The Japanese-European ASACUSA experiment at CERN’s Antiproton Decelerator (AD) has reported a new measurement of the antiproton’s mass, accurate to about one part in a thousand million. This means that the measurement of the antiproton’s mass relative to the electron is now almost as accurate as that of the proton.

To make these measurements, the ASACUSA team first traps antiprotons inside antiprotonic helium, in which the negatively charged antiproton takes the place of an electron and occupies a Rydberg state, keeping it relatively far from the nucleus. The antiprotonic helium atoms thus live long enough to allow the frequencies of atomic transitions to be measured by laser spectroscopy. The frequencies depend on the ratio of the antiproton mass to the electron mass and ASACUSA has already used this technique to achieve record precision (CERN Courier July/August 2006 p8).

However, an important source of imprecision comes from Doppler broadening of the resonance observed when the laser is tuned to the transition frequency. The atoms move around, so that those moving towards and away from the laser beam experience slightly different frequencies. In the previous measurement in 2006, the ASACUSA team used just one laser beam, and the achievable accuracy was dominated by this effect. This time they have used two beams moving in opposite directions, with the result that the broadening for the two beams partly cancels out.

The resulting narrow spectral lines allowed the team to measure three transition frequencies with fractional precisions of 2.3–5 parts in 109. By comparing the results with three-body QED calculations, they find an antiproton-to-electron mass ratio of 1836.1526736(23), where the error (23) represents one standard deviation. This agrees with the proton-to-electron value, which is known to a similar precision.