397 research outputs found
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
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.
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
On the pion cloud of the nucleon
We evaluate the two--pion contribution to the nucleon electromagnetic form
factors by use of dispersion analysis and chiral perturbation theory. After
subtraction of the rho--meson component, we calculate the distributions of
charge and magnetization in coordinate space, which can be interpreted as the
effects of the pion cloud. We find that the charge distribution of this pion
cloud effect peaks at distances of about 0.3 fm. Furthermore, we calculate the
contribution of the pion cloud to the isovector charges and radii of the
nucleon.Comment: 7 pages, latex, 3 ps figures, minor change
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
Novel evaluation of the two-pion contribution to the nucleon isovector form factors
We calculate the two-pion continuum contribution to the nucleon isovector
spectral functions drawing upon the new high statistics measurements of the
pion form factor by the CMD-2, KLOE, and SND collaborations. The general
structure of the spectral functions remains unchanged, but the magnitude
increases by about 10%. Using the updated spectral functions, we calculate the
contribution of the two-pion continuum to the nucleon isovector form factors
and radii. We compare the isovector radii with simple rho-pole models and
illustrate their strong underestimation in such approaches. Moreover, we give a
convenient parametrization of the result for use in future form factor
analyses.Comment: 9 pages, 2 eps figures, revtex4, CMD-2 and SND data included,
conclusions unchanged, version to appear in Phys. Lett.
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
Dispersion analysis of the nucleon form factors including meson continua
Dispersion relations provide a powerful tool to analyse the electromagnetic
form factors of the nucleon for all momentum transfers. Constraints from
meson-nucleon scattering data, unitarity, and perturbative QCD can be included
in a straightforward way. In particular, we include the 2pi, rho-pi, and KKbar
continua as independent input in our analysis and provide an error band for our
results. Moreover, we discuss two different methods to include the asymptotic
constraints from perturbative QCD. We simultaneously analyze the world data for
all four form factors in both the space-like and time-like regions and
generally find good agreement with the data. We also extract the nucleon radii
and the omega-NN coupling constants. For the radii, we generally find good
agreement with other determinations with the exception of the electric charge
radius of the proton which comes out smaller. The omega-NN vector coupling
constant is determined relatively well by the fits, but for the tensor coupling
constant even the sign can not be determined.Comment: 24 pages, 9 figure
Vacancy complexes with oversized impurities in Si and Ge
In this paper we examine the electronic and geometrical structure of
impurity-vacancy complexes in Si and Ge. Already Watkins suggested that in Si
the pairing of Sn with the vacancy produces a complex with the Sn-atom at the
bond center and the vacancy split into two half vacancies on the neighboring
sites. Within the framework of density-functional theory we use two
complementary ab initio methods, the pseudopotential plane wave (PPW) method
and the all-electron Kohn-Korringa-Rostoker (KKR) method, to investigate the
structure of vacancy complexes with 11 different sp-impurities. For the case of
Sn in Si, we confirm the split configuration and obtain good agreement with EPR
data of Watkins. In general we find that all impurities of the 5sp and 6sp
series in Si and Ge prefer the split-vacancy configuration, with an energy gain
of 0.5 to 1 eV compared to the substitutional complex. On the other hand,
impurities of the 3sp and 4sp series form a (slightly distorted) substitutional
complex. Al impurities show an exception from this rule, forming a split
complex in Si and a strongly distorted substitutional complex in Ge. We find a
strong correlation of these data with the size of the isolated impurities,
being defined via the lattice relaxations of the nearest neighbors.Comment: 8 pages, 4 bw figure
Multichannel parametrization of \pi N scattering amplitudes and extraction of resonance parameters
We present results of a new multichannel partial-wave analysis for \pi N
scattering in the c.m. energy range 1080 to 2100 MeV. This work explicitly
includes \eta N and K \Lambda channels and the single pion photoproduction
channel. Resonance parameters were extracted by fitting partial-wave amplitudes
from all considered channels using a multichannel parametrization that is
consistent with S-matrix unitarity. The resonance parameters so obtained are
compared to predictions of quark models
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