G. Y. Sun

Evidence for a narrow resonance at 1530MeV/c2 in the K0p-system of the reaction pp→Σ+K0p from the COSY-TOF experiment

The hadronic reaction pp -->Sigma+ K0 p was measured exclusively at a beam momentum of 2.95 GeV/c using the TOF detector at the COSY storage ring. A narrow peak was observed in the invariant mass spectrum of the K0 p subsystem at 1530 +- 5 MeV/c2 with a significance of 4 - 6 standard deviations, depending on background assumptions. The upper limit of 18 +- 4 MeV/c2 (FWHM) for its width is given by the experimental resolution. The corresponding total cross section is estimated to be about 0.4 +- 0.1(stat) +- 0.1(syst) microbarn. Since a resonance in this subsystem must have strangeness S = + 1 we claim it to be the Theta+ state for which very recently evidence was found in various experiments.

Measurement of the eta production in proton proton collisions with the COSY time-of-flight spectrometer

Abstract:The reaction pp → ppη was measured at excess energies of 15 and 41 MeV at an external target of the Jülich Cooler Synchrotron COSY with the time-of-flight spectrometer. About 25000 events were measured for the excess energy of 15 MeV and about 8000 for 41 MeV. Both protons of the process ppη were detected with an acceptance of nearly 100% and the η was reconstructed by the missing-mass technique. For both excess energies the angular distributions are found to be nearly isotropic. In the invariant-mass distributions strong deviations from the pure phase space distributions are seen.

Influence of N*-resonances on hyperon production in the channel pp->K+ Lambda p at 2.95, 3.20 and 3.30 GeV/c beam momentum

Abstract Hyperon production in the threshold region was studied in the reaction pp → K + Λ p using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three different beam momenta of p beam = 2.95 , 3.20 and 3.30 GeV/ c , corresponding to excess energies of e = 204 , 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9 ± 0.8 ± 2.0 μb , 28.4 ± 1.3 ± 2.2 μb and 35.0 ± 1.3 ± 3.0 μb . Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence of the N ( 1650 ) -resonance at p beam = 2.95 GeV / c , whereas for the higher momenta an increasing relative contribution of the N ( 1710 ) - and/or of the N ( 1720 ) -resonance was observed. In addition, the pΛ -final-state interaction turned out to have a significant influence on the Dalitz plot distribution.

Two-pion production in proton-proton collisions with a polarized beam

The two-pion production reaction pp → ppπ+π− was measured with a polarized proton beam at Tp ≈ 750 and 800MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed-pulse technique for Quirl and central calorimeter provided positive π+ identification in addition to the standard particle identification, energy determination as well as time-of-flight and angle measurements. Thus all four-momenta of the emerging particles could be determined with 1–4 overconstraints. Total and differential cross-sections as well as angular distributions of the vector analyzing power have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find significant analyzing-power values up to Ay = 0.3. The data taken in the energy region of the excitation of the Roper resonance confirm that its dominant ππ decay channel is N* → Nσ.

The COSY-TOF barrel detector

Abstract A barrel-shaped scintillator hodoscope was developed as part of the time-of-flight spectrometer TOF operated at an external beam line of the cooler synchrotron COSY. The COSY-TOF spectrometer is used mainly to perform kinematically complete experiments on light meson production and proton–proton bremsstrahlung in proton–proton collisions for laboratory energies up to 2.5 GeV. The newly developed scintillator hodoscope (BARREL1), one of several detector segments of the COSY-TOF spectrometer, consists of one layer of 96 scintillator bars of 15 mm thickness arranged to form a barrel with 3 m in diameter and 2.85 m in length. For any given interaction process the TOF spectrometer allows to determine the velocity vectors of all charged ejectiles emerging from the target by measuring the time of flight between the start and the stop detector as well as the point of impact on the stop detector. For the BARREL1 detector the position information is obtained by two-sided light read-out of the scintillator bars. For minimum ionizing particles the spatial resolution was measured to be Δ z( FWHM )=8 cm , corresponding to a polar angle resolution in the range 0.57°≤Δ Θ ≤2.86°, depending on the hit position. The binning in azimuthal angular direction amounts to Δ Φ =360°/96=3.75°. For the time-of-flight resolution of minimum ionizing particles a value of Δ t( FWHM )=0.61 ns was obtained.

On the production of \pi^{+}_{} \pi^{+}_{} pairs in pp collisions at 0.8 GeV

Data accumulated recently for the exclusive measurement of the pp

Systematic study of the pp

\rightarrow

\rightarrow

pp

pp

\pi^{+}_{}