Study of eta and omega meson mass shift in nuclei using recoil-less reactions

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Whether or not the masses of hadrons are modified in nuclear matter is one of the central problems of modern nuclear physics. In order to study this problem, there are many on-going efforts world wide to produce mesons in nuclei via high-energy heavy ion collisions, and to reconstruct the meson masses from the dilepton decays. The present study is based on an entirely different approach of "recoilless" production of mesons in nuclei, and of determination of their masses from the analyzes of Q-value spectra.We analyzed the deeply-bound pionic atom data (π^- meson bound in the 1s level of pionic ^<205>Pb atom), and unambiguously showed that the pi^- mass in lead nucleus is heavier by about 26 MeV as compared with its vacuum value. This result has been published in Physical Review Letters. In order to further study the contribution of the neutron/proton distribution difference on the meson mass, we performed another experiment using a series of Sn isotopes (different neutron numbers). The data are being analyzed, but preliminary results already show that the expected isotope effects are clearly seen.Our proposal to study eta (η) and omega (ω) meson mass shift in nuclei had already been approved at GSI laboratory in Darmstadt, Germany, but the beam time was deferred due to the fact that we chose to run the Sn experiment first. Meanwhile, we constructed a special focal-plane detector, sensitive to ^3He (our signal) but insensitive to protons (our background) based on the Cherenkov detection. The detector was tested at KEK, and was shown to perform as expected.