78 research outputs found
The Longitudinal Polarimeter at HERA
The design, construction and operation of a Compton back-scattering laser
polarimeter at the HERA storage ring at DESY are described. The device measures
the longitudinal polarization of the electron beam between the spin rotators at
the HERMES experiment with a fractional systematic uncertainty of 1.6%. A
measurement of the beam polarization to an absolute statistical precision of
0.01 requires typically one minute when the device is operated in the
multi-photon mode. The polarimeter also measures the polarization of each
individual electron bunch to an absolute statistical precision of 0.06 in
approximately five minutes. It was found that colliding and non-colliding
bunches can have substantially different polarizations. This information is
important to the collider experiments H1 and ZEUS for their future
longitudinally polarized electron program because those experiments use the
colliding bunches only.Comment: 21 pages (Latex), 14 figures (EPS
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.
Near-Threshold eta Meson Production in Proton-Proton Collisions
The production of eta mesons has been measured in the proton-proton
interaction close to the reaction threshold using the COSY-11 internal facility
at the cooler synchrotron COSY. Total cross sections were determined for eight
different excess energies in the range from 0.5 MeV to 5.4 MeV. The energy
dependence of the total cross section is well described by the available
phase-space volume weighted by FSI factors for the proton-proton and proton-eta
pairs.Comment: 9 pages, 1 table, 5 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
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
Monitoring of the accelerator beam distributions for internal target facilities
We describe a direct method for monitoring the geometrical dimensions of a
synchrotron beam at the target position for internal target installations. The
method allows for the observation of the proton beam size as well as the
position of the beam relative to the target. As a first demonstration of the
technique, we present results obtained by means of the COSY-11 detection system
installed at the cooler synchrotron COSY. The influence of the stochastic
cooling on the COSY proton beam dimensions is also investigated.Comment: 7 pages, 8 figures, Submitted to Nucl. Inst. & Meth.
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
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
Near Threshold K+K- Meson-Pair Production in Proton-Proton Collisions
The near threshold total cross section and angular distributions of K+K- pair
production via the reaction pp --> ppK+K- have been studied at an excess energy
of Q = 17 MeV using the COSY-11 facility at the cooler synchrotron COSY. The
obtained cross section as well as an upper limit at an excess energy of Q = 3
MeV represent the first measurements on the K+K- production in the region of
small excess energies where production via the channel pp --> pp Phi --> ppK+K-
is energetically forbidden. The possible influence of a resonant production via
intermediate scalar states f0(980) and a0(980) is discussed.Comment: 8 pages, 6 figures, replaced with revised version, accepted for
publication in Phys. Lett.
Energy Dependence of the Near-Threshold Total Cross-Section for the pp --> pp eta' Reaction
Total cross sections for the pp --> pp eta' reaction have been measured in
the excess energy range from Q = 1.53 MeV to Q = 23.64 MeV. The experiment has
been performed at the internal installation COSY-11 using a stochastically
cooled proton beam of the COoler SYnchrotron COSY and a hydrogen cluster
target. The determined energy dependence of the total cross section weakens the
hypothesis of the S-wave repulsive interaction between the eta' meson and the
proton. New data agree well with predictions based on the phase-space
distribution modified by the proton-proton final-state-interaction (FSI) only.Comment: 12 pages, 1 table, 4 figure
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