469 research outputs found
Model-independent approach to eta -> pi+ pi- gamma and eta' -> pi+ pi- gamma
We present a new, model-independent method to analyze radiative decays of
mesons to a vector, isovector pair of pions of invariant mass square below the
first significant pion-pion threshold in the vector channel. It is based on a
combination of chiral perturbation theory and dispersion theory. This allows
for a controlled inclusion of resonance physics without the necessity to
involve vector meson dominance explicitly. As an example, the method is applied
to an analysis of the reactions eta -> pi+ pi- gamma and eta'->pi+ pi- gamma.Comment: 16 pages, 4 figure
Dispersion relations and speeds of sound in special sectors for the integrable chain with alternating spins
Based on our previous analysis \cite{doerfel3} of the anisotropic integrable
chain consisting of spins and we compare the dispersion relations
for the sectors with infinite Fermi zones. Further we calculate the speeds of
sound for regions close to sector borders, where the Fermi radii either vanish
or diverge, and compare the results.Comment: 11 pages, LaTeX2e, uses iopart.cls,graphicx.sty and psfrag.sty, 2
figure
Ab initio calculation of the Hoyle state
The Hoyle state plays a crucial role in the hydrogen burning of stars heavier
than our sun and in the production of carbon and other elements necessary for
life. This excited state of the carbon-12 nucleus was postulated by Hoyle [1]
as a necessary ingredient for the fusion of three alpha particles to produce
carbon at stellar temperatures. Although the Hoyle state was seen
experimentally more than a half century ago [2,3], nuclear theorists have not
yet uncovered the nature of this state from first principles. In this letter we
report the first ab initio calculation of the low-lying states of carbon-12
using supercomputer lattice simulations and a theoretical framework known as
effective field theory. In addition to the ground state and excited spin-2
state, we find a resonance at -85(3) MeV with all of the properties of the
Hoyle state and in agreement with the experimentally observed energy. These
lattice simulations provide insight into the structure of this unique state and
new clues as to the amount of fine-tuning needed in nature for the production
of carbon in stars.Comment: 4 pp, 3 eps figs, version accepted for publication in Physical Review
Letter
Improved Analysis of J/psi Decays into a Vector Meson and Two Pseudoscalars
Recently, the BES collaboration has published an extensive partial wave
analysis of experimental data on J/psi -> phi pi+pi-, J/psi -> omega pi+pi-,
J/psi -> phi K+K- and J/psi -> omega K+K-. These new results are analyzed here,
with full account of detection efficiencies, in the framework of a chiral
unitary description with coupled-channel final state interactions between pi-pi
and K-bar K pairs. The emission of a dimeson pair is described in terms of the
strange and nonstrange scalar form factors of the pion and the kaon, which
include the final state interaction and are constrained by unitarity and by
matching to the next-to-leading-order chiral expressions. This procedure allows
for a calculation of the S-wave component of the dimeson spectrum including the
f_0(980) resonance, and for an estimation of the low-energy constants of Chiral
Perturbation Theory, in particular the large N_c suppressed constants L_4^r and
L_6^r. The decays in question are also sensitive to physics associated with OZI
violation in the 0++ channel. It is found that the S-wave contributions to phi
pi+pi-, phi K+K- and omega pi+pi- given by the BES partial-wave analysis may be
very well fitted up to a dimeson center-of-mass energy of ~1.2 GeV, for a large
and positive value of L_4^r and a value of L_6^r compatible with zero. An
accurate determination of the amount of OZI violation in the J/psi -> phi
pi+pi- decay is achieved, and the S-wave contribution to omega K+K- near
threshold is predicted.Comment: 18 pages, 6 figures, title changed, accepted version for PR
Dynamical coupled-channels model study of pion photoproduction
The photoproduction of pion off nucleon is investigated within a dynamical
coupled-channels approach based on the Juelich pi-N model, which has been quite
successful in the description of pi-N to pi-N scattering for center-of-mass
energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed
to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge
invariant. The calculated differential cross sections and photon spin
asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to
pi^+ n, gamma p to pi^0 p and gamma n to pi^- p are in good agreement with the
experimental data.Comment: Talk given at The 8th International Workshop on the Physics of
Excited Nucleons (NSTAR2011), May 17-20, 2011, Thomas Jefferson National
Accelerator Facility, Newport News, Virginia, US
Photocouplings at the Pole from Pion Photoproduction
The reactions and are analyzed in a
semi-phenomenological approach up to GeV. Fits to differential cross
section and single and double polarization observables are performed. A good
overall reproduction of the available photoproduction data is achieved. The
J\"ulich2012 dynamical coupled-channel model -which describes elastic
scattering and the world data base of the reactions ,
, and at the same time - is employed as the hadronic
interaction in the final state. The framework guarantees analyticity and, thus,
allows for a reliable extraction of resonance parameters in terms of poles and
residues. In particular, the photocouplings at the pole can be extracted and
are presented.Comment: 37 pages, 31 figures. Angles of the photocouplings at the pole
adapted to the convention of Phys. Rev. C 87, 068201 (2013
Coupled-channel dynamics in the reactions piN --> piN, etaN, KLambda, KSigma
Elastic piN scattering and the world data of the family of reactions pi^- p
--> eta n, K^0 Lambda$, K^0 Sigma^0, K^+ Sigma^-, and pi^+ p --> K^+ Sigma^+
are described simultaneously in an analytic, unitary, coupled-channel approach.
