72 research outputs found
New results on the pd --> 3He eta production near threshold
Measurements on the eta meson production in proton-deuteron collisions have
been performed using the COSY-11 facility at COSY Juelich. Here we present
preliminary results on total and differential cross sections for the pd --> 3He
eta reaction at five excess energies between Q = 5.1 and Q = 40.6 MeV. The
obtained angular distributions for the emitted eta mesons in the center of mass
system expose a transition from an almost isotropic emission to a highly
anisotropic distribution. The extracted total cross sections support a strong
eta-3He final state interaction and will be compared with model predictions.Comment: Presented at MESON 2004: 8th International Workshop on Meson
Production, Properties and Interactions, Cracow, Poland, 4-8 Jun 2004, 3
pages, Submitted to Int. J. Mod. Phys.
Threshold Hyperon Production at COSY-11
The Lambda, Sigma0 and Sigma+ hyperon production in NN collisions is studied
at the COSY - 11 installation in order to investigate the production mechanism
as well as to extract information about the Y-N interaction.Comment: 3 pages, 2 figure
Search for Bremsstrahlung radiation in quasi-free n p --> n p gamma reactions
Due to the high sensitivity of the N N --> N N gamma reaction to the
nucleon-nucleon potential, Bremsstrahlung radiation is used as a tool to
investigate details of the nucleon-nucleon interaction. Such investigations can
be performed at the cooler synchrotron COSY in the Research Centre Juelich, by
dint of the COSY-11 detection system. The results of the identification of
Bremsstrahlung radiation emitted via the d p --> d p gamma reaction in data
taken with a proton target and a deuteron beam are presented and discussed.Comment: 3 pages; Presented at Meson 2004: 8th International Workshop on Meson
Production, Properties and Interaction, Krakow, Poland, 4-8 June 2004;
Submitted to International Journal of Modern Physics
Associated strangeness production at threshold
The associated strangeness dissociation at threshold has been studied at the
COSY-11 facility measuring the hyperon - and the K+K- meson pair production.
Measurements of the near threshold Lambda and Sigma0 production via the pp ->
pK+ Lambda / Sigma0 reaction at COSY-11 have shown that the Lambda / Sigma0
cross section ratio exceeds the value at high excess energies (Q >= 300 MeV) by
an order of magnitude. For a better understanding additional data have been
taken between 13 MeV and 60 MeV excess energy.
The near threshold production of the charged kaon-antikaon pair is related to
the discussion about the nature of the scalar states in the 1 GeV/c^2 mass
range, i.e. the f0(980) and a0(980). The interpretation as a K anti-K molecule
is strongly dependent on the K anti-K interaction which can be studied via the
production channel. A first total cross section value on the reaction pp ->
ppK+K- at an excess energy of 17 MeV i.e. below the phi production threshold
was measured.Comment: 3 pages, 2 figures, Presented at Conference on Quarks and Nuclear
Physics (QNP 2002), Julich, Germany, 9-14 Jun 200
Measurement of the invariant mass distributions for the pp -> ppeta' reaction at excess energy of Q = 16.4 MeV
The proton-proton and proton-eta' invariant mass distributions have been
determined for the pp -> ppeta' reaction at an excess energy of Q = 16.4 MeV.
