Laser Spectroscopy of Antiprotonic Helium Atoms

%PS205 %title\\ \\Following the discovery of metastable antiprotonic helium atoms ($\overline{p}He^{+} $) at KEK in 1991, systematic studies of their properties were made at LEAR from 1991 to 1996. In the first two years the lifetime of $\overline{p}He^{+}$ in liquid and gaseous helium at various temperatures and pressures was measured and the effect of foreign gases on the lifetime of these atoms was investigated. Effects were also discovered which gave the antiproton a 14\% longer lifetime in $^4$He than in $^3$He, and resulted in important differences in the shape of the annihilation time spectra in the two isotopes.\\ \\Since 1993 laser spectroscopy of the metastable $\overline{p}He^{+}$ atoms became the main focus of PS205. Transitions were stimulated between metastable and non-metastable states of the $\overline{p}He^{+}$ atom by firing a pulsed dye laser beam into the helium target every time an identified metastable atom was present (Figure 1). If the laser frequency matched the transition energy, the \={p} were deexcited to the non-metastable level where they immediately annihilated and produced a sharp spike in the antiproton annihilation time spectrum. The technique was first used with the slow-extracted 200 MeV/c \={p} beam from LEAR, but was later extended to make use of fast extracted \={p} bunches. This bunched operation of LEAR was better suited for weak resonances and for studies made early in the lifetime of the $\overline{p}He^{+}$ atoms, since the lasers could be fired in advance of the arrival of the \={p} bunch. In all 10 resonant transitions in $^4$He and 3 in $^3$He have now been observed. The measured energies show agreement at the remarkable level of 10 ppm with the most accurate theoretical calculations available (cf Figure~2~a). More recently the hyperfine structure arising from the interaction of the electron spin and the \={p} orbital angular momentum has been resolved in the $(n,l)=(37,35)\rightarrow(38,34)$ transition ($\lambda = 726.097(4)$ nm, cf.~Figure~2b).\\ \\The laser spectroscopy technique also allows investigations of the influence of the surrounding helium atoms on specific states of the $\overline{p}He^{+} $ atoms. It turns out that both single-level lifetimes and (to a small extent) energy levels depend on the density of helium, and that small admixtures of hydrogen and oxygen shorten the level lifetimes drastically. The fact that this effect is state-dependent means that certain metastable states can be transformed into short-lived ones by the addition of carefully controlled quantities of hydrogen. This feature has been used in the most recent experiments as to detect still more resonances.\\ \\\\ \\\\ \\