Spectroscopy of Antihydrogen

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The goal of this project was to test the CPT invariance (e. g., equality of particle vs antiparticle mass & charge) by means of precision spectroscopy of antihydrogen atoms and antiprotonic helium atoms.We studied in detail the formation mechanism of antihydrogen atoms, and found : 1) the dominance of 3-body recombination process, 2) produced antihydrogen atoms are likely to be in highly excited states, 3) antihydrogen atoms have much higher kinetic energy than was expected from the ambient temperature of some 10 K. In other words, although we can now produce antihydrogen atoms at a rate of a few hundred Hz, none of them is suitable for high-precision spectroscopy. In order to overcome this difficulty, we (ALPHAcollaboration) constructed a new setup comprising a superconducting Penning trap, a superconducting octupole coils and a pair of mirror coils, with which we are now attempting to magnetically trap antihydrogen atoms. As of today, none has been found yet.High-precision laser spectroscopy of antiprotonic helium atoms has been quite successful. We achieved a new CPT limit of proton/antiproton mass and charge comparison of 2x10-9, by the following methods : 1) we eliminated collisional effects by stopping ultra-low-energy antiprotons (decelerated by a linear antiproton decelerator) in an ultra-low-density helium gas target, 2) we developed a new laser system based on pulsed amplification of single-mode cw lasers, 3) the optical frequency comb technology was used to calibrate the laser frequencies. Our measurement resulted in the determination of proton-to-electron mass ratio to be 1836.152 674 ?} 0.000 005. This is almost as precise as the known proton-to-electron mass ratio of 1836.152 672 47 ?} 0.000 000 80 (codata 2006). Methods to further improve the precision have been identified, so that our measurements are likely to become more precise than the proton-mass measurement in a few years.