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

Abstract

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