441 research outputs found
Singularity structure of the pi N scattering amplitude in a meson-exchange model up to energies W < 2.0 GeV
Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the
singularity structure of the pi N scattering amplitudes has been investigated.
For all partial waves up to F waves and c.m. energies up to W = 2 GeV, the
T-matrix poles have been calculated by three different techniques: analytic
continuation into the complex energy plane, speed-plot and the regularization
method. For all 4-star resonances, we find a perfect agreement between the
analytic continuation and the regularization method. We also find resonance
poles for resonances that are not so well established, but in these cases the
pole positions and residues obtained by analytic continuation can substantially
differ from the results predicted by the speed-plot and regularization methods.Comment: 21 pages, 4 figures, 4 table
Poles, the only true resonant-state signals, are extracted from a worldwide collection of partial wave amplitudes using only one, well controlled pole-extraction method
Each and every energy dependent partial-wave analysis is parameterizing the
pole positions in a procedure defined by the way how the continuous energy
dependence is implemented. These pole positions are, henceforth, inherently
model dependent. To reduce this model dependence, we use only one,
coupled-channel, unitary, fully analytic method based on the isobar
approximation to extract the pole positions from the each available member of
the worldwide collection of partial wave amplitudes which are understood as
nothing more but a good energy dependent representation of genuine experimental
numbers assembled in a form of partial-wave data. In that way, the model
dependence related to the different assumptions on the analytic form of the
partial-wave amplitudes is avoided, and the true confidence limit for the
existence of a particular resonant state, at least in one model, is
established. The way how the method works, and first results are demonstrated
for the S11 partial wave.Comment: 22 pages, 8 figures, 2 table
Stability of the Zagreb Carnegie-Mellon-Berkeley model
In ref. [1] we have used the Zagreb realization of Carnegie-Melon-Berkeley
coupled-channel, unitary model as a tool for extracting pole positions from the
world collection of partial wave data, with the aim of eliminating model
dependence in pole-search procedures. In order that the method is sensible, we
in this paper discuss the stability of the method with respect to the strong
variation of different model ingredients. We show that the Zagreb CMB procedure
is very stable with strong variation of the model assumptions, and that it can
reliably predict the pole positions of the fitted partial wave amplitudes.Comment: 25 pages, 12 figures, 19 table
Soft-core meson-baryon interactions. II. and scattering
The potential includes the t-channel exchanges of the scalar-mesons
and f_0, vector-meson , tensor-mesons f_2 and f_2' and the
Pomeron as well as the s- and u-channel exchanges of the nucleon N and the
resonances , Roper and S_{11}. These resonances are not generated
dynamically. We consider them as, at least partially, genuine three-quark
states and we treat them in the same way as the nucleon. The latter two
resonances were needed to find the proper behavior of the phase shifts at
higher energies in the corresponding partial waves. The soft-core -model
gives an excellent fit to the empirical S- and P-wave phase shifts up
to T_{lab}=600 MeV. Also the scattering lengths have been reproduced well and
the soft-pion theorems for low-energy scattering are satisfied. The
soft-core model for the interaction is an SU_f(3)-extension of the
soft-core -model. The potential includes the t-channel exchanges
of the scalar-mesons a_0, and f_0, vector-mesons , and
, tensor-mesons a_2, f_2 and f_2' and the Pomeron as well as u-channel
exchanges of the hyperons and . The fit to the empirical S-, P- and D-wave phase shifts up to T_{lab}=600 MeV is reasonable and
certainly reflects the present state of the art. Since the various
phase shift analyses are not very consistent, also scattering observables are
compared with the soft-core -model. A good agreement for the total and
differential cross sections as well as the polarizations is found.Comment: 24 pages, 20 PostScript figures, revtex4, submitted to Phys. Rev.
