A. Khoukaz

Precision study of the eta He-3 system using the dp ---> He-3 eta reaction

The differential and total cross sections for the d+p->3He+eta reaction have been measured in a high precision high statistics COSY-ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy Q of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of Q to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for Q>4 MeV. The very rapid rise of the total cross section to its maximum value within 0.5 MeV of threshold implies a very large eta-3He scattering length and hence the presence of a quasi-bound state extremely close to threshold.

Measurement of the dp→He3η reaction near threshold

Total and differential cross sections for the He 3 center-of-mass momenta in the range from 17.1 MeV/ c to 87.5 MeV/ c . The data were taken during a slow ramping of the COSY internal deuteron beam scattered on a proton target detecting the He 3 ejectiles with the COSY-11 facility. The forward–backward asymmetries of the differential cross sections deviate clearly from zero for center-of-mass momenta above 50 MeV/ c indicating the presence of higher partial waves in the final state. Below 50 MeV/ c center-of-mass momenta a fit of the final state enhancement factor to the data of the total cross sections results in the He 3 – η scattering length of |a|=4.3±0.5xa0fm | a | = 4.3 ± 0.5 fm .

Comparison of Lambda and Sigma**0 threshold production in proton proton collisions

Threshold measurements of the associated strangeness production reactions pp --> p K(+) Lambda and pp --> p K(+) Sigma(0) are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Sigma(0) production appears to be about a factor of 28 smaller than the one for the Lambda particle. It is concluded that strong Sigma(0)-p final state interactions, and in particular the Sigma-N --> Lambda-p conversion reaction, are the likely cause of the depletion for the yield in the Sigma signal. This hypothesis is in line with other experimental evidence in the literature.

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.

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.

Precision study of the dp -> He-3 eta reaction for excess energies between 20 and 60 MeV

The differential and total cross sections for the dp -> He-3 eta reaction have been measured at COSY-ANKE at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. The results are in line with trends apparent from the detailed near-threshold studies and also largely agree with those from CELSIUS, though the present data have much better angular coverage near the forward and backward directions, thus allowing firmer conclusions to be drawn. While at 19.5 MeV the results can be described in terms of s- and p-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global s- and p-wave fit gives a poorer overall description of the data though the position of the pole in the eta He-3 scattering amplitude, corresponding to the quasibound or virtual state, is hardly changed.

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.

Determination of deuteron beam polarizations at COSY

The vector and tensor polarizations of a deuteron beam have been measured using elastic deuteron-carbon scattering at 75.6 MeV and deuteron-proton scattering at 270 MeV. After their acceleration to 1170 MeV inside the COSY ring, the polarizations of the deuterons were checked by studying a variety of nuclear reactions using a cluster target at the ANKE magnet spectrometer placed at an internal target position of the storage ring. All these measurements were consistent with the absence of depolarization during acceleration and provide us with a number of secondary standards that can be used in subsequent experiments at the facility.

Differential cross section and analysing power of the pp→{pp}sπ0 reaction at 353 MeV

In order to establish links between p-wave pion production in nucleon-nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is detected at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}s. We now report on measurements of the differential cross section and analysing power of the ~pp → {pp}s π 0 reaction at 353 MeV. Both observables can be described in terms of sand d-wave pion production and, by using the phase information from elastic pp scattering, unique solutions can be obtained for the corresponding amplitudes. This information is vital for the partial wave decomposition of the corresponding pn → {pp}s π − reaction and hence for the extraction of the p-wave terms.

Generalized Dalitz plot analysis of the near-threshold pp → pp K + K − reaction in view of the K + K − final state interaction

The excitation function for the pp{yields}ppK{sup +}K{sup -} reaction revealed a significant enhancement close to threshold that may plausibly be assigned to the influence of the pK{sup -} and K{sup +}K{sup -} final-state interactions. In an improved reanalysis of COSY-11 data for the pp{yields}ppK{sup +}K{sup -} reaction at excess energies of Q=10 and 28 MeV, including the proton-K{sup -} interaction, the enhancement is confirmed. Invariant mass distributions for the two- and three-particle subsystems allow us to test at low excess energies the ansatz and parameters for the description of the interaction in the ppK{sup +}K{sup -} system as derived from the COSY-ANKE data. Finally, based for the first time on the low-energy K{sup +}K{sup -} invariant mass distributions and the generalized Dalitz plot analysis, we estimate the scattering length for the K{sup +}K{sup -} interaction to be |Re(a{sub K{sup +}}{sub K{sup -}})|=0.5{sub -0.5}{sup +4.0} fm and Im(a{sub K{sup +}}{sub K{sup -}})=3.0{+-}3.0 fm.