An improved limit on the charge of antihydrogen from stochastic acceleration

Stochastic acceleration applied to 1,000 trapped antihydrogen atoms yields a 20-fold reduction of the experimental upper bound on the magnitude of the charge of antihydrogen, which is expected to be charge neutral. One of the most puzzling current questions in physics is why we see so much more matter than antimatter in the Universe. Studying the properties of antimatter might give hints about the reasons for this imbalance. According to the Standard Model of particle physics, the charge of antihydrogen should be neutral, but it is challenging to test this experimentally as it is difficult to produce antimatter and to measure its properties. Now a team from the ALPHA collaboration at CERN has measured the charge of antihydrogen, confirming charge neutrality to a precision of a factor of 20 greater than that achieved previously. As the charge of the antiproton is known to similar precision, this result also delivers an improved limit on the positron charge anomaly. Antimatter continues to intrigue physicists because of its apparent absence in the observable Universe. Current theory requires that matter and antimatter appeared in equal quantities after the Big Bang, but the Standard Model of particle physics offers no quantitative explanation for the apparent disappearance of half the Universe. It has recently become possible to study trapped atoms1–4 of antihydrogen to search for possible, as yet unobserved, differences in the physical behaviour of matter and antimatter. Here we consider the charge neutrality of the antihydrogen atom. By applying stochastic acceleration to trapped antihydrogen atoms, we determine an experimental bound on the antihydrogen charge, Qe, of |Q| < 0.71 parts per billion (one standard deviation), in which e is the elementary charge. This bound is a factor of 20 less than that determined from the best previous measurement5 of the antihydrogen charge. The electrical charge of atoms and molecules of normal matter is known6 to be no greater than about 10−21e for a diverse range of species including H2, He and SF6. Charge–parity–time symmetry and quantum anomaly cancellation7 demand that the charge of antihydrogen be similarly small. Thus, our measurement constitutes an improved limit and a test of fundamental aspects of the Standard Model. If we assume charge superposition and use the best measured value of the antiproton charge8, then we can place a new limit on the positron charge anomaly (the relative difference between the positron and elementary charge) of about one part per billion (one standard deviation), a 25-fold reduction compared to the current best measurement8,9.