First direct observation of strong interaction spin-orbit effects in antiprotonic atoms

The strong -p-nucleus spin-orbit interaction was investigated in a measurement of the strong-interaction effects of the 9→8 transition in -p¹⁷⁴Yb at the Low-Energy Antiproton Ring (LEAR) at CERN. This measurement was part of an experimental programme where, for the first time, the fine-structure components of the last observable X-ray transition in a -p atom, which carries information on the strong -p-nucleus interaction, were resolved and studied individually. The observed splitting ΔE_exp=2408±26 eV consists of the electromagnetic fine-structure splitting ΔE_FS=2350 eV and an additional splitting Δe{open}=58±26 eV. In addition, one finds a significant difference in the level widths of Δ=195±59 eV with the larger value_{{two downward arrows}}=1216±41 eV for the lower fine-structure level. This experiment follows an earlier measurement on -p¹³⁸Ba, where the transition 8→7 is influenced by the strong interaction. In this case, however, the fine-structure components could not be resolved. The results for¹⁷⁴Yb may be attributed to a spin-orbit (LS) term in the complex strong-interaction potential.