P. Winter

Experimental study of pp eta dynamics in the pp ---> pp eta reaction

A high statistics measurement of the pp --> ppeta reaction at an excess energy of Q = 15.5 MeV has been performed at the internal beam facility COSY-11. The stochastically cooled proton beam and the used detection system allowed to determine the momenta of the outgoing protons with a precision of 4 MeV/c (sigma) in the center-of-mass frame. The determination of the four-momentum vectors of both outgoing protons allowed to derive the complete kinematical information of the ppeta-system. An unexpectedly large enhancement of the occupation density in the kinematical regions of low proton-eta relative momenta is observed. A description taking the proton-proton and the eta-proton interaction into account and assuming an on-shell incoherent pairwise interaction among the produced particles fails to explain this strong effect. Its understanding will require a rigorous three-body approach to the ppeta system and the precise determination of contributions from higher partial waves. We also present an invariant mass spectrum of the proton-proton system determined at Q = 4.5 MeV. Interestingly, the enhancement at large relative momenta between protons is visible also at such a small excess energy. In contrast to all other determined angular distributions, the orientation of the emission plane with respect to the beam direction is extracted to be anisotropic.

Kaon pair production close to threshold

Abstract The total cross section of the reaction p p → p p K + 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 p K − system towards lower masses which could at least be partially connected to the influence of the Λ ( 1405 ) resonance.

Energy dependence of the Λ/Σproduction cross-section ratio in p-p interactions

Abstract.The production of the Λ- and Σ0-hyperons has been measured via the pp → pK+Λ/Σ0 reaction at the internal COSY-11 facility in the excess energy range between 14 and 60 MeV. The transition of the Λ/Σ0 cross-section ratio from about 28 at Q≤13 MeV to the high-energy level of about 2.5 is covered by the data showing a strong decrease of the ratio between 10 and 20 MeV excess energy. Effects from the final-state interactions in the p-Σ0 channel seem to be much smaller than in the p-Λ channel. Estimates of the effective range parameters are given for the NΛ and the NΣ systems.

Threshold hyperon production in proton-proton collisions at COSY-11

Abstract The Σ + hyperon production was measured at the COSY-11 spectrometer via the p p → n K + Σ + 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 + Λ / Σ 0 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 Λ and Σ 0 production we have performed the measurement on the Σ + 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 Σ + is identified via the missing mass technique. Details of the method and the measurement will be given and discussed in view of theoretical models.

A method to disentangle single- and multi-meson production in missing mass spectra from quasi-free pn → pnX reactions

A method to Disentangle Single- and Multi-Meson Production in Missing Mass Spectra from Quasi-Free pn->pn X reactions

Hadronic interaction of η and η′ mesons with nucleons

Abstract Due to their short life-time, flavour-neutral mesons cannot be utilized as free secondary beams or targets, and therefore a study of their interaction with nucleons is not possible via direct scattering experiments. It is, however, accessible via its influence on the energy dependence — and on the phase space distributions of the cross sections for reactions in which these mesons are produced. In case of the pp → ppη reaction the experimentally determined distributions of the differential cross sections close to the production threshold cannot be described by taking into account the S-wave proton-proton and proton-η interaction only. Here we show that the angular distributions determined at the COSY-11 facility reveal some evidence for P-wave admixture in the proton-proton subsystem already at an excess energy as low as Q = 15.5 MeV. We also present that one can estimate the relative strength of the η-nucleon and η′-nucleon interactions by comparison of the η and η′ production yield.

Drift chamber with a c-shaped frame

We present the construction of a planar drift chamber with wires stretched between two arms of a c-shaped aluminum frame. The special shape of the frame allows to extend the momentum acceptance of the COSY-11 detection system towards lower momenta without suppressing the high-momentum particles. The proposed design allows for construction of tracking detectors covering small angles with respect to the beam, which can be installed and removed without dismounting the beam-pipe. For a three-dimensional track reconstruction a computer code was developed using a simple algorithm of hit pre-selection.

Upper limit for the cross-section of the overlapping scalar resonances f0(980) and a0(980) produced in proton–proton collisions in the range of the reaction threshold

Utilizing a missing mass technique we investigate the pp → ppX reaction scanning beam energies in the range permitting to create a mass close to that of the f0(980) and a0(980) scalar resonances, but still below the K+K− threshold where they decay dominantly into ππ and πη mesons, respectively. Prior to the data analysis we introduce a notion of the close to threshold total cross-section for broad resonances. We estimated for the overlapping mesons a0 and f0 the total cross-section to be smaller than 430 nb at excess energy of Q = 5 MeV. The experiment has been performed at the Cooler Synchrotron (COSY) using the COSY-11 facility.

Experimental results on strangeness production in proton–proton collisions at COSY

The production of K+ and K− mesons in elementary proton–proton collisions has been investigated at the Cooler synchrotron COSY in Julich. A high quality proton beam with low emittance and small momentum spread permitted study of the creation of these mesons very close to the kinematical threshold. The energy dependence of the total cross section is investigated using internal beam facilities providing a high accuracy particle momentum determination as well as an external non-magnetic detection setup with a large geometrical acceptance. The determination of the four-momentum vectors for all ejectiles of each registered event gives the complete kinematical information allowing study of the interaction of the outgoing particles. Results on the performed studies of the pp → ppK+K−, pp → pΛK+ and pp → pΣ0K+ reactions will be presented and their relevance to the interpretation of heavy ion collisions will be discussed.

First close-to-threshold measurement of the analysing power A y in the reaction \(\vec pp \rightarrow pp\eta\)

Abstract.At the internal facility COSY-11 a first measurement of the reaction