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تسجيل مستخدم جديدHadronic interaction of the eta meson with two nucleons
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The COSY-11 collaboration has conducted experiments aiming at the determination of the excitation function and phase-space population of the p p --> p p eta reaction close to the kinematical threshold. The precise data obtained with the stochastically cooled proton beam of the cooler synchrotron COSY and the high resolution zero-degree magnetic spectrometer allowed for the observation of the significant deviations - in the shape of the excitation function and two-particle invariant masses - from the predictions based on the assumption that the reaction phase space is homogenously populated. Comparison of the shape of the excitation function for the p p --> p p eta and p p --> p p eta-prime reaction allows to distinquish in the model independent way an influence originating from the proton-proton and proton-eta interaction. For the comparison the full data set from experiments performed at COSY and other laboratories is used.
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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. This interaction is, however, ac
cessible 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 p p --> p p eta 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-eta 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 eta-nucleon and eta-prime-nucleon interactions by comparison of the eta and eta-prime production yield.
We report on the status of the search for eta-mesic nuclei and the studies of the interaction of the eta meson with nucleons. Recently we have completed the analysis of the new WASA-at-COSY data on the production of the eta meson with polarized proto
n beam. New results on the analyzing power for the pp->ppeta reaction with more than an order of magnitude improved precision shed a new light on the production mechanism of the eta meson in nucleon-nucleon collisions. Also, the latest results of the search for eta-mesic nuclei are discussed.
The $eta$-meson production in photon- and hadron-induced reactions, namely, $gamma p to p eta$, $pi^- p to n eta$, $pp to ppeta$, and $pn to pneta$, are investigated in a combined analysis in order to learn about the relevant production mechanisms an
d the possible role of nucleon resonances in these reactions. We consider the nucleonic, mesonic, and nucleon resonance currents constructed within an effective Lagrangian approach and compare the results with the available data for cross sections and spin asymmetries for these reactions. We found that the reaction $gamma p to p eta$ could be described well with the inclusion of the well-established $S_{11}(1535)$, $S_{11}(1650)$, $D_{13}(1520)$, and $D_{13}(1700)$ resonances, in addition to the mesonic current. Consideration of other well-established resonances in the same mass region, including the spin-5/2 resonances, $D_{15}(1675)$ and $F_{15}(1680)$, does not further improve the results qualitatively. For the reaction $pi^- p to n eta$, the $P_{13}(1720)$ resonance is found to be important for reproducing the structure observed in the differential cross section data. Our model also improves the description of the reaction $NN to NNeta$ to a large extent compared to the earlier results by Nakayama textit{et al.} [Phys. Rev. C textbf{68}, 045201 (2003)]. For this reaction, we address two cases where either the $S_{11}(1535)$ or the $D_{13}$ dominates. Further improvement in the description of these reactions and the difficulty to uniquely determine the nucleon resonance parameters in the present type of analysis are discussed.
We present the results of measurements of the analysing power for the p(pol)p --> pp eta reaction at the excess energies of Q=10 and 36 MeV, and interpret these results within the framework of the meson exchange models. The determined values of the a
nalysing power at both excess energies are consistent with zero implying that the eta meson is produced predominantly in s-wave.
We measure the mass of the eta meson using psi(2S) --> eta J/psi events acquired with the CLEO-c detector operating at the CESR e+e- collider. Using the four decay modes eta --> gamma gamma, 3pi0, pi+pi-pi0, and pi+pi-gamma, we find M(eta)=547.785 +-
0.017 +- 0.057 MeV, in which the first uncertainty is statistical and the second systematic. This result has an uncertainty comparable to the two most precise previous measurements and is consistent with that of NA48, but is inconsistent at the level of 6.5sigma with the much smaller mass obtained by GEM.
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