Sessions - DPG-Tagungen

Nuclear Physics Tuesday
HK 11.13 Tue 13:30 Foyer
Study of the Bremsstrahlung radiation in the quasi-free np → np
γ reaction — •Joanna Przerwa for the COSY-11 collaboration —
Institute of Physics, Jagellonian University, Cracow, PL-30-059 Poland
Using a deuteron beam and a proton target the quasi-free np → np
γ reaction has been studied at the COSY-11 facility, an internal magnetic
spectrometer system at the cooler synchrotron COSY in Jülich.
Data have been taken at a beam momentum of 3.165 GeV/c close to the
threshold of the dp → dp η process. Events corresponding to the dp → p
n γ ps reaction have been identified by measuring the outgoing charged
as well as neutron ejectiles. The fast protons are detected by means of
drift chambers and scintillator hodoscopes and the slow spectator protons
are measured in a Si-pad arrangement close to the target. Neutrons
and photons are registered in a scintillator/lead sandwich detector. The
momentum vectors of the protons are reconstructed by tracking back
the trajectories to the target point and the momentum of the neutron
is calculated from the time-of-flight between target and neutron detector.
For the calibration of the neutron detector the produced photons
are used. The efficiency is determined by Monte Carlo studies supported
by calibration measurements performed simultaneously to other reaction
studies by selecting special reaction channels relevant for the neutron
detector.
Results of the data analysis and the experimental techniques will be
presented and discussed.
Supported by Forschungszentrum Jülich.
HK 11.14 Tue 13:30 Foyer
Study of the d−η interaction via dp → dpη reaction — •C. Piskor-
Ignatowicz for the COSY-11 collaboration — Institute of Physics, Jagellonian
University, Cracow, PL-30-059 Poland
Recently the deuteron-η interaction was intensively studied on theoretical
ground. This topic is of special interest due to the possible existence
of η-nucleus bound or quasibound states. The COSY-11 collaboration
preformed measurements of the near-threshold η meson production in
the dp → dpη reaction. The aim of the experiment was the investigation
of the d−η interaction by studying the energy dependence of the total reaction
cross section as well as Dalitz plot distributions for the final state
particles. Measurements were preformed for four excess energies in the
range from 0.2 MeV to 9.6 MeV. The momenta of the outgoing protons
and deuterons were registered with the COSY-11 detection system and
the η-mesons were identified via the missing mass method. Preliminary
results of these measurements will be presented.
Supported by Forschungszentrum Jülich.
HK 11.15 Tue 13:30 Foyer
Threshold Production of Σ + at COSY-11 — •T. Ro˙zek for the
COSY-11 collaboration — Institue of physics, Silesian University Katowice,
IKP Forschungszentrum Jülich
At the cooler synchrotron COSY the threshold production of hyperons
has been studied via the pp → pK + Λ(Σ 0 ) reaction [1] resulting in a
cross section ratio R = σ(Λ)/σ(Σ 0 ) of about 28 which exceeds the ratio
at high energies by an order of magnitude. Possible explanations within
different models are proposed [2] but a definite conclusion is up to now
not possible. Further studies in other isospin channels like the Σ + production
will help in the clarification of the relevant dominant reaction
mechanisms.
The Σ + production was measured at the COSY-11 installation via the
pp → nK + Σ + reaction at Q = 13 MeV and Q = 60 MeV. COSY-11 is
an internal magnetic spectrometer experiment at the COoler SYnchroton
and storage ring COSY in Jülich. It is equipped with scintillator
hodoscopes and drift chambers for charged particle detection and a scintillator/lead
sandwich detector for neutrons.
The status of the current analysis will be presented and discussed.
[1] J.T.Balewski et al.,PLB420(1998) 211, S. Sewerin et al.,PRL 83 (1999)
682., P. Kowina, PhD thesis 2002.
