M. Janusz

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.

Near-threshold production of the eta meson via the quasifree pn -g pn eta reaction

Total cross sections for the quasifree pn -> pn eta reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pn -> pn eta reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio s(pn -> pn eta)/s=sigma(pn -> pn eta) toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton- neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn -> pn eta reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.

Mechanism of Near-Threshold Production of the {eta} Meson

Measurements of the analyzing power for the pp-->pp eta reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S11(1535) resonance is predominantly due to the exchange of the pi meson between the colliding nucleons.

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

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.

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