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Register a new userDependence of calculated binding energies and widths of $eta$-mesic nuclei on treatment of subthreshold $eta$-nucleon interaction
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We demonstrate that the binding energies and widths of eta-mesic nuclei depend strongly on subthreshold eta-N interaction. This strong dependence is made evident from comparing three different eta-nucleus optical potentials: (1) a microscopic optical potential taking into account the full effects of off-shell eta-nucleon interactions; (2) a factorization approximation to the microscopic optical potential where a downward energy shift parameter is introduced to approximate the subthreshold eta-nucleon interaction; and (3) an optical potential using on-shell eta-nucleon scattering length as the interaction input. Our analysis indicates that the in-medium $eta$N interaction for bound-state formation is about 30 MeV below the free-space $eta$N threshold, which causes a substantial reduction of the attractive force between the $eta$ and nucleon with respect to that implied by the scattering length. Consequently, the scattering-length approach overpredicts the binding energies and caution must be exercised when these latter predictions are used as guide in searching for $eta$-nucleus bound states. We also show that final-state-interaction analysis cannot provide an unequivocal determination of the existence of $eta$-nucleus bound state. More direct measurements are, therefore, necessary.
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Eta-mesic nucleus or the quasibound nuclear state of an eta ($eta$) meson in a nucleus is caused by strong-interaction force alone. This new type of nuclear species, which extends the landscape of nuclear physics, has been extensively studied since its prediction in 1986. In this paper, we review and analyze in great detail the models of the fundamental $eta$--nucleon interaction leading to the formation of an $eta$--mesic nucleus, the methods used in calculating the properties of a bound $eta$, and the approaches employed in the interpretation of the pertinent experimental data. In view of the successful observation of the $eta$--mesic nucleus $^{25}$Mg$_{eta}$ and other promising experimental results, future direction in searching for more $eta$--mesic nuclei is suggested.
Coherent photoproduction of $eta$-mesons off $^3$He, i.e. the reaction $gamma ^3{He}rightarrow eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $etarightarrow gammagamma$ and $etarightarrow 3pi^0rightarrow 6gamma$ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong $eta {^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $pi^0$-proton back-to-back emission in the $gamma ^3{He}rightarrow pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.
We study eta meson properties in the infinite nuclear matter and in atomic nuclei with an emphasis on effects of the eta coupling to N*(1535)--nucleon-hole modes. The N*(1535) resonance, which dominates the low-energy eta-nucleon scattering, can be seen as a chiral partner of the nucleon. The change of the chiral mass gap between the N* and the nucleon in a nuclear medium has an impact on the properties of the eta-nucleus system. If the N*-nucleon mass gap decreases with a density increase (chiral symmetry restoration) the calculations show the existence of the resonance state at the energy about 60 MeV and two bound eta-nucleus states with the binding energies about -80 MeV. These states can have strong effect on predicted cross sections of the ^12C (gamma,p) ^11B reaction with eta-meson production.
We calculate theoretically the formation spectra of eta(958)-nucleus systems in the (p,d) reaction for the investigation of the in-medium modification of the eta mass. We show the comprehensive numerical calculations based on a simple form of the eta optical potential in nuclei with various potential depths. We conclude that one finds an evidence of possible attractive interaction between eta and nucleus as peak structure appearing around the eta threshold in light nuclei such as 11C when the attractive potential is stronger than 100 MeV and the absorption width is of order of 40 MeV or less. Spectroscopy of the (p,d) reaction is expected to be performed experimentally at existing facilities, such as GSI. We also estimate the contributions from the omega and phi mesons, which have masses close to the eta meson, concluding that the observation of the peak structure of the eta-mesic nuclei is not disturbed although their contributions may not be small.
We are going to perform an inclusive spectroscopy experiment of eta mesic nuclei with the 12C(p,d) reaction to study in-medium properties of the eta meson. In nuclear medium, the eta meson mass may be reduced due to partial restoration of chiral symmetry. In case of sufficiently large mass reduction and small absorption width of eta at normal nuclear density, peak structures of eta mesic states in 11C will be observed near the eta emission threshold even in an inclusive spectrum. The experiment will be carried out at GSI with proton beam supplied by SIS using FRS as a spectrometer. The detail of the experiment is described.
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