Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Thursday
pretation will be descussed.
[1] T. Wise et al., Phys. Rev. Lett. 87, 042701 (2001).
[2] B. Lorentz et al., Phys. Rec. C 61 54002(2000).
HK 42.5 Thu 15:15 F
A Lamb-shift Polarimeter for the Polarized Gas Target at
ANKE/COSY — •Ralf Engels 1 , Reinhard Emmerich 1 , Jürgen
Ley 1 , Hans Paetz gen. Schieck 1 , Maxim Mikirtytchiants 2,3 ,
Frank Rathmann 2 , Hellmut Seyfarth 2 ,andAlexandre Vassiliev
3 — 1 Institut für Kernphysik der Universität zu Köln, Zülpicher
Str.77, 50937 Köln — 2 Institut für Kernphysik, FZ Jülich, Leo-Brandt-
Str., 52425 Jülich — 3 High Energy Physics Dept., St. Petersburg Nucl.
Phys. Inst., 188350 Gatchina, Russia
With a Lamb-shift polarimeter it is possible to measure the occupation
HK43 Theory VII
numbers of the individual hyperfine substates in a beam of hydrogen or
deuterium atoms. Therefore one can calculate the nuclear polarization of
the atomic beam in magnetic fields of varying strength. The polarization
of a slow (500-2000 eV) ion beam can be measured as well.
The modular components of the polarimeter consisting of a Glavishtype
ionizer, a Wienfilter, a Cs cell, a spinfilter, and a quenching region
were designed, produced and then tested at the Universität zu Köln with
an unpolarized (horizontal) ion beam. At the Forschungszentrum Jülich
the tests were completed by measuring the necessary correction factors
and the polarization of the vertical atomic beam of the source for the
polarized gas target at ANKE/COSY with a 90˚ deflector behind the
ionizer. First results obtained with a polarized hydrogen and deuterium
beam are presented. Planned studies to investigate the polarization of
the gas in storage cells are discussed as well.
Time: Thursday 16:00–18:00 Room: A
Group Report HK 43.1 Thu 16:00 A
Two-pion production on the nucleon — •Sonja Schneider,
Siegfried Krewald, andJosef Speth — Institut für Kernphysik,
FZ Jülich
We developed a meson-theoretical model for the pion-induced two-pion
production on the nucleon. In a first step, our model was formulated
strictly at the tree level. Accounting for ππ final state interaction, the
exchange of a σ- oraρ-meson was replaced by the exchange of correlated
two-pion states. With this second version we already achieved a good
description of the data except for the ∆33-dominated π + p → π + π + n and
π + p → π + π 0 p channels. In a third step, a simple treatment of threebody
unitarity has to be implemented, accounting in particular for the
unitarization of the πN P33 partial wave.
Group Report HK 43.2 Thu 16:30 A
The baryon spectrum in a covariant quark model with
instanton-induced forces — •Ulrich Löring, Dirk Merten,
Bernard Metsch, Herbert-R. Petry, andChristian Haupt —
Institut für Theoretische Kernphysik, Univerität Bonn, Nußallee 14-16,
53115 Bonn
On the basis of the Bethe-Salpeter equation in instantaneous approximation
we formulated a relativistic quark model for baryons. Motivated
by the success of the non-relativistic quark model, the full quark propagators
are replaced by their free forms with constituent quark masses,
and the interactions of the quarks are described by unretarded, static potentials.
To generate the hyperfine structure of the baryon spectrum we
adopt ’t Hooft’s force which is derived from QCD-instanton-effects. This
relativistic model allows a very successful description of the complete
baryon spectrum up to 3 GeV with spins up to J =15/2. In particular
several prominent features such as the linear Regge-trajectories, the low
position of the Roper resonance (and its strange counterparts) as well
as the striking phenomenon of approximate parity doublets can be uniformly
explained. In this respect the specific role of instanton-effects is
discussed, and we demonstrate that the alternative one-gluon-exchange,
however, is not able to reproduce the excited baryon spectrum correctly.
