Hyperfine splitting is a familiar phenomenon in atomic physics in which magnetic interactions between the nucleus and the electrons split the electronic energy levels into sublevels. Eberhard Widmann of the University of Tokyo and colleagues in Japan and Europe have confirmed that helium atoms in which one of the electrons is replaced by an antiproton exhibit hyperfine splitting as well as a more subtle phenomenon known as “superhyperfine” splitting. This effect results from the magnetic interaction between the orbital angular momentum of the anti-proton, the spin of the electron and the spin of the anti-proton (E. Widmann et al 2002 Phys. Rev. Lett. 89 243402).
Widmann and co-workers have previously observed hyperfine splitting within anti-protonic helium. This is caused by the magnetic interaction between the orbital angular momentum of the anti-proton and the spin of the electron. The latest results provide a more accurate measurement of this phenomenon and also confirm the theoretical prediction made by Dimitar Bakalov of the Bulgarian Academy of Sciences in Sofia and Vladimir Korobov of the Joint Institute for Nuclear Research in Dubna that the two levels in each hyperfine “doublet” are themselves split into two further sublevels. This superhyperfine effect arises from the weaker magnetic interaction between the spin of the anti-proton and the other angular momenta. In the two sublevels within each half of the doublet, the electron spins are parallel and the anti-proton spins antiparallel.
The researchers created antiprotonic helium atoms by directing pulses of antiprotons from CERN’s Antiproton Decelerator into helium gas. Initially the two levels in each hyperfine doublet were equally populated. Then by firing a laser pulse into the helium the researchers were able to reduce the population of the lower level relative to the higher level by sending antiprotons in atoms lying in the lower level into atomic states from which they instantly annihilated. They then sent a microwave pulse into the gas to stimulate repopulation of the lower level, and used a second laser to measure the new population of the lower level.
The frequency of the microwave pulse was chosen to match the two theoretically allowed transitions that occur between sublevels with the same anti-proton spin in different halves of the doublet. By recording a repopulation of the lower half of the doublet, the researchers verified that these transitions do indeed occur and, therefore, that anti-protonic helium atoms undergo hyperfine splitting. Their results agreed with theory to a level of six parts in 105. They also confirm the expected superhyperfine splitting with a precision of 1.6%.