SU(3) flavor symmetry is used to relate both the t- and the u- channel
exchanges that drive the meson-baryon interaction in the different channels.
Angular distributions, polarizations, and spin-rotation parameters are compared
with available experimental data. Partial-wave amplitudes are determined and
the resonance content is extracted from the analytic continuation, including
resonance positions and branching ratios, and possible sources of uncertainties
are discussed. The results provide the final-state interactions for the ongoing
analysis of photo- and electroproduction data.Comment: 53 pages, 48 figures; results updated to published versio
Vernetzt-kooperative Planung mit Computern - Grundlagen und Methoden der Bauinformatik
In Anbetracht der leistungsstarken Computernetzwerke und der intensiven Globalisierung von Wissenschaft und Technik im Bauwesen ist es das Ziel der Bauinformatik, die Planungsprozesse des Konstruktiven Ingenieurbaus für die Nutzung verteilter Ressourcen zu gestalten, geeignete Kooperationsmodelle für die Fachplanung im Informationsverbund der Projektbeteiligten zu entwickeln und die kooperative Projektbearbeitung unter Nutzung verteilter Fachmodelle in Netzen zu ermöglichen. Dazu werden neue methodische Grundlagen geschaffen, welche die tiefgreifenden Veränderungen der Arbeits- und Organisationsstrukturen berücksichtigen. Vor diesem Hintergrund zeigt der Beitrag aktuelle Forschungsinhalte der Bauinformatik auf, die insbesondere im Rahmen des DFG-Schwerpunktprogramms 1103 'Vernetzt-kooperative Planungsprozesse im Konstruktiven Ingenieurbau' von verschiedenen Projekten arbeitsteilig entwickelt werden. Es wird ein integratives Prozessmodell vorgestellt, das die Grundlage für die Unterstützung der vernetzt-kooperativen Planung mit modernen Methoden der Informationsverarbeitung und Kommunikationstechnik zusammenführt. An einem Beispiel aus dem baulichen Brandschutz werden die methodischen Ansätze der Prozessmodellierung mit Petri-Netzen und der mobilen Softwareagenten zur Unterstützung der Kooperation in der Fachplanung des Bauwesens dargestellt und erläutert
Towards an understanding of isospin violation in pion-nucleon scattering
We investigate isospin breaking in low-energy pion-nucleon scattering in the
framework of chiral perturbation theory. This work extends the systematic
analysis of [1] to the energy range above threshold. Various relations, which
identically vanish in the limit of isospin symmetry, are used to quantify
isospin breaking effects. We study the energy dependence of the S- and P-wave
projections of these ratios and find dramatic effects in the S-waves of those
two relations which are given in terms of isoscalar quantities only. This
effect drops rather quickly with growing center-of-mass energy.Comment: 12 pp, REVTeX, 8 figs, FZJ-IKP(TH)-2000-2
Ein Dokumentmodell für Kursdokumente in Webbasierten Virtuellen Lernumgebungen
aus der Einführung:
'Virtuelle Lernumgebungen ermöglichen Aus- und Weiterbildung unabhängig von Zeit und Raum, z.B. können Berufstätige während wie auch nach der Arbeitszeit Kurse über ihren PC mit Internetzugang absolvieren und dabei verteilte Lerngruppen und Projektteams bilden. Eine Vielzahl anderer Szenerien virtueller Lernumgebungen sind denkbar. Faktoren, die diese Entwicklung massiv unterstützen, sind der Verbreitungsgrad multimedialer PCs - ca. 7 Mio. Personen über 14 Jahren hatten 1998 in Deutschland beruflich oder privat Zugriff auf Onlinedienste bzw. das Internet - und insbesondere die Dynamik, mit der sich das Wissen in bestimmten Fachgebieten verändert und damit ständige berufliche Weiterbildung erfordert. Die Informations- und Multimediatechnik sind hierfür typische Beispiele. Dieser Entwicklung kann mit klassischen Lehrmethoden und -medien, z.B. Fortbildungsseminaren und dem Lehrbuch, nicht adäquat Rechnung getragen werden. Virtuelle Lernumgebungen bieten deshalb eine zeitgemäße Möglichkeit der kostengünstigen und effizienten Aus- und Weiterbildung
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