The hyperfine puzzle of strong field bound-state QED
Prof. Dr. Wilfried Nörtershäuser (Institut für Kernphysik, Technische Universität Darmstadt)

The so-called "specific difference" between the ground state hyperfine splittings in hydrogen-like and lithium-like heavy ions was suggested for a test of bound-state QED in the strongest magnetic fields available in the laboratory.
The isotope 209Bi is the ideal candidate since it has in both charge states ground-state hyperfine splittings that can be probed by laser spectroscopy. The 1s hyperfine splitting in hydrogen-like Bi-209 was measured by laser spectroscopy at the experimental storage ring (ESR) at the GSI Helmholtz-Center for Heavy Ion Research in Darmstadt already in 1994 [1]. But then it took 17 years before the corresponding transition in Li-like Bi was observed for the first time [2]. We have recently improved the accuracy of the measurements in both hyperfine transitions by more than an order of magnitude and could therefore perform the first significant test of a specific difference in a heavy element. Our result exhibits a deviation of more than 7 sigma from the theoretical prediction [4]. I will report on these measurements and further activities that have started to solve this "hyperfine puzzle" [3] of strong-field QED.

[1] I. Klaft et al., Phys. Rev. Lett. 73, 2425 (1994)

[2] M. Lochmann et al., Phys. Rev. A 90, 030501R (2014)

[3] J. Ullmann et al., Nature Comm. 8, 15484 (2017)

[4] P. Karr, Nature Physics 13, 553 (2017)