in previous studies between the experimental and theoretical
polarization observables in radiative capture.Comment: 48 pages, 25 figures, 4 tables, revtex4. Submitted to Phys. Rev.
Local chiral interactions and magnetic structure of few-nucleon systems
The magnetic form factors of H, H, and He, deuteron
photodisintegration cross sections at low energies, and deuteron threshold
electrodisintegration cross sections at backward angles in a wide range of
momentum transfers, are calculated with the chiral two-nucleon (and
three-nucleon) interactions including intermediate states that have
recently been constructed in configuration space. The =3 wave
functions are obtained from hyperspherical-harmonics solutions of the
Schr\"odinger equation. The electromagnetic current includes one- and two-body
terms, the latter induced by one- and two-pion exchange (OPE and TPE,
respectively) mechanisms and contact interactions. The contributions associated
with intermediate states are only retained at the OPE level, and are
neglected in TPE loop (tree-level) corrections to two-body (three-body) current
operators. Expressions for these currents are derived and regularized in
configuration space for consistency with the interactions. The low-energy
constants that enter the contact few-nucleon systems. The predicted form
factors and deuteron electrodisintegration cross section are in excellent
agreement with experiment for momentum transfers up to 2--3 fm. However,
the experimental values for the deuteron photodisintegration cross section are
consistently underestimated by theory, unless use is made of the Siegert form
of the electric dipole transition operator. A complete analysis of the results
is provided, including the clarification of the origin of the aforementioned
discrepancy.Comment: 24 pages, 13 figure
Probing the arrangement of hyperplanes
AbstractIn this paper we investigate the combinatorial complexity of an algorithm to determine the geometry and the topology related to an arrangement of hyperplanes in multi-dimensional Euclidean space from the âprobingâ on the arrangement. The âprobingâ by a flat means the operation from which we can obtain the intersection of the flat and the arrangement. For a finite set H of hyperplanes in Ed, we obtain the worst-case number of fixed direction line probes and that of flat probes to determine a generic line of H and H itself. We also mention the bound for the computational complexity of these algorithms based on the efficient line probing algorithm which uses the dual transform to compute a generic line of H.We also consider the problem to approximate arrangements by extending the point probing model, which have connections with computational learning theory such as learning a network of threshold functions, and introduce the vertical probing model and the level probing model. It is shown that the former is closely related to the finger probing for a polyhedron and that the latter depends on the dual graph of the arrangement.The probing for an arrangement can be used to obtain the solution for a given system of algebraic equations by decomposing the ÎŒ-resultant into linear factors. It also has interesting applications in robotics such as a motion planning using an ultrasonic device that can detect the distances to obstacles along a specified direction
Two-Loop QCD Corrections to the Heavy Quark Form Factors: Anomaly Contributions
We present closed analytic expressions for the order triangle
diagram contributions to the matrix elements of the singlet and non-singlet
axial vector currents between the vacuum and a quark-antiquark state. We have
calculated these vertex functions for arbitrary momentum transfer and for four
different sets of internal and external quark masses. We show that both the
singlet and non-singlet vertex functions satisfy the correct chiral Ward
identities. Using the exact expressions for the finite axial vector form
factors, we check the quality and the convergence of expansions at production
threshold and for asymptotic energies.Comment: 24 pages, 6 figure
Continuum Coupling and Single-Nucleon Overlap Integrals
The presence of a particle continuum, both of a resonant and non-resonant
character, can significantly impact spectroscopic properties of weakly bound
nuclei and excited nuclear states close to, and above, the particle emission
threshold. In the framework of the continuum shell model in the complex
momentum-plane, the so-called Gamow Shell Model, we discuss salient effects of
the continuum coupling on the one-neutron overlap integrals and the associated
spectroscopic factors in neutron-rich helium and oxygen nuclei. In particular,
we demonstrate a characteristic near-threshold energy dependence of the
spectroscopic factors for different l-waves. We show also that the realistic
radial overlap functions, which are needed for the description of transfer
reactions, can be generated by single-particle wave functions of the
appropriately chosen complex potential.Comment: 9 figures; 23 pages; corrected version; accepted in Nuclear Physics
Electroproduction of pseudoscalar mesons on the deuteron
A general analysis of polarization phenomena for coherent meson
electroproduction on deuterons, , where is a
pseudoscalar or -meson, is presented. The spin structure of the
electromagnetic current for -production at threshold is parametrized in
terms of specific (inelastic) threshold electromagnetic form factors which
depend on the momentum transfer squared and the effective mass of the produced
hadronic system. We give expressions for the structure functions of the
reaction (where the deuteron target is polarized) in
terms of these threshold form factors. The spin and isospin structures of the
amplitudes (where is a virtual photon) is
established in the framework of the impulse approximation and relationships
between meson electroproduction on deuterons and on nucleons are given. The
reaction of electroproduction on deuterons is investigated in detail
both at threshold and in the region of -isobar excitation, using the
effective Lagrangian approach for the calculation of the amplitudes of the
elementary process . Special attention is devoted to the
analysis of all standard contributions to the exclusive cross section for
, which are functions of the momentum transfer square, , of
the excitation energy of the produced hadrons and of the pion production angle,
in a region of relatively large momentum transfer. The sensitivity of these
contributions to different parametrizations of the form
factor as well as to the choice of potential is discussed.Comment: 44 pages 19 figure
The Pion Cloud of the Nucleon: Facts and popular Fantasies
I discuss the concept of the pion cloud surrounding the nucleon and other
hadrons - and its limitations.Comment: 9 pp, 3 figs, invited talk, to appear in the proceedings of the
workshop ``Shape of Hadrons'' (Athens, Greece, 2006
Analytic continuation of nucleon electromagnetic form factors in the time-like region
The possibility to compute nucleon electromagnetic form factors in the
time-like region by analytic continuation of their space-like expressions has
been explored in the framework of the Skyrme model. We have developed a
procedure to solve analytically Fourier transforms of the nucleon
electromagnetic current and hence to obtain form factors defined in all
kinematical regions and fulfilling the first-principles requirements. The
results are discussed and compared to data, both in space-like and time-like
region.Comment: 34 pages, 15 figure
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