Laser spectroscopy of antiprotonic helium and stringent constraint on the antiproton charge and mass

The antiprotonic helium atom-molecule (atomcule in short), pe/sup -/ He/sup 2+/=pHe/sup +/, first discovered at KEK and studied in detail at LEAR, is a unique metastable existence interfacing between matter and antimatter. Our recent high-resolution laser spectroscopy of pHe /sup +/ has reached a precision of 0.5 ppm, and the agreement between our experimental values of transition energies and the calculations has become better than 2 ppm. This agreement in turn sets a severe constraint on the antiproton charge and mass. Future possibilities at the new antiproton decelerator (AD) are also discussed. (12 refs)