High-precision spectroscopy of antiprotonic helium atoms using femtosecond frequency comb, and related topics in low-energy antiproton physics

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.