Anti-Hydrogen Production Under Laser Control

Anti-hydrogen production under laser control has been achieved in an experiment conducted at the CERN lab in Geneva. Cold anti-hydrogen (Hbar) atoms are the antimatter counterparts of hydrogen atoms. Previously antihydrogen was formed when positrons cooled antiprotons within the carefully designed electric and magnetic fields of a nested Penning trap. That the anti-atoms had formed at all was verified, but they’re not yet cold enough to be held in place.

The ultimate goal is to make a goodly supply of anti-atoms, store them, and then probe their internal structure with laser light to determine whether they have the same quantum behavior as ordinary hydrogen. An incremental step would be not just to make the anti-atoms but to see to it that they are in specific internal energy states, and this is what the ATRAP (http://hussle.harvard.edu/~atrap/ ) collaboration has now done. To gain some extra control over anti-H production, they have to make the production process a bit more complicated. Where the lasers come into the picture is to initiate a three-step process.

First, laser light selectively excites cesium atoms into special “Rydberg” states. Second, positrons collide with the Cs atoms, an encounter which cedes one of the atom’s electrons to the positron; the positron-electron pair, which constitutes a sort of atom-like entity of its own, known as positronium (abbreviated Ps), inherits the cesium atom’s excitation. (By the way, this excited Ps is a thousand times bigger than plain Ps). Third, the positron part of the Ps can occasionally be captured by an antiproton moving in the same direction. In the process the anti-hydrogen atoms assumes the same binding energy as the former Ps.

The rate for producing anti-H this way is still lower than with the older methods, but the use of the intermediate cesium process and laser excitation offers an extra measure of control over atomic conditions within the trap (useful in experiments yet to come) and, furthermore, may have resulted, in this case, in the coldest anti-atoms ever created in a lab. (Storry et al., Physical Review Letters, 31 December 2004; contact Gerald Gabrielse, 617-495-4381, gabrielse@physics.harvard.edu)