Antiprotonic helium is an exotic three-body system consisting of a helium nucleus, an electron and an antiproton. The antiproton occupies highly excited metastable states, where the influence of the strong interaction with the nucleus is negligibly small. The metastable states can be explored by precision spectroscopy and, by comparing the results to three-body QED calculations, tests of CPT symmetry can be performed.
The hyperfine structure of antiprotonic helium is unique because the antiproton occupies states with extremely large angular momentum quantum numbers L ~ 33...37, leading to a dominant splitting arising from the interaction of the antiproton angular momentum and the electron spin called hyperfine (HF) splitting. The antiproton spin leads to a further splitting called super hyperfine (SHF) splitting, yielding a quadruplet structure. Measuring the HF structure and comparing it to calculations has the potential to determine the spin magnetic moment of the antiproton, which is known to 0.3%, to higher precision.
The Stefan Meyer Institute is currently performing measurements within the ASACUSA collaboration at CERN-AD of the (n,L)=(37,35) state of antiprotonic helium-4, aiming at improving the value of the antiproton magnetic moment by a factor 3 or more. We propose here to extend these measurements to a second state of antiprotonic helium-4 and for the first time ever investigate a state in antiprotonic helium-3, where the spin of the helium-3 nucleus leads to a further splitting resulting in an octuplet structure which will pose an even more rigorous test on the validity of the three-body calculations. The proposal is made in collaboration with two Russian teams of scientists who will improve the current calculation of the energy intervals of the states under investigation (V.I. Korobov, JINR Dubna) and further investigate collisional processes (G. Ya. Korenman, Moscow State University) that might lead to shifts of the resonance lines under experimental conditions and are needed to describe the resonance line shape for a precision determination of the line centers.
The proposal will lead to a thorough and systematic study of the hyperfine structure of antiprotonic helium-3 and helium-4 and allow a reliable determination of the antiproton magnetic moment with higher precision than currently available. It thus constitutes a precision test of theoretical description of antiprotonic helium and its interaction with the surrounding medium and ultimately of CPT symmetry.