Newly Created Antihydrogen Atoms

Newly created antihydrogen atoms have been caught speeding for the first time. Owing to the vast preponderance of ordinary matter over antimatter in the visible universe, and the propensity of any antimatter around to annihilate hastily with any conventional particulate matter in the vicinity, the only place anti-atoms exist on Earth for more than a microsecond is in a chambered vault at the CERN Antiproton Decelerator (AD) lab in Geneva. There, antiprotons created artificially in high-energy proton collisions and anti-electrons (positrons) from a radioactive source are cooled and brought together in a bratwurst-sized vessel filled with electrodes at various voltages. By careful husbandry (first of all, the antiprotons have to be slowed by a factor of 10 billion, from an energy of 5 MeV to .3 meV) anti-hydrogen (or H-bar) atoms are made from antiprotons and positrons.

Although the anti-h's haven't yet been definitely fixed in space or produced in their lowest quantum state (which is what you need to do laser spectroscopy), there are still other studies that can be made on these very rare atoms as they mill about. (For some previous CERN anti-H results see Update 605 and Update 611.) One thing that can be done is to measure the speeds of the anti-atoms by seeing how many of them emerge from a region of oscillating electric fields without being ionized. The ATRAP collaboration, one of the CERN H-bar groups, has done exactly this. They have determined that the anti-atoms are moving with an average energy of 200 meV, which corresponds to a velocity only about 20 times that of the thermal speed of an equivalent sample of atoms kept at a temperature of 4.2 K. This is still too warm for the purpose of holding the anti-atoms in a trap, but the researchers suspect that their current crop of anti-atoms contains some with much lower velocities and that there will be a way to cull an ever colder allotment in the future now that there is a speedometer for antihydrogen atoms. (Gabrielse et al., Physical Review Letters, 13 August; gabrielse@physics.harvard.edu, 33-450-28-38-95.)