Johannes Ullmann

High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED

Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209Bi82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209Bi82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.

An improved value for the hyperfine splitting of hydrogen-like 209Bi82+

We report an improved measurement of the hyperfine splitting in hydrogen-like bismuth (209Bi82+) at the experimental storage ring ESR at GSI by laser spectroscopy on a coasting beam. Accuracy was improved by about an order of magnitude compared to the first observation in 1994. The most important improvement is an in situ high voltage measurement at the electron cooler (EC) platform with an accuracy at the 10 ppm level. Furthermore, the space charge effect of the EC current on the ion velocity was determined with two independent techniques that provided consistent results. The result of nm provides an important reference value for experiments testing bound-state quantum electrodynamics in the strong magnetic field regime by evaluating the specific difference between the splittings in the hydrogen-like and lithium-like ions.

Laser cooling of relativistic heavy-ion beams for FAIR

Laser cooling is a powerful technique to reduce the longitudinal momentum spread of stored relativistic ion beams. Based on successful experiments at the experimental storage ring at GSI in Darmstadt, of which we show some important results in this paper, we present our plans for laser cooling of relativistic ion beams in the future heavy-ion synchrotron SIS100 at the Facility for Antiproton and Ion Research in Darