Antihydrogen

Future precision experiments with trapped cold antihydrogen promise to provide extremely stringent tests of the fundamental symmetry between matter and antimatter (CPT symmetry) in the hadron as well as in the lepton sector. Ultrahigh-resolution Doppler-free two-photon laser-spectroscopy of ordinary hydrogen and antihydrogen might be used to compare matter and antimatter at unprecedented levels of experimental precision. In addition, there is the fascinating prospect to directly observe the gravitational force on antimatter, because antihydrogen is a pure antimatter system which is both stable and electrically neutral. For both types of experiment it is essential to cool the antihydrogen atoms. Laser-cooling of antihydrogen can be done on the transition from the 1S ground state to the 2P excited state (Lyman-alpha) at a wavelength of 122 nm in the Vacuum-Ultraviolet. Continuous coherent radiation at this wavelength can be produced by four- wave sum-frequency mixing in mercury vapour. First experiments used three argon-ion-lasers, one of which was frequency-doubled in an external cavity (257 nm wavelength), the second one was used to pump a dye-laser (545 nm wavelength) and the third one was used to pump a titanium: sapphire laser, again with external frequency doubling (399 nm wavelength). Future laser-cooling of antihydrogen, however, requires reliable Lyman-alpha generation in the beam-time environment of an accelerator facility. We have thus set out to replace the dye laser and the argon-ion laser. Recent first results from a second-generation continuous-wave Lyman-alpha source, based on solid-state lasers, will be discussed. Current antihydrogen experiments use the Antiproton Decelerator (AD) at CERN and this talk will review the status of the ATRAP experiment. An exciting new horizon is FLAIR, the Facility for Low-Energy Antiproton and Ion Research. This is a next-generation low-energy antiproton source that will make use of the high flux of antiprotons at the - upcoming international FAIR research center near GSI/Darmstadt.