Antiprotonic helium is a unique atomic three-body system which is a mixture of particles and an antiparticle and
yet shows metastability with lifetimes of a few microseconds before annihilation of the constituent antiproton.
Using the antiprotons provided from the Antiproton Decelerator facility at CERN, Geneva, we the ASACUSA
collaboration (standing for Atomic Spectroscopy And Collisions Using Slow Antiprotons) has been pursuing
high precision in laser spectroscopy of this exotic atom, to test CPT invariance between matter and antimatter.
Recently, we have measured 12 different transition frequencies with fractional precisions in the order of a ppb.
A femtosecond optical frequency comb has found application here in measurement of absolute frequencies of a
continuous-wave pulse-amplified laser light ranging from blue to infrared. Comparison with three-body QED
calculations yielded an antiproton-to-electron mass ratio and antiproton-to-proton mass ratio with a precision
of a few ppb, contributing to the precise determination of the fundamental constant and to the test of CPT.
As a pioneering work of atomic physics with low-energy antiprotons, we have developed techniques of electromagnetic
trapping of antiprotons in ultra-high vacuum and produced ultra-slow antiproton beams at energies
ranging from 10 eV to 20 keV. (Note the great orders of magnitude of deceleration and cooling from the GeV
energies at which antiprotons are produced at accelerator facilities.) This unique beam is used for our research
on atomic collision dynamics between an antiproton and an ordinary atom, and also for future production and
microwave spectroscopy of cold antihydrogen atoms.
Laser is a powerful tool for precise spectroscopy and has been used in many different fields of physics. In this paper we present interesting and important examples of laser spectroscopy applied in the field of atomic physics involving an exotic particle `antiproton'. We have performed laser spectroscopy of metastable antiprotonic helium atoms (or atom-molecules) (pHe+) and have observed a density dependence of the resonance vacuum wavelengths for the known transitions (n,1) equals *(39,35) yields (38,34) and (37,34) yields (36,33).
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