HK 43.3 Thu 17:00 A
Relativistic Mean Field Theory with Generalized Nucleon-
Meson Couplings — •Stefan Typel 1 und Hermann Wolter 2 —
1 NSCL/Michigan State University, USA — 2 University of Munich, Ger-
many
Nuclear matter and finite nuclei can be qualitatively well described in
quantum hadrodynamics (QHD) with constant meson-nucleon couplings
in the mean field approximation. For a quantitative description an effective
medium dependence has to be introduced, e.g. by assuming nonlinear
meson self-couplings or a density dependence of the nucleon-meson couplings.
From Dirac-Brueckner theory of nuclear matter it is well known
on the other hand that the nuclear self energies are both density and
momentum dependent. The momentum dependence is essential for a
description of elastic proton-nucleus scattering. We therefore introduce
generalized nucleon-mesons couplings in the QHD-Lagrangian which lead
to density and momentum dependent self energies. Two models are considered.
In the first model we investigate couplings of the mesons fields
to derivatives of the nucleon field. This leads to new source terms in the
field equations of the mesons and density-dependent meson masses. In
the second model the meson-nucleon couplings are assumed to be functions
of derivative densities generating rearrangement contributions in the
self-energies. We suggest parametrizations of the generalized couplings
and study their effect on the equation of state of nuclear matter and the
energy dependence of the Schrödinger equivalent optical potential.
HK 43.4 Thu 17:15 A
Short-ranged Central and Tensor Correlations in the Nuclear
Many-Body System — •Thomas Neff, Hans Feldmeier, and
Robert Roth — Gesellschaft für Schwerionenforschung, Darmstadt
Realistisc nucleon-nucleon interactions have a strong repulsive core and
a strong tensor force. We introduce a unitary correlation operator that
takes care of the short-ranged central and tensor correlations induced by
the nuclear force. This unitary correlation operator allows us to perform
ab initio calculations of nuclei up to A ≈ 50 in a mean-field or
shell model many-body approach. The unitary correlation operator is
the product of a central correlation operator that performs radial shifts
in the two-body density and a tensor correlation operator that aligns the
two-body density with the total spin of two nucleons. An effective interaction
can be defined by correlating the bare nuclear interaction. The
unitary correlation operator method provides a method to extract the
common low-energy behavior of realistic nuclear interactions.
HK 43.5 Thu 17:30 A
Nucleon-Nucleon potential from two body Dirac equations at
medium energies — •Davaadorj Bayansan 1 , Andreas Funk 1 ,
Bin Liu 2 , Horace W. Crater 2 , Heinrich V. von Geramb 1 ,and
Hugo Arellano 3 — 1 Nuclear Theory , University Hamburg — 2 Space
Institute, University Tennessee, Tullahoma — 3 Fisica, Universidad de
Chile, Santiago
We investigate the implication and use of Dirac’s instant form dynamics
in applications of two-body Dirac equations. First, it yields a center
of momentum reduction of the two-body Dirac equations to Schrödingerlike
equations with effective energy dependent potentials. Second, a link
to known and optimally fitted NN potentials (Nijmegen,AV18) as well as
quantum inversion potentials is made. Third, the effective potentials are
extented, above meson production threshold, as NN optical potentials.
The latest NN phase shifts up to 3 GeV are fitted. Some application of
the resulting NN potentials in medium energy nucleon-nucleus scattering,
formulated in terms of NA full folding optical models, are also shown.
HK 43.6 Thu 17:45 A
Production of the f0(980) in the reaction π − p → π 0 π 0 n — •Felix
Sassen, Siegfried Krewald, andJosef Speth — Forschungszentrum
Jülich, IKP (Th), D-52425 Jülich
Lately the E852-Collaboration at Brookhaven published data on pion
production in the charge exchange reaction π − p → π 0 π 0 n. [1] The published
S-wave mass spectrum taken at low momentum transfer t to the
nucleon shows a sharp dip at mππ ≈ 1 GeV where as the same spectrum
taken at high t exhibits a sharp peak at the same position.
This behaviour sheded some doubt on the interpretation of the f0(980)
as a K ¯ K molecule.[2] We show using a model with two production mechanisms
(π- anda1-exchange) that the above experimental results do not
oppose the K ¯ K interpretation of the f0(980). In our model the final state