W. Eyrich

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.

Study of the reaction pp→ΛΛ near threshold

Abstract Results are presented from a study of the reaction p p→ Λ Λ near threshold. Over 3000 events recorded at s values 14.6 and 25.5 MeV above the Λ Λ threshold (2231.2 MeV) have been analysed. Results for the production cross section, differential cross section, and the Λ and Λ polarization are given at both energies and are compared with recent theoretical calculations of this process.

Study of the Reaction

Abstract Results are presented from a study of the reaction p p→ Λ Λ near threshold. Over 3000 events recorded at s values 14.6 and 25.5 MeV above the Λ Λ threshold (2231.2 MeV) have been analysed. Results for the production cross section, differential cross section, and the Λ and Λ polarization are given at both energies and are compared with recent theoretical calculations of this process.

\bar{p} p \to \bar{\Lambda} \Lambda

Abstract The PS185 experiment at the CERN Low Energy Antiproton Ring (LEAR) has studied the reaction p p → \ gLΛ at several momenta. In this paper results from two runs with high statistics at 1.546 GeV/c and 1.695 GeV/c are described. Based on 4063 and 11362 analysed events, respectively, differential and integrated cross sections, polarizations and spin correlations are presented. The singlet fraction, extracted from the spin correlations, is consistent with zero at both momenta, showing that the \ gLΛ pairs are produced in a pure triplet state. A comparison of the decay asymmetry parameters of Λ and \ gL reduces the upper limits for the violation of the CP invariance for this system.

Near Threshold

Abstract The excitation function of the reaction p p→ Λ Λ in the threshold region has been measured at LEAR. Sixteen measurements of the total cross section, in the energy range between 0.85 MeV below threshold and 4.05 MeV above, are presented. The shapes of the measured differential cross sections indicate a remarkably strong p-wave contribution even down to the reaction threshold. We also report here the measurement of significant polarizations in the threshold region; these are compared with previous higher-energy data.

Study of the reaction p̄p → \̄gLΛ at 1.546 and 1.695 GeV/c

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.

Threshold measurement of the reaction pp→ΛΛ at lear

Abstract The PS185 experiment at the CERN Low Energy Antiproton Ring (LEAR) has studied the reaction p p → \ gLΛ at several momenta. In this paper results from two runs with high statistics at 1.546 GeV/c and 1.695 GeV/c are described. Based on 4063 and 11362 analysed events, respectively, differential and integrated cross sections, polarizations and spin correlations are presented. The singlet fraction, extracted from the spin correlations, is consistent with zero at both momenta, showing that the \ gLΛ pairs are produced in a pure triplet state. A comparison of the decay asymmetry parameters of Λ and \ gL reduces the upper limits for the violation of the CP invariance for this system.

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 The excitation function of the reaction p p→ Λ Λ in the threshold region has been measured at LEAR. Sixteen measurements of the total cross section, in the energy range between 0.85 MeV below threshold and 4.05 MeV above, are presented. The shapes of the measured differential cross sections indicate a remarkably strong p-wave contribution even down to the reaction threshold. We also report here the measurement of significant polarizations in the threshold region; these are compared with previous higher-energy data.

Study of the reaction anti-p p ---> anti-Lambda Lambda at 1.546-GeV/c and 1.695-GeV/c

Abstract In a study of hyperon production with antiprotons at the CERN Low-Energy Antiproton Ring (LEAR), we have measured the ratio A=(α + α )/(α− α ) . Here α and α are the decay asymmetry parameters of the Λ and Λ non-leptonic charged decays. The hyperons were produced with identical polarization in the reaction p p→ Λ Λ at an incident p momentum of 1.546 GeV/c. The result, A=− 0.07 ± 0.09, is consistent with zero, as required from CP invariance.

Threshold Measurement of the Reaction

Abstract The p p → p K 0 Σ + reaction was investigated with the TOF spectrometer at COSY at 3.059 GeV / c incident beam momentum. The main objective was to clarify whether or not a narrow exotic S = + 1 resonance, the Θ + pentaquark, is populated at 1.53 GeV / c 2 in the p K 0 subsystem with a data sample of much higher statistical significance compared to the previously reported data in this channel. An analysis of these data does not confirm the existence of the Θ + pentaquark. This is expressed as an upper limit for the cross section σ ( p p → Σ + Θ + ) 0.15 μb at the 95% confidence level.