Ultra-low energy antihydrogen

The study of CPT invariance with the highest achievable precision in all pa rticle sectors is of fundamental importance for physics, Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved at the Low-Energy An tiproton Ring (LEAR) at CERN in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low-energy positron or p ositronium beams have been generated. Finally, steady progress has been mad e in trapping and cooling neutral atoms. Thus the ingredients to form antih ydrogen at rest are at hand. This report will describe the techniques available to produce, decelerate, and accumulate antiprotons at low energy, how to generate high-density plas mas of low-energy positrons, and how to combine these two species into anti hydrogen. Once antihydrogen atoms have been formed, they can be captured in magnetic gradient traps and standard spectroscopic methods applied to interrogate th eir atomic structure with extremely high precision for comparison with the hydrogen atom, In particular, the 1S-2S transition, with a lifetime of the excited state of 122 ms and thereby a natural linewidth of five parts in 10 (16), offers in principle the possibility to directly compare matter and an timatter properties at a level of one part in 10(18). Other quantities of i nterest, such as the hyperfine structure splitting of the ground state, wil l also be discussed. Finally, we will give a brief outlook into the future and comment on some o f the possible antiproton facilities which could be used to continue this f ield of research well into the next century.