Improved Study of the Antiprotonic Helium Hyperfine Structure
  We report the initial results from a systematic study of the hyperfine (HF) structure of antiprotonic helium (n,l) = (37,~35) carried out at the Antiproton Decelerator (AD) at CERN. We performed a laser-microwave-laser resonance spectroscopy using a continuous wave (cw) pulse-amplified laser system and microwave cavity to measure the HF transition frequencies. Improvements in the spectral linewidth and stability of our laser system have increased the precision of these measurements by a factor of five and reduced the line width by a factor of three compared to our previous results. A comparison of the experimentally measured transition frequencies with three body QED calculations can be used to determine the antiproton spin magnetic moment, leading towards a test of CPT invariance. 
T. Pask, D. Barna, A. Dax, R. S. Hayano, M. Hori, D. Horv\'ath, B.   Juh\'asz, C. Malbrunot, J. Marton, N. Ono, K. Suzuki, J...