Atoms made entirely of antimatter: Two methods produce slow antihydrogen

An antihydrogen (H-) atom-a positron (e⁺) in orbit about an antiproton (p-)-is the simplest atom made entirely of antimatter. Producing H- atoms that are cold enough to be trapped for precise laser spectroscopy, to compare antihydrogen and hydrogen, is a goal that has been pursued for many years. A prequel to this review summarized the techniques for accumulating cold p- and e⁺ that opened the way to slow H- production, along with crucial devices like the nested Penning trap that was developed to bring the p- and e⁺ together. Several exciting years have seen the first production, observations and studies of slow H- atoms-so far by two different methods. The demonstrations of e⁺ cooling of p- in a nested Penning trap led to observations of slow H- atoms produced in this way (method I) using two detection techniques. Field ionization detection of H- produced by method I makes it possible to go beyond the simple counting of H- atoms via charge exchange collisions-a method that seems to naturally produce H- atoms with essentially the low energy distribution of the p- from which they form.