Atomic Spectroscopy and Collisions Using Slow Antiprotons \\ ASACUSA Collaboration

ASACUSA (\underline{A}tomic \underline{S}pectroscopy \underline{A}nd \underline{C}ollisions \underline{U}sing \underline{S}low \underline{A}ntiprotons) is a collaboration between a number of Japanese and European research institutions, with the goal of studying bound and continuum states of antiprotons with simple atoms.\\ Three phases of experimentation are planned for ASACUSA. In the first phase, we use the direct $\overline{p}$ beam from AD at 5.3 MeV and concentrate on the laser and microwave spectroscopy of the metastable antiprotonic helium atom, $\overline{p}$He$^+$, consisting of an electron and antiproton bound by the Coulomb force to the helium nucleus. Samples of these are readily created by bringing AD antiproton beam bunches to rest in helium gas. With the help of techniques developed at LEAR for resonating high precision laser beams with antiproton transitions in these atoms, ASACUSA achieved several of these first-phase objectives during a few short months of AD operation in 2000. Six atomic transitions of the metastable antiprotonic helium atom were thereby detected, three of which had not previously been observed. They included two transitions in the UV region of the spectrum. If we assume CPT invariance between the properties of the proton and the antiproton, as the theoretical calculations of transition frequencies do, the agreement is a signature of the excellence of theoretical treatments and calculation techniques of three-body Coulomb system including QED corrections. If, on the other hand, we take the calculated values to be correct, the agreement gives a stringent test of the fundamental constants of the antiproton and therefore tests the CPT theorem. Taking the second point of view, the results from the year 2000 together constrain any difference between the antiproton and proton charges and masses by a further factor of eight beyond the limits obtained by our PS205 experiments at LEAR.\\ The year 2000 also saw the first success of the ASACUSA triple-resonance laser/ microwave experiment. In this, both laser and microwave beams are to be used to measure the peculiar hyperfine structure of $\overline{p}$He$^+$, associated with the interaction of the electron spin magnetic moment with the orbital magnetic moment of the antiproton.\\ In phase 2 experiments, we add an RFQ linear decelerator (RFQD), which decelerates antiprotons from 5.3 MeV to few tens of keV. By enabling us to stop the antiprotons in very low density gas targets, this makes it possible to study in detail the imperfectly understood processes by which antiprotonic atoms are created. It also allows us to measure the antiproton's specific energy loss dE/dx in a variety of materials. After a test period with 5 MeV protons spent in a proton beam in Aarhus, the RFQD was installed in the AD beam line in November, and reached its design energy in the tens of keV range, with deceleration efficiency approaching the expected value of 45$\%$. The RFQD beam was then used to produce extensive new data on the stopping power of carbon and gold for antiprotons in the crucial unexplored region between 60 keV and 8 keV.\\ In phase 3, a multielectrode antiproton trap, another powerful tool for studying the antiproton, will be placed downstream of the RFQD. At the time of writing (March 2001), this had been assembled in the AD beam line after extensive testing in Tokyo. Antiprotons will be captured and cooled (by collisions with electrons) in the trap, and then extracted from it at and below $\sim 1 $ keV (eventually down to $\sim 10 $ eV). Such ultra-low-energy beams have hitherto been unavailable. They will allow us to produce antiprotonic atoms at such low target pressures that they are almost unperturbed by further collisions with target atoms. This will permit study of their production mechanisms in still closer detail, and the performance of extremely high precision spectroscopy.\\