IOP PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 70 (2007) 1995–2065 doi:10.1088/0034-4885/70/12/R01 Antiprotonic helium and CPT invariance Ryugo S Hayano1, Masaki Hori1, Dezso˝ Horvath´ 2,4 and Eberhard Widmann3 1 Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 2 KFKI Research Institute for Particle and Nuclear Physics, H-1525 Budapest, Hungary 3 Stefan Meyer Institute for Subatomic Physics, Boltzmanngasse 3, A-1090 Vienna, Austria E-mail:
[email protected] Received 12 March 2007, in final form 31 July 2007 Published 1 November 2007 Online at stacks.iop.org/RoPP/70/1995 Abstract We review recent progress in the laser and microwave spectroscopy of antiprotonic helium atoms (pHe¯ + ≡ e− − p¯ − He++) carried out at CERN’s Antiproton Decelerator facility (AD). Laser transitions were here induced between Rydberg states (n, ) and (n ± 1, − 1) of pHe¯ + (n ∼ 40 and n − 1 being the principal and orbital angular momentum quantum numbers of the antiproton orbit). Successive refinements in the experimental techniques improved the fractional precision on the pHe¯ + frequencies from 3 parts in 106 to ∼1 part in 108. These included a radiofrequency quadrupole decelerator, which reduced the energy of the antiprotons from 5.3 MeV (the energy of the beam emerging from AD) to ∼100 keV. This enabled the production of pHe¯ + in ultra-low density targets, where collisional effects with other helium atoms are negligible. A continuous wave pulse-amplified dye laser, stabilized against a femtosecond optical frequency comb, was then used to measure the pHe¯ + frequencies with ppb-scale precision.