Collisional and Spectroscopic Studies of Exotic Atoms Using Ultra-Slow Antiprotons

Antiproton, the antiparticle of proton, is a unique projectile in the study of atomic collision physics, which can be treated theoretically either as a 'negative proton' or a 'heavy electron'. Atomic capture of an antiproton will result in formation of a highly excited exotic atom. Antiprotonic helium atom has been studied intensively by means of precision laser spectroscopy, which has led to a stringent determination of antiproton mass and charge to a level of ppb. Comparison of these values with those of proton gives one of the best tests of CPT invariance, the most fundamental symmetry in physics. However, the dynamic processes of antiproton capture remain unclarified, except for some indirect information given by those optical and X-ray observations. With an aim to produce an antiproton beam at atomic-physics energies for 'pure' collision experiments, we have so far developed techniques to decelerate, cool and confine antiprotons in vacuo, using a sequential combination of the Antiproton Decelerator (AD) at CERN, a Radio-Frequency Quadrupole Decelerator (RFQD), and an electromagnetic trap. Our recent success in stable extraction of monoenergetic ultra-slow antiprotons, about 3x105 in number available every 5 minutes, has opened up the possibility to study ionization and atomic capture processes between an antiproton thus provided as a beam and an atom prepared in the form of a supersonic gas-jet target, at an unprecedented low energy from 10 eV to 1 keV under the single-collision condition. Our design and strategy of the cross-beam experiments are discussed.