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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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A novel method is proposed to measure eta(958) meson bound states in 11C nuclei by missing mass spectroscopy of the 12C(p,d) reaction near the eta production threshold. It is shown that peak structures will be observed experimentally in an inclusive measurement in case that the in-medium eta mass reduction is sufficiently large and that the decay width of eta mesic states is narrow enough. Such a measurement will be feasible with the intense proton beam supplied by the SIS synchrotron at GSI combined with the good energy resolution of the fragment separator FRS.
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.
The total cross section of the p d -> p d eta reaction has been measured at two energies near threshold by detecting the final proton and deuteron in a magneti spectrometer. The values are somewhat larger than expected on the basis of two simple theoretical estimates.
The excitation function for the $pdrightarrow$ $d p pi^{0}$ reaction has been measured by WASA-at-COSY experiment with the aim of searching for $^{3}hspace{-0.03cm}mbox{He}$-$eta$ mesic nuclei. The measurement in the vicinity of $eta$ meson production was performed using a ramped proton beam. The data analysis and interpretation was carried out with the assumption that the $eta$-mesic Helium decays via the formation of an intermediate N$^{*}$(1535) resonance. No direct signal of the $eta$-mesic nucleus is observed in the excitation function. We determine a new improved upper limit for the total cross section for the bound state production and decay in the $pdrightarrow$ $(^{3}hspace{-0.03cm}mbox{He}$-$eta)_{bound} rightarrow$ $d p pi^{0}$ process. It varies between 13 nb to 24 nb for the bound state with width in the range $Gamma in (5,50)$ MeV.
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.
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