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Register a new userStudies of eta-mesic nuclei at the LPI electron synchrotron
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V. A. Baskov
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A brief review of searches for eta-mesic nuclei is presented with emphasis on photoreactions and results of a new experiment done at the LPI electron synchrotron are reported. Decay products of eta-mesic nuclei presumably formed by photons in a carbon target, namely correlated back-to-back (pi+ n) and (p n) pairs, have been studied with a two-arm time-of-flight setup. The obtained data show that apart from previously observed (pi+ n) pairs from one-nucleon annihilation of stopped etas (via eta N --> pi N) correlated (p n) pairs from two-nucleon absorption of eta in the nucleus (via eta NN --> NN) are also ejected with a comparable rate. The cross section of eta-mesic nuclei photoproduction off carbon was estimated from the obtained data to be less than 10 microbarn.
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A new experiment is proposed with the aim to study eta-mesic nuclei and low-energy interactions of eta with nuclei. Two decay modes of eta produced by a photon beam inside a nucleus will be observed, namely a collisional decay eta N to pi N inside the nucleus and the radiative decay eta to gamma gamma outside. In addition, a collisional decay of stopped S_{11}(1535) resonance inside the nucleus, S_{11}(1535) N to N N, will be studied. The experiment can be performed using the tagged photon beam at ESRF with the end-point energy 1000 MeV and the GRAAL detector which includes a high-resolution BGO calorimeter and a large acceptance lead-scintillator time-of-flight wall. Some results of simulation and estimates of yields are given.
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.
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.
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.
An approved experiment at the internal proton beam of the JINR nuclotron on a search for eta-mesic nuclei in the reaction pA --> np + eta(A-1) --> np + pi-p + X is briefly presented.
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