126 research outputs found

    Courbes d'excitation des réactions 18O(p,p1γ) 18O* et 18O(p,α1,2,3γ)15N* de 3,2 à 5,4 MeV

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    Les courbes d'excitation des réactions 18O(p, p1) 18O* et 18O(p, α1,2,3)15N* ont été mesurées pour les énergies incidentes de 3,2 à 5,4 MeV. Les courbes montrent, pour certaines énergies, des anticorrélations entre les voies de sortie proton et alpha. Si cet effet peut être attribué à la conservation du spin isobarique dans les réactions, il détermine le spin isobarique de quelques états excités du noyau composé 19F

    SEARCH FOR NARROW ISOVECTOR DIBARYONS CLOSE TO LOW MASSES THRESHOLD

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    For masses larger than 2 MN + Mπ, narrow peaks in two proton invariant (missing) masses spectra : Mpp (Mx) are now well established. The existence of such narrow peaks is not so unquestionable below 2 MN + Mπ. Tensor analyzing power T20 of the invariant mass Mpp has been studied using the p ([MATH], pp) reaction. Depending on the assumption done for background substraction -a hole or an oscillation may appear at Mpp = 1941 MeV. Additional data are under analysis and will improve the statistics

    Comparison of the Near-Threshold Production of eta- and K-Mesons in Proton-Proton Collisions

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    The pp -> pp eta and pp -> pLambda K^+ reactions near threshold are dominated by the first and second S_11 resonance respectively. It is shown that a one-pion-exchange model exciting these isobars reproduces well the ratio of the production cross sections. The consequences for this and other channels are discussed.Comment: 10 pages, LaTeX2e, 1 eps-figur

    Threshold J/ψJ/\psi- production in nucleon-nucleon collisions

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    We analyze J/ψJ/\psi- production in nucleon-nucleon collisions near threshold in the framework of a general model independent formalism, which can be applied to any reaction N+NN+N+V0N+N\to N+N+V^0, where V0=ωV^0=\omega, ϕ\phi, or J/ψJ/\psi. Such reactions show large isotopic effects: a large difference for pppp- and pnpn-collisions, which is due to the different spin structure of the corresponding matrix elements. The analysis of the spin structure and of the polarization observables is based on symmetry properties of the strong interaction. Using existing experimental data on the different decays of J/ψJ/\psi-meson, we suggest a model for N+NN+N+J/ψN+N\to N+N+J/\psi, based on tt-channel η+π\eta+\pi-exchanges. We predict polarization phenomena for the n+pn+p+J/ψn+p\to n+p+J/\psi-reaction and the ratio of cross sections for npnp and pppp-collisions. For the processes η(π)+NN+J/ψ\eta(\pi)+N\to N+J/\psi we apply two different approaches: vector meson exchange and local four-particle interaction. In both cases we find larger J/ψJ/\psi-production in npnp-collisions, with respect to pppp-collisions.Comment: 17 pages, 6 figure

    Near-Threshold Production of omega Mesons in the pp -> pp omega Reaction

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    The total cross section for omega production in the pp -> pp omega reaction has been measured at five c.m. excess energies from 3.8 to 30 MeV. The energy dependence is easily understood in terms of a strong proton-proton final state interaction combined with a smearing over the width of the state. The ratio of near-threshold phi and omega production is consistent with the predictions of a one-pion-exchange model and the degree of violation of the OZI rule is similar to that found in the pi-p -> n omega/phi reactions.Comment: Report in LaTeX2e. 12 pages with 2 eps figure

    S-wave eta'-proton FSI; phenomenological analysis of near-threshold production of pi0, eta, and eta' mesons in proton-proton collisions

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    We describe a novel technique for comparing total cross sections for the reactions pp --> pp pi(0), pp --> pp eta, and pp --> pp eta' close to threshold. The initial and final state proton-proton interactions are factored out of the total cross section, and the dependence of this reduced cross section on the volume of phase space is discussed. Different models of the proton-proton interaction are compared. We argue that the scattering length of the S-wave eta'-proton interaction is of the order of 0.1 fm.Comment: 10 pages, 5 figure

    On the close to threshold meson production in neutron-neutron collisions

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    A method of measuring the close to threshold meson production in neutron-neutron collisions is described where the momenta of the colliding neutrons can be determined with the accuracy obtainable for the proton-proton reaction. The technique is based on the double quasi-free nn --> nn X^0 reaction, where deuterons are used as a source of neutronsComment: 6 pages, 2 figures, to be published in Phys. Lett.

    Energy Dependence of the Near-Threshold Total Cross-Section for the pp --> pp eta' Reaction

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    Total cross sections for the pp --> pp eta' reaction have been measured in the excess energy range from Q = 1.53 MeV to Q = 23.64 MeV. The experiment has been performed at the internal installation COSY-11 using a stochastically cooled proton beam of the COoler SYnchrotron COSY and a hydrogen cluster target. The determined energy dependence of the total cross section weakens the hypothesis of the S-wave repulsive interaction between the eta' meson and the proton. New data agree well with predictions based on the phase-space distribution modified by the proton-proton final-state-interaction (FSI) only.Comment: 12 pages, 1 table, 4 figure

    Production of η\eta\prime Mesons in the ppppηpp \to pp\eta\prime Reaction at 3.67 GeV/c

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    The ratio of the total exclusive production cross sections for η\eta\prime and η\eta mesons has been measured in the pppp reaction at pbeam=3.67p_{beam}=3.67 GeV/c. The observed η/η\eta\prime/\eta ratio is (0.83±0.110.18+0.23)×102(0.83\pm{0.11}^{+0.23}_{-0.18})\times 10^{-2} from which the exclusive η\eta\prime meson production cross section is determined to be (1.12±0.150.31+0.42)μb(1.12\pm{0.15}^{+0.42}_{-0.31})\mu b. Differential cross section distributions have been measured. Their shape is consistent with isotropic η\eta\prime meson production.Comment: 14 pages, 5 figures, accepted by Phys.Lett.
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