The measurement was carried out using the COSY-11 detector setup and the proton
beam of the cooler synchrotron COSY. The shapes of the determined invariant
mass distributions are similar to those of the pp -> ppeta reaction and reveal
an enhancement for large relative proton-proton momenta. This result, together
with the fact that the proton-eta interaction is much stronger that the
proton-eta' interaction, excludes the hypothesis that the observed enhancement
is caused by the interaction between the proton and the meson
Strangeness Production in Proton-Proton Collisions Close to Threshold
Exclusive data on the reactions pp -> ppK+K- and pp -> pK+ Lambda/Sigma0 have
been taken at the cooler synchrotron COSY close to threshold. At equal excess
energies, an enhancement of the Lambda/Sigma0 ratio by one order of magnitude
has been observed compared to data at higher excess energies. New results
obtained at the COSY-11 facility explore the transition region between this
low-energy Sigma0 suppression and excess energies of 60 MeV. A first total
cross section for elementary antikaon production below the phi threshold has
been determined, two orders of magnitude smaller compared to kaon production at
the same excess energy.Comment: 4 pages, 2 figures, Talk given by P. Moskal at 16th International
Conference on Particles and Nuclei (PANIC 02), Osaka, Japan, 30 Sep - 4 Oct
200
Threshold hyperon production in proton-proton collisions at COSY-11
Sigma+ hyperon production was measured at the COSY-11 spectrometer via the p
p --> n K+ Sigma+ reaction at excess energies of Q = 13 MeV and Q = 60 MeV.
These measurements continue systematic hyperon production studies via the p p
--> p K+ Lambda/Sigma0 reactions where a strong decrease of the cross section
ratio close-to-threshold was observed. In order to verify models developed for
the description of the Lambda and Sigma0 production we have performed the
measurement on the Sigma+ hyperon and found unexpectedly that the total cross
section is by more than one order of magnitude larger than predicted by all
anticipated models.
After the reconstruction of the kaon and neutron four momenta, the Sigma+ is
identified via the missing mass technique. Details of the method and the
measurement will be given and discussed in view of theoretical models.Comment: 7 pages, 8 figures, Phys. Lett. B in print, in the second version a
minor editorial changes have been made by a native speaking proofreader and a
note in proof has been added with an additional referenc
Kaon pair production close to threshold
The total cross section of the reaction pp->ppK+K- has been measured at
excess energies Q=10 MeV and 28 MeV with the magnetic spectrometer COSY-11. The
new data show a significant enhancement of the total cross section compared to
pure phase space expectations or calculations within a one boson exchange
model. In addition, we present invariant mass spectra of two particle
subsystems. While the K+K- system is rather constant for different invariant
masses, there is an enhancement in the pK- system towards lower masses which
could at least be partially connected to the influence of the Lambda(1405)
resonance.Comment: accepted for publication in Phys. Lett.
Measurement of Longitudinal Spin Transfer to Lambda Hyperons in Deep-Inelastic Lepton Scattering
Spin transfer in deep-inelastic Lambda electroproduction has been studied
with the HERMES detector using the 27.6 GeV polarized positron beam in the HERA
storage ring. For an average fractional energy transfer = 0.45, the
longitudinal spin transfer from the virtual photon to the Lambda has been
extracted. The spin transfer along the Lambda momentum direction is found to be
0.11 +/- 0.17 (stat) +/- 0.03 (sys); similar values are found for other
possible choices for the longitudinal spin direction of the Lambda. This result
is the most precise value obtained to date from deep-inelastic scattering with
charged lepton beams, and is sensitive to polarized up quark fragmentation to
hyperon states. The experimental result is found to be in general agreement
with various models of the Lambda spin content, and is consistent with the
assumption of helicity conservation in the fragmentation process.Comment: 8 pages, 3 figures; new version has an expanded discussion and small
format change
Evidence for Quark-Hadron Duality in the Proton Spin Asymmetry
Spin-dependent lepton-nucleon scattering data have been used to investigate
the validity of the concept of quark-hadron duality for the spin asymmetry
. Longitudinally polarised positrons were scattered off a longitudinally
polarised hydrogen target for values of between 1.2 and 12 GeV and
values of between 1 and 4 GeV. The average double-spin asymmetry in
the nucleon resonance region is found to agree with that measured in
deep-inelastic scattering at the same values of the Bjorken scaling variable
. This finding implies that the description of in terms of quark
degrees of freedom is valid also in the nucleon resonance region for values of
above 1.6 GeV.Comment: 5 pages, 1 eps figure, table added, new references added, in print in
Phys. Rev. Let
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