Study of deeply-bound pionic atoms

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The purpose of the research is to study theoretically as well as experimentally the deeply-bound pionic atom states, in particular is and 2p states, and to obtain information on the effective pion mass in nuclei. We also developed a new method to study the in-medium masses of eta and omega mesons. The major achievements during the two years are as follows :1. In order to search for the is bound state of pionic atom, we carried out an experiment using the ^<206>Pb(d, ^3He) reaction. The wavefunction of the pionic is state bound to a heavy nucleus has a large overlap with the nucleus, hence suitable to study the behavior of pions in nuclei. However, no such states have been experimentally observed. Theoretical studies have suggested that it is possible to identify 2p as well as is states as clear peaks in the excitation-energy spectrum, if Pb2O6 is chosen as the target. We therefore carried out the expriment, and successfully observed the 2p and is peaks. When the detailed analysis, being carried out, is completed, we expect to determine the pion effective mass with a high precision.2. The success of the pionic atom experiment motivated us to study the feasibility of studying the behavior of other mesons, such as eta and omega, by using the recoilless (d, ^3He) reaction. We first estimated the meson-nucleus optical potential, and determined the binding energies and widths of mesons by solving the Klein-Gordon equation. In both cases, the potentials were strongly attractive, and the existence of bound states are predicted. We then used the Green function method and the Eikonal approximation to estimate the reaction cross sections. We found that the bound-state peaks can be observed, even after taking account of various background processes. An experimental proposal based on this study was submitted to GSI, and was approved as S214.