High precision laser spectroscopy of antiprotonic helium atoms

Antiprotonic helium, a neutral three-body system p̄e−He2+(=p̄He+), produced when antiprotons (p̄) are captured in various phases of helium, comprises a series of long-lived (∼μs lifetime) states with large principal and orbital quantum numbers (n, l). In recent experiments at the CERN antiproton decelerator, comparing experimental values with three-body quantum electro-dynamics calculations, we deduced that the antiproton and proton charges and masses agree to within 6×10−8.