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 $v=n-\ell -1=0\mathrm{}$ and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between $1.3\times {10}^{-7\mathrm{}}$ and $1.6\times {10}^{-6\mathrm{}}$. By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within $6\times {10}^{-8\mathrm{}}$ with a confidence level of $90\%$.