Study of Electro-magnetic Interaction and Search for Axion Using Decay Processes of Positronium.

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In order to detect efficiently many gamma-rays emerging from electron-positron annihilation at rest, we constructed "Multi-gamma ray Spectrometer". This detector is so unique that 32 pieces of scintilation counters are placed on the sphere and thus one can observe individual gamma-ray emittel from the positron source at the center. It will be expected that this detector has the detection efficiency of multi-gamma rays hundred times better than any detectors available. Furthermore since we can measure the energy and emission angle of each gamma ray, background is well distinguished using energy-monentum conservation.The radio isotope ^<63>Ge provides positrons which creates an positronium decaying into many gamma rays. We first observed four gamma-rays so as to check higher order (alpha^4) effect of QED and to explore new particles such as an axion. The large background probably 100 times as much as four gamma events can be suppressed. We could successfully extract signals of four gamma processes using the particular aspect of this detector which gives us information on the energy-momentum conservation.Then we measured for the first time the decay rate for e^+e^- -> gamma and the branching ratio of 4gamma to 2gamma was obtained to be (1.48<plus-minus>0.13 <plus-minus>0.12) x10^<-6> being consistent with the lowest order QED calculation, However we could not observe anay significant signal indication the production of axion.