Laser spectroscopy of antiprotonic helium atoms

The PS205 experiment at LEAR was initiated after we discovered at KEK that about 3% of p's stopped in liquid helium show anomalously long lifetime of about 3 μs. Our success in 1993 to observe laser-induced resonant p annihilation firmly established that the anomaly is due to the formation of an exotic 3-body ''atomcule'', consisting of an electron, an antiproton and an alpha particle. Subsequent progress in experimental and theoretical studies of this interesting system is discussed. (orig.)