Confirming antihydrogen neutrality with voltage bias

Neutral atoms appear to have literally zero net electric charge. The consistency of quantum field theory requires that the proton and electron have the same charge magnitude e, and observations of several atoms and molecules have confirmed neutrality to 10⁻²¹ e. Theory demands that antiatoms are likewise strictly neutral. But direct experimental confirmation of the theoretical expectation has been problematic. Now the ALPHA collaboration at CERN has measured the charge of antihydrogen to be −1.3 × 10⁻⁸ e, consistent with zero to within about one standard deviation and six orders of magnitude more precise than the previous best antihydrogen measurement. To obtain its result, the collaboration returned to data it had taken a few years ago on antihydrogen in its cryogenic trap, a detail of which is shown in the figure. As part of those experiments, group members released antihydrogen atoms and then detected where 386 of them annihilated on the trap walls. Since their device might have inadvertently trapped antiprotons that could be misidentified as antihydrogen, they had first applied an electric field to clear those charged particles out. In retrospect, collaboration members realized that the old antihydrogen position distributions would reveal how their antiproton-sweeping field influenced the antiatoms’ motion and thus enable them to determine the antihydrogen charge. ALPHA’s finding complements indirect measurements obtained by separately determining the charge of the antiproton and positron. But the ALPHA result is more stringent by about a factor of two and could be improved by procedures designed with charge testing in mind. (C. Amole et al., ALPHA collaboration, Nat. Commun. 5, 3955, 2014 doi:10.1038/ncomms4955 .)