Dear colleagues and friends of the HI Jena,

welcome to the May 2017 issue of the HI Jena newsletter.
Below you find informations and news about recent activities of our institute

Kind Regards,
Helmholtz Institute Jena

The specific difference between the ground state hyperfine splittings (hfs) in hydrogen-like and lithium-like ions of the same isotope was suggested about fifteen years ago as the ultimate tool to prove bound-state QED in the strong magnetic field generated by the heavy nucleus [1].
The isotope of bismuth, ²⁰⁹Bi, exhibits both groundstate hyperfine transitions very close to the visible spectrum and therefore they can be probed by laser spectroscopy. The 1s hfs in hydrogen-like bismuth (²⁰⁹Bi⁸²⁺) was measured by direct laser spectroscopy at the experimental storage ring (ESR) in 1994 [2]. Seventeen years later and using an improved laser spectroscopic technique [3] at the same ring we found the 2s hfs in lithium-like bismuth (²⁰⁹Bi⁸⁰⁺) [4]. Combined with a new measurement of the 1s hfs in the
hydrogen-like we found good agreement with the theoretical prediction. Yet the accuracy of our result was limited at that time by the calibration of the electron-cooler voltage, determining the velocity of the ions in the ring [4].
We have repeated this experiment in 2014. This time we were able to monitor the electron-cooler voltage in situ using a voltage divider provided by PTB in Braunschweig. First results on the 1s hfs in hydrogen-like bismuth using a coasting beam have been already published in [5].

After two years for studying potential systematic errors we have now achieved relative accuracies at the 10^-5 level for both hyperfine transitions [6]. These are the most accurate transition wavelengths measured in a heavy highly charged ion so far. This improvement has finally allowed us to improve the accuracy on the specific difference by an order of magnitude. A significant fact is that our new result shows now a 7σ-difference to the latest theoretical prediction [7]. Such a large discrepancy was not expected by theory and therefore it has put the specific difference into question as a tool to test QED in strong fields.
It has, however, been pointed out that the specific difference is still sensitive to the nuclear magnetic moment of μ(²⁰⁹Bi) [7,]. A small variation from the tabulated value could bring theory and experiment into agreement. Therefore, there is also a need to remeasure this ground state nuclear property. A new measurement of μ(²⁰⁹Bi) via nu clear magnetic resonance is in preparation at TU Darmstadt and on a long-term perspective, a measurement of the hfs in both ion species is planned at the SpecTrap Penning trap [8] and of the magnetic moment directly on hydrogen-like bismuth at the Penning trap ARTEMIS, which are both installed at HITRAP/GSI.
In order to confirm both, the reliability of the proposed nuclear structure independence in the specific difference as well as any assumption of a different nuclear magnetic moment value, we are now considering to measure the hfs in hydrogen-like and lithium-like of a second isotope. Two candidates are envisaged: ²⁰⁷Bi and ²⁰⁸Bi. Their magnetic moments are ascribed to the single proton outside the ²⁰⁸Pb-core and the additional neutron holes below the N = 126 shell-closure. The specific difference in ²⁰⁷Bi is expected to have a deviation that scales with the magnetic moment compared to that in ²⁰⁹Bi because of the similar
magnetic moment and spin. In the case of ²⁰⁸Bi, a new measurement of the magnetic moment relative to μ(²⁰⁹Bi) has recently been performed at COLLAPS/ISOLDE by collinear laser spectroscopy [9]. If the moment of ²⁰⁹Bi is really different from the literature value, then we expect a similar disagreement for the specific difference in the case of ²⁰⁸Bi. In case that our disagreement with theory is an artifact due to an incomplete cancellation of the Bohr-Weisskopf effect in the specific difference, the deviation for ²⁰⁸Bi should not scale with the magnetic moment since a considerably different Bohr-Weisskopf effect is expected for this isotope with a different spin and nuclear magnetism distribution. Bound-state strong-field QED can only be proven if this cancellation works as proposed in [1].

[1] V. M. Shabaev, A. N. Artemyev, V. A. Yerokhin, O. M. Zherebtsov, and G. Soff, Phys. Rev. Lett 86 3959 (2001).
[2] I. Klaft, et al., Phys. Rev. Lett. 73 2425 (1994).
[3] R. Sánchez et. al. J. Phys. B. 50 085004 (2017).
[4] M. Lochmann et al. Phys. Rev. A 90 030501 (2014).
[5] J. Ullmann et al. J. Phys. B 48 144022 (2015).
[6] J. Ullmann, et al., Nature Communications 8, 15484 (2017).
[7] A. V. Volotka et al. Phys. Rev. Lett. 108 073001 (2012).
[8] T. Murböck et al. Phys. Rev. A 94 043410 (2016).
[9] S. Schmidt et al. to be published.

Semiannual Palaver of the HI Jena

On March 23th the semi annual palaver of the HI Jena took place. We would like to thank all speakers for their nice presentations as well as all participants for their questions and comments triggering stimulating scientific discussions.

HI Jena participates in Open Day event of GSI and FAIR

The open day 2017 of GSI and FAIR was the first after many years, and the first presenting GSI and FAIR together. The almost eleven thousand visitors were handled perfectly, and all comments very extremely positive. It was a marvelous opportunity to communicate what we are and do to the general public, including our own families and friends who could finally see what we are up to all day, and why we are proud of it.

The event was very well prepared, publicized and organized, and all could see what the lab has achieved and is achieving and how tangible and concrete is the progress of FAIR. But even more, we have been able to transmit to the visitors the enthusiasm, the passion, the knowledge, the creativity and the commitment of a whole community, the qualities which make ours much more than a workplace, but a fantastic adventure to advance human knowledge. That is what a world class research center is, and that is what our visitors yesterday could experience and see.

Call for Proposals for Beamtime in 2018/2019 - deadline prolonged to 19. June 2017

We would like to inform you that the deadline for submission of beam time proposals has been prolonged from 31. May to 19. June 2017. For more information and instructions on this ‘Cell for Proposals’ please also see the G-PAC webpage (www.gsi.de/g-pac) - or www.gsi.de/phelix_committee for PHELIX beamtimes respectively. And please remember contacting your GSI-contact person on technical feasibility prior to submission!