Laser spectroscopy of the antiprotonic helium atom – its energy levels and state lifetimes

The antiprotonic atom is a three-body exotic system consisting of an antiproton, an electron and a helium nucleus. Its surprising longevity was found and has been studied for more than 10 years. In this work, transition energies and lifetimes of this exotic atom were systematically studied by using the antiproton beam of AD(Antiproton Decelerator) facility at CERN, with an RFQ antiproton decelerator, a narrow-bandwidth laser, Cerenkov counters with fast-response photomultiplier tubes, and cryogenic helium target systems. Thirteen transition energies were determined with precisions of better than 200 ppb by a laser spectroscopy method, together with the elimination of the shift effect caused by collisions with surrounding atoms. Fifteen lifetimes (decay rates) of short-lived states were determined from the time distributions of the antiproton-annihilation signals and the resonance widths of the atomic spectral lines. The relation between the magnitude of the decay rates and the transition multipolarity was investigated, and some states were found to have anomalously short lifetimes. The reasons were explained as state-mixing and collisional effects. The experimental transition energies and lifetimes were compared with latest precise three-body variational calculations, and we succeeded in putting an upper limit (CPT limit) of 10 ppb on the proton-antiproton mass and charge differences.