The ALPHA collaboration has just published a new measurement of the charge of the antihydrogen atom. Although the Standard Model predicts that antihydrogen must be strictly neutral, only a few actual direct measurements have been performed so far to test this conjecture.

A glance at the Particle Data Book reveals that, according to the latest measurements, the antiproton charge can differ from the charge of the electron by at most 7 × 10–10 times the fundamental charge. The comparable number for the positron is somewhat larger, at 4 × 10–8. Note that studies with atoms of normal matter show that they are neutral to about one part in 1021. We are, therefore, unsurprisingly, way behind in our ability to study antimatter. Given that we still do not understand the baryon asymmetry, it is generally a good idea to take a hard look at antimatter, if you can get your hands on some.

Antihydrogen is unique in the laboratory in that it should be neutral, stable antimatter. Indeed, the charge–parity–time (CPT) symmetry requires antihydrogen to have the same properties as hydrogen, including charge neutrality. In ALPHA, we can produce antihydrogen atoms and catch them in a trap formed by superconducting magnets, and we can hold them for at least 1000 s.

The current article in Nature results from experiments in the recently commissioned ALPHA-2 machine, and uses a new technique proposed by ALPHA member Joel Fajans and colleagues at UC Berkeley. The new method, known as stochastic acceleration, involves subjecting the trapped antihydrogen atoms to electric-field pulses at various time intervals. If the antihydrogen is not really neutral, it will be "heated" by the repeated pulses until it finally escapes the trap and annihilates. Comparing the results of trials with and without the pulsed field, we can derive a limit on how "charged" antihydrogen might be. The answer so far: antihydrogen is neutral to 0.7 ppb (one standard deviation) of the fundamental charge. This is a factor of 20 improvement over our previous limit, set by using static electric fields to try to deflect antihydrogen when it is released from the trap.

If we take another approach and assume that antihydrogen is indeed neutral, we can combine this result with ASACUSA’S measurement of the antiproton charge anomaly to improve the limit on the positron charge anomaly by a factor of about 25. Of course, we are looking for signs of new physics in the antihydrogen system – it is probably best not to assume anything.