Extraction of P11 resonances from pi N data
We show that two P11 nucleon resonance poles near the pi Delta threshold,
obtained in several analyses, are stable against large variations of parameters
within a dynamical coupled-channels analysis based on meson-exchange
mechanisms. By also performing an analysis based on a model with a bare nucleon
state, we find that this two-pole structure is insensitive to the analytic
structure of the amplitude in the region below pi N threshold. Our results are
M_pole = (1363^{+9}_{-6} -i79^{+3}_{-5}) MeV and (1373^{+12}_{-10}
-i114^{+14}_{-9}) MeV. We also demonstrate that the number of poles in the 1.5
GeV < W < 2 GeV region could be more than one, depending on how the structure
of the single-energy solution of SAID is fitted. For three-pole solutions, our
best estimated result of a pole near N(1710) listed by Particle Data Group is
(1829^{+131}_{-65} -i192^{+88}_{-110}) MeV which is close to the results of
several previous analyses. Our results indicate the need of more accurate pi N
reaction data in the W > 1.6 GeV region for high precision resonance
extractions.Comment: 15 pages, 4 figure
Relativistic two-pion exchange nucleon-nucleon potential: configuration space
We have recently performed a relativistic chiral expansion of the
two-pion exchange potential, and here we explore its configuration space
content. Interactions are determined by three families of diagrams, two of
which involve just and , whereas the third one depends on
empirical coefficients fixed by subthreshold data. In this sense, the
calculation has no adjusted parameters and gives rise to predictions, which are
tested against phenomenological potentials. The dynamical structure of the
eight leading non-relativistic components of the interaction is investigated
and, in most cases, found to be clearly dominated by a well defined class of
diagrams. In particular, the central isovector and spin-orbit, spin-spin, and
tensor isoscalar terms are almost completely fixed by just and .
The convergence of the chiral series in powers of the ratio (pion mass/nucleon
mass) is studied as a function of the internucleon distance and, for 1 fm,
found to be adequate for most components of the potential. An important
exception is the dominant central isoscalar term, where the convergence is
evident only for 2.5 fm. Finally, we compare the spatial behavior of the
functions that enter the relativistic and heavy baryon formulations of the
interaction and find that, in the region of physical interest, they differ by
about 5%.Comment: 27 pages, 33 figure
Slow viscoelastic relaxation and aging in aqueous foam
Like emulsions, pastes and many other forms of soft condensed matter, aqueous
foams present slow mechanical relaxations when subjected to a stress too small
to induce any plastic flow. To identify the physical origin of this
viscoelastic behaviour, we have simulated how dry disordered coarsening 2D
foams respond to a small applied stress. We show that the mechanism of long
time relaxation is driven by coarsening induced rearrangements of small bubble
clusters. These findings are in full agreement with a scaling law previously
derived from experimental creep data for 3D foams. Moreover, we find that the
temporal statistics of coarsening induced bubble rearrangements are described
by a Poisson process.Comment: 7 pages, 3 figure
Exact results on the Kondo-lattice magnetic polaron
In this work we revise the theory of one electron in a ferromagnetically
saturated local moment system interacting via a Kondo-like exchange
interaction. The complete eigenstates for the finite lattice are derived. It is
then shown, that parts of these states lose their norm in the limit of an
infinite lattice. The correct (scattering) eigenstates are calculated in this
limit. The time-dependent Schr\"odinger equation is solved for arbitrary
initial conditions and the connection to the down-electron Green's function and
the scattering states is worked out. A detailed analysis of the down-electron
decay dynamics is given.Comment: 13 pages, 9 figures, accepted for publication in PR
Cd-vacancy and Cd-interstitial complexes in Si and Ge
The electrical field gradient (EFG), measured e.g. in perturbed angular
correlation (PAC) experiments, gives particularly useful information about the
interaction of probe atoms like 111In / 111Cd with other defects. The
interpretation of the EFG is, however, a difficult task. This paper aims at
understanding the interaction of Cd impurities with vacancies and interstitials
in Si and Ge, which represents a controversial issue. We apply two
complementary ab initio methods in the framework of density functional theory
(DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method
and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct
local geometry. Surprisingly we find that both in Si and Ge the substitutional
Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the
Cd on the bond-center site. This complex has a very small EFG, allowing a
unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si.
Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split
configuration with large EFGs, being in reasonable agreement with experiments
Model Dependence of the Properties of S11 Baryon Resonances
The properties of baryon resonances are extracted from a complicated process
of fitting sophisticated, empirical models to data. The reliability of this
process comes from the quality of data and the robustness of the models
employed. With the large of amount of data coming from recent experiments, this
is an excellent time for a study of the model dependence of this extraction
process. A test case is chosen where many theoretical details of the model are
required, the S11 partial wave. The properties of the two lowest N* resonances
in this partial wave are determined using various models of the resonant and
non-resonant amplitudes.Comment: 24 pages, 10 figures; revised fits with error estimates, expanded
comparison between CMB and K-matrix model
- …