Results of a search for quasi-bound states of the η meson and a target nucleus obtained during several years using the internal target at the Nuclotron d beam are presented. Formation of η-mesic nuclei was studied in the reaction d + C in the energy interval 1.5Ä2.2 GeV/nucleon Decay products of S11 resonances excited by η mesons captured by nucleons in the nuclear target have been analyzed. Measurements were performed with a scintillation spectrometer SCAN. Analysis of obtained data is presented.
Recent results from the LPI experiment on searching for η-mesic nuclei in photoreactions are discussed and further perspectives are summarized.This talk concerns experimental investigations of new objects of the nuclear physics, η-mesic nuclei ( η A) which are bound systems of the η-meson and a nucleus. Discovery and learning properties of these objects are of fundamental significance for understanding interactions of the η-meson with nucleons and nucleon resonances, for understanding a behavior of hadrons in nuclear matter.A mechanism of creation of η-mesic nuclei in photoreactions and their decay into a πN pair is shown in Fig. 1. At the first stage of the reaction, an incoming photon produces a slow η-meson and a fast nucleon N 1 escaping the nuclear target. Then the η-meson propagates in the nuclear matter and undergoes multiple rescattering off nucleons. At last, η annihilates producing an πN 2 pair escaping the nucleus. The S 11 (1535) nucleon resonance plays a fundamental role in all that dynamics. It mediates creation and annihilation of η and it makes the η-meson bound in the nucleus due to an S 11 (1535)-induced ηN interaction (attraction). Knowing energies/momenta of the ingoing (γ) and outgoing (N 1 , πN 2 ) particles, one can determine energies/momenta of the η and S 11 (1535) propagating in the nuclear matter. The width and binding energy of η in the nucleus can be determined as well.Theoretical estimates given in Refs. 1,2 show that the binding effects lead to a full dominance of the reaction mechanism related with the multiple rescattering of η and a formation of the intermediate η-nucleus over a mechanism of nonresonance background production of the πN pairs in the subthreshold invariant-mass region √ s πN < m η + m N of the subprocess η + N → π + N . A peak in the mass distribution of πN is theoretically expected in this region. This is illustrated in Fig. 2 where the spectral function S(E) of the (kinetic) energy E of η in the nucleus is shown which is proportional to the number of ηN collisions which the η experiences when travels through the nucleus. Another spectral function, S(E, q), shows a distribution of the produced πN pairs over their total energy (E + m η + m N ) and momentum q. The presence
A brief review of searches for η-mesic nuclei is presented with emphasis on photoreactions. Results of a new experiment done at the LPI electron synchrotron are reported. They are as follows.New data on photoproduction of η-mesic nuclei off 12 C have been collected at the bremsstrahlung photon beam of E γ max = 850 MeV. An experimental setup with two plastic time-of flight spectrometers detected correlated π + n and pn pairs from annihilation of η-mesons stopped in the nuclear matter and measured their velocity distributions. Data analysis was performed using an Intra Nuclear Cascade Model in the GEANT-3 framework in order to take into account properties of the setup and physical background. A separation between charged pions and protons was achieved using information on velocities and ranges of the particles in plastic detectors. The obtained data show, apart from previously observed π + n pairs from one-nucleon annihilation of etas (via ηN → πN), a presence of emitted correlated pn pairs with velocities corresponding to the kinematics of the near-threshold reaction of two-nucleon absorption of the η-meson in the nucleus (ηNN → NN). Assuming that such π + n and pn pairs are mostly produced through formation and decay of quasi-bound states of the η-meson and a nucleus (i.e. η-mesic nuclei η A), the cross section of η-mesic nuclei formation was estimated as σ (γ + 12 C → η A + X) 10 µb.
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