CERN experiment weighs antimatter with unprecedented accuracy

The ASACUSA experiment


“This is a very satisfying result,” said Masaki Hori, a project leader in the ASACUSA collaboration. “It means that our measurement of the antiproton’s mass relative to the electron is now almost as accurate as that of the proton.”


To make these measurements antiprotons are first trapped inside helium atoms, where they can be ‘tickled’ with a laser beam. The laser frequency is then tuned until it causes the antiprotons to make a quantum jump within the atoms, and from this frequency the antiproton mass can be calculated.  However, an important source of imprecision comes from the fact that the atoms jiggle around, so that those moving towards and away from the beam experience slightly different frequencies.  A similar effect is what causes the siren of an approaching ambulance to apparently change pitch as it passes you in the street. In their previous measurement in 2006, the ASACUSA team used just one laser beam, and the achievable accuracy was dominated by this effect. This time they used two beams moving in opposite directions, with the result that the jiggle for the two beams partly cancelled out, resulting in a four times better accuracy.

“Imagine measuring the weight of the Eiffel tower” said Hori. “The accuracy we’ve achieved here is roughly equivalent to making that measurement to within less than the weight of a sparrow perched on top. Next time it will be a feather.”

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