Sub-ppm Laser Spectroscopy of Antiprotonic Helium and a CPT-Violation Limit on the Antiprotonic Charge and Mass

Six laser-resonant transitions have been detected in metastable antiprotonic helium atoms produced at the CERN Antiproton Decelerator. They include UV transitions from the last metastable states in the v = n-l-1 = 0 and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between 1.3×10^-7 and 1.6×10^-6. By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within 6×10^-8 with a confidence level of 90%.