Studies of Anomalously Long-Lived Hadronic Atoms

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This research was aimed at 1) precise laser spectroscopy of antiprotonic helium atoms using the LEAR facility at CERN (Switzerland) and 2) search for deeply-bound pionic atoms using the FRS spectrometer at GSI (Germany). For the former we have discovered many laser-induced resonances in antiprotonic helium-3 and -4 atoms thus establishing knowledges on the structure and dynamics of these atoms, while for the latter we have succeeded in observing formation of 2p and 1s states for the first time, in the (d, 3He) reaction on Pb-208 target.1) High precision laser spectroscopy of antiprotonic helium atoms was performed at the CERN low energy antiproton ring (LEAR). In 1996, our emphasis was on the high-precision measurement of the resonance profile. Recent development in the theoretical calculation of antiprotonic helium level structure has made it possible to predict the resonance wavelengths in a better-than 50 ppm accuracy. In addition, a significant progress is being made in understandi … More ng the fine and hyperfine structure of antiprotonic helium. Using the high-resolution mode, we successfully resolved a hyperfine doublet structure, and the observed splitting was in good agreement with the theoretical prediction using a high-precision relativistic variational calculation. We also discovered a significant pressure dependence of the resonance wavelengths, widths and level lifetimes.So far, we used the slow-extraction mode of LEAR,and accepted one antiproton at a time in our target, and irradiated a single antiprotonic helium atom with a high power laser. From 1995, we started to develop a novel technique of pulsed-beam laser resonance method. Namely, more than 10 million antiprotons were extracted from LEAR in a short burst, and a single laser pulse was applied to observe the resonance. This method turned out to be a very powerful tool to find weak resonance lines. Seven new resonance lines were discovered using this new method.2) Deeply-bound pionic atoms were discovered for the first time in the (d, 3He) reaction on Pb-208 target, using the FRS spectrometer at GSI (Germany). Conventionally, pionic atoms spectroscopy was done by observing X-rays emitted during deexcitation. However, as the overlap between the pion wavefunction and the nucleus becomes larger, which is true for the deeply-bound (1s, 2p) states, the pion absorption occurs more frequently than X-ray emission, hence it is impossible to obtain information on the deeply-bound states in the conventional method.We therefore developed a new method of directly producing a pion in the deeply-bound atomic orbit using a nuclear reaction, such as (n, d) or (d, 3He). In the (d, 3He) spectrum we measured at GSI in 1996, the peaks due to the formation of 2p and 1s pionic Pb atoms were clearly seen. From the peak locations and the peak widths, it will become possible to better understand the pion-nucleus interaction. Less