Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Tuesday
HK 12.30 Tue 10:30 Foyer Chemie
Search for Narrow Nucleon Resonances Below Pion Threshold
— •M. Kohl, C. Rangacharyulu, A. Richter, andG. Schrieder
for the A1 collaboration — Institut f. Kernphysik, TU Darmstadt
The question if resonance states exist in the invariant mass region between
the nucleon mass and the pion threshold is of particular importance
for the structure of the nucleon. Since the decay energy of such states is
not large enough to produce a pion, only electromagnetic or weak decay
is possible, leading to extremely narrow decay widths. Recently, some
experimental indications for such resonances have been reported. In pp
scattering, narrow excited states of the nucleon at 1004, 1044, and 1094
MeV/c 2 have been observed [1]. The high-resolution three-spectrometer
facility of the A1 collaboration at the Mainz Microtron MAMI is particularly
suited to verify the existence of such narrow nucleon resonances. A
test experiment in the 1 H(e,e’π + )X reaction provided first indication for
a state at MX = 1007 MeV/c 2 - yet with small significance. The status
of the measurements will be reported.
[1] B. Tatischeff et al., Phys. Rev. Lett. 79 (1997) 601
Supported by DFG under contract RI 242/15-2.
HK 12.31 Tue 10:30 Foyer Chemie
Investigation of the Polarisation Asymmetry in Eta-
Photoproduction on Protons in the GDH-Experiment at ELSA
— •Melitta Godo and Thilo Michel for the GDH-Kollaboration
collaboration — Physikalisches Institut IV, Erwin-Rommel-Str. 1, 91058
Erlangen
An analysis of the data taken by the GDH-Experiment at ELSA in
the energy range between 700 and 1000 MeV has shown the possibility
to extract information about the asymmetry of the eta-photoproduction.
The reaction is identified by a missing-mass-method. With regard to
this method, the acceptance of the GDH-detector for this particular reaction
channel has been simulated. This presentation outlines the acceptance
calculations and first experimental data of an asymmetry in
eta-photoproduction.
HK 12.32 Tue 10:30 Foyer Chemie
Strange decays from excited states of the nucleon — •Ralph
Castelijns, J. Bacelar, and H. Löhner for the Crystal Barrel/TAPScollaboration
— Kernfysisch Versneller Instituut, Groningen,
The Netherlands
The spectrum of excited states of the nucleon can provide essential
information on the quark-gluon structure of hadrons. Complete information
on various decay channels is mandatory, however some channels have
hardly been explored. In particular, decays with many photons in the
final state are difficult to access. We exploit the opportunity of a joint
HK13 Poster Session: Heavy Ions
experiment with the TAPSphoton spectrometer (528 BaF2 crystals) and
the Crystal Barrel detector (1290 CsI detectors) at the tagged photon
beam of ELSA in Bonn to study the barely known decay of excited nucleon
states into a neutral K meson and a Sigma hyperon. This decay
channel may reveal unknown resonances, in particular one expected near
the production threshold. The experiment requires the measurement of
six photons and a proton in the final state. We report on the feasibility
studies and present first results from a commissioning experiment with
photonuclear reactions on the proton.
HK 12.33 Tue 10:30 Foyer Chemie
High-Precision Measurements of Proton-Proton
Bremsstrahlung — •M. Mahjour-Shafiei 1 , H. Amir-Ahmadi 1 ,
J.C.S. Bacelar 1 , R. Castelijns 1 , K. Ermisch 1 , I. Gaˇsparić 2 ,
M.N. Harakeh 1 , N. Kalantar-Nayestanaki 1 , M. Kiˇs 1 , and
H. Löhner 1 — 1 Kernfysisch Versneller Instituut, Groningen, The
Netherlands — 2 Institut Rugjer Boˇsković, Zagreb, Croatia
The understanding of the strong force acting between nucleons is one
of the most fundamental problems addressed by nuclear physics. One
of the simplest reactions to study this force, besides elastic scattering,
is proton-proton Bremsstrahlung. In 1996 a series of experiments were
set up at KVI to study real and virtual proton-proton bremsstrahlung
at 190 MeV in which the kinematics was chosen such that one goes as
far away as possible from the elastic channel, thereby producing higher
energy photons. In continuation of that work and in order to cover a
much larger area of the available phase space, a new set up employing
the SALAD and the Plastic-Ball detectors together was used with which
much smaller photon energies were measured, thus moving toward the
elastic channel. The preliminary results of the first measurements will
be presented.
