Laser spectroscopy at ESR: a QED-test is finally in reach!

Recent tests of QED in bound states of light ions reach unprecedented accuracies. In heavy ions however, such tests can be performed in the extremely strong fields of the large nucleus. In the 1990s, the first attempts to test QED by in-ring laser spectroscopy of the ground-state hyperfine splitting energies of highly charged heavy ions were hampered by insufficient knowledge of nuclear corrections. By measuring the transition energies of two charges states of the same nucleus, these corrections can be canceled. Therefore, the known transition energy of hydrogen-like bismuth had to be complemented by the one in the lithium-like charge state. The big success of finding this resonance in 2011 was overshadowed again, but this time by the inaccurately known voltage of the electron cooler, which is needed to transform the measurement to the ions’ rest-frame. In the latest attempt in 2014 the voltage was measured in-situ for the first time, which removed the main systematic uncertainty and also served as a diagnostic tool. First results of this beam time will be presented and systematic effects will be discussed.