[2] A. Gasparian et al., PLB 480 (2000) 273, NPA684:397,2001, R. Shyam
et al., PRC63 (2001) 022202, A. Sibirtsev et al. NPA646 (1999) 427, nuclth/0004022
v2 (2000).
Supported by Forschungszentrum Jülich.
HK 11.16 Tue 13:30 Foyer
The ANKE Silicon Tracking Telescopes as a tool for Polarimetry
and Luminosity Determination — •A. Mussgiller for the
ANKE collaboration — Institut für Kernphysik, Forschungszentrum
Jülich, Germany
The first two ANKE Silicon Spectator Tracking Telescopes have been
installed several centimeter close to the COSY beam inside the accelerator
vacuum. Their design is optimized for identification and tracking of
low energy spectator protons emitted from an internal deuterium target.
In addition, the large acceptance at large scattering angles, allows to extend
the ANKE capabilities to study pp- and pd-elastic reactions and
to cross-check the obtained data, detecting both particles. The pp-free
and quasi-free reaction is an appropriate tool to determine the luminosity
over the complete COSY energy range. Moreover, it seems to be
well-suited as a beam-polarimeter. This technique is exploited together
with the possibilities offered by the acceleration of polarized deuterons
at COSY, to measure the � dp → (pp)1 S0 n charge–exchange reaction near
the forward direction.
This work is supported by the FZJ.
HK 11.17 Tue 13:30 Foyer
Strangeness production in proton - nucleus interactions at
ANKE/COSY — •Y. Valdau for the ANKE collaboration —
Forschungszentrum Jülich, 52425 Jülich
ANKE is a magnetic spectrometer and detection system at an internal
target position of COSY-Jülich. The device permits to momentum
analyze ejectiles from hadronic interactions with forward emission angles
around 0 ◦ . A major goal of the experimental program at ANKE
is the investigation of proton-induced strangeness production in the nuclear
medium. In a recent measurements the production of K + -mesons
accompanied by protons or deuteron has been investigated at different
beam energies Tp = 1.0, 2.0, 2.3 GeV. These data may shed light on the
reaction mechanism leading to kaon production in nuclei. Particularly,
they supply direct evidence for the two-step reaction mechanism, with
formation of intermediate pions, leading to kaon production. In addition,
measurements with a deuterium target suggest that production cross section
on neutron exceeds that on proton, and the corresponding ratio is
close to 4. Supported by FZJ, WTZ, DFG.
HK 11.18 Tue 13:30 Foyer
Investigation of the Scalar Resonance a + 0 (980) at ANKE ∗ — •V.
Kleber for the ANKE collaboration — Institut für Kernphysik, Universität
zu Köln, 50937 Köln / Forschungszentrum Jülich, 52425 Jülich
The scalar resonances a0/f0(980) have been observed in many experiments.
However, their nature is not yet understood and they are interpreted,
e.g., as genuine mesons (and thus members of the scalar nonet),
K ¯K molecules or compact qq–¯q¯q states. At COSY–Jülich an experimental
program has been started, using the ANKE spectrometer, which aims
at exclusive data on a0/f0 production from pp, pn, pd and dd interactions
close to the K ¯ K threshold. The final goal will be the extraction
of the charge-symmetry breaking a0/f0 mixing amplitude to shed light
on the nature of the light scalar resonances. Here, we report about
the first exclusive study of the reactions pp→dK + ¯ K 0 and pp→dπ + η.
The measurements were performed at beam energies of Tp=2.65 and
2.83GeV, corresponding to excess energies of 46 and 103MeV above the
K ¯ K threshold. From the measurement at the lower energy the total cross
section of the reaction pp→dK + ¯ K 0 as well as differential distributions
have been extracted. The main contribution to this reaction is interpreted
as a0→K + ¯ K 0 decays. The large background contribution from
multiple-pion production to the reaction pp→dπ + η makes the analysis
more complex than in the case of kaon production. The data at the
higher energies are still under analysis, and the current status will be
presented. ∗ supported by FZJ, BMBF and DPG