HK 12.34 Tue 10:30 Foyer Chemie
Investigation of the Polarisation Asymmetry in Eta-Photoproduction
on Protons in the GDH-Experiment at ELSA —
•Melitta Godó and Thilo Michel — Physikalisches Institut IV,
Erwin-Rommel-Str. 1, 91058 Erlangen
An analysis of the data taken by the GDH-Experiment at ELSA in
the energy range between 700 and 1000 MeV has shown the possibility
to extract information about the asymmetry of the eta-photoproduction.
The reaction is identified by a missing-mass-method. With regard to
this method, the acceptance of the GDH-detector for this particular reaction
channel has been simulated. This presentation outlines the acceptance
calculations and first experimental data of an asymmetry in
eta-photoproduction.
Time: Tuesday 10:30–12:45 Room: Foyer Chemie
HK 13.1 Tue 10:30 Foyer Chemie
Close dilepton pair rejection strategies for HADES — •Jaroslav
Bielcik —GSI,Darmstadt
The second-generation dilepton spectrometer HADESat GSI has recently
started to take physics data from heavy-ion induced reactions in
the 1-2 AGeV energy range. The main contribution to the combinatorial
background in the measured e + e− continuum comes from conversion
pairs induced by π0 decay photons.The identification of these pairs and
their removal from the track sample decreases significantly the combinatorial
background to be subtracted from the reconstructed e + e − mass
distribution in order to isolate the vector meson e + e − signal. A full simulation
of the HADESspectrometer response to conversion pairs has been
performed and was used to develop efficient strategies based on a cluster
analysis in MDC and RICH subdetectors to identify such pairs.Our
methods will be presented and results will be discussed.
HK 13.2 Tue 10:30 Foyer Chemie
The HADES second level trigger algorithm: principles and first
results from experiments with 12 C beam — •Alberica Toia,
Wolfgang Kuehn, James Ritman, Joerg Lehnert, Markus
Petri, Michael Traxler, Ingo Froehlich, Daniel Kirschner,
Daniel Schaefer, andAdrian Gabriel for the HADEScollaboration
— II. Physikalisches Institut, Giessen
The dilepton spectrometer HADESat GSI Darmstadt investigates lepton
decays of vector mesons in elementary and heavy ion reactions. Since
the branching ratio of these decays is in the order of 10 −5 , a highly selective
real time trigger is needed in order to improve the quality of data and
the statistics. This trigger consists of 3 Image Processing Units which
perform pattern recognition to detect lepton signatures in different subdetectors
and a Matching Unit which combines the position information
into tracks to make the final decision. To investigate and characterize
the functionality of this trigger, all hardware components have been emulated
with software. After testing the emulation with simulated and
real data, the performance of the hardware trigger has been investigated
with data from recent beamtimes. The goals, methods and results of this
analysis will be presented.
HK 13.3 Tue 10:30 Foyer Chemie
Correlation studies with INDRA@GSI — •C. Schwarz for the
ALADIN-INDRA collaboration —
GSI, Darmstadt
Correlation funcions, constructed from light-particle coincidences, give
access to the spatial extension of the emitting source. This is due to the
final-state interaction of the emitted particle pairs which depend on their
spatial proximity. Using this technique, the interaction zone as well as
the quasi-projectile and quasi-target sources of light particles were inves-