294 research outputs found
Meson-meson correlations in baryon-baryon and antibaryon-baryon interactions
Recent work of the J\"ulich group about the role of meson-meson correlations
in baryon-baryon and antibaryon-baryon interactions is reviewed.Comment: Lecture given at the Erice School 1995, TEX, 10 pages, 15 figure
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
Proton Sea Quark Flavour Asymmetry and Roper Resonance
We study the proton and the Roper resonance together with the meson cloud
model, by constructing a Hamiltonian matrix and solving the eigenvalue
equation. The proton sea quark flavour asymmetry and some properties of the
Roper resonance are thus reproduced in one scheme
Consistent analysis of the reaction and
The production of mesons in the reactions and
is described consistently within a relativistic meson exchange
model of hadronic interactions. The photoproduction can be described quite well
over the entire energy range of available data by considering an and a
resonance, in addition to the -channel mesonic current. The
observed angular distribution is due to the interference between the
-channel and the nucleon resonance - and -channel contributions. Our
analysis yields positions close to 1650 MeV and 1870 MeV for the and
resonances, respectively. We argue that, at present, identifying these
states with the known resonance and the missing
resonance predicted at 1880 MeV, respectively, would be premature. It is found
that the nucleonic current is relatively small and that the
coupling constant cannot be much larger than . As for the
reaction, different current contributions are
constrained by a combined analysis of this and the photoproduction reaction.
Difficulties to simultaneously account for the 47-MeV and 144-MeV angular
distributions measured by the COSY-11 and DISTO collaborations, respectively,
are addressed.Comment: minor revision, scheduled to a appear in Phys. Rev. C 69 (May 2004),
revtex, 17 pages, 10 figures, 3 table
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
The Role of in Two-pion Exchange Three-nucleon Potential
In this paper we have studied the two-pion exchange three-nucleon potential
using an approximate chiral symmetry of the
strong interaction. The off-shell pion-nucleon scattering amplitudes obtained
from the Weinberg Lagangian are supplemented with contributions from the
well-known -term and the exchange. It is the role of the
-resonance in , which we have investigated in detail in the
framework of the Lagrangian field theory. The -contribution is quite
appreciable and, more significantly, it is dependent on a parameter Z which is
arbitrary but has the empirical bounds . We find that the
-contribution to the important parameters of the depends
on the choice of a value for Z, although the correction to the binding energy
of triton is not expected to be very sensitive to the variation of Z within its
bounds.Comment: 14 pages, LaTe
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
Covariant Quark Model for the Baryons
A family of simply solvable covariant quark models for the baryons is
presented. With optimal parameter choices the models reproduce the empirical
spectra of the baryons in all flavor sectors to an accuracy of a few percent.
Complete spectra are obtained for all states of the strange, charm and beauty
hyperons with . The magnetic moments and axial coupling constants of
the ground state baryons correspond to those of conventional quark models. We
construct current-density operators that are consistent with empirical nucleon
form factors at low and medium energies.Comment: 32pages, LateX, 3 figures(postscript
reaction near threshold
We analyze the total cross section data for near threshold
measured recently at SATURNE. Using an effective range approximation for the
on-shell S-wave final state interaction we extract from these data the
modulus fm of the threshold transition amplitude
. We present a calculation of various (tree-level) meson exchange
diagrams contributing to . It is essential that -emission from
the anomalous -vertex interferes destructively with
-emission from the proton lines. The contribution of scalar
-meson exchange to turns out to be negligibly small. Without
introducing off-shell meson-nucleon form factors the experimental value
fm can be reproduced with an -coupling constant
of . The results of the present approach agree qualitatively
with the J\"ulich model. We also perform a combined analysis of the reactions
and near threshold.Comment: Latex-file 6 pages, 2 Figure
Present status of the nonstrange and other flavor partners of the exotic Theta+ baryon
Given the existing empirical information about the exotic Theta+ baryon, we
analyze possible properties of its SU(3)F-partners, paying special attention to
the nonstrange member of the antidecuplet N*. The modified piN partial-wave
analysis presents two candidate masses, 1680 MeV and 1730 MeV. In both cases,
the N* should be rather narrow and highly inelastic. Our results suggest
several directions for experimental studies that may clarify properties of the
antidecuplet baryons, and structure of their mixing with other baryons. Recent
experimental evidence from the GRAAL and STAR Collaborations could be
interpreted as observations of a candidate for the Theta+ nonstrange partner.Comment: 4 pages, 1 figure, talk given at the Topical Group on Hadron Physics
(Fermilab, Oct. 24-26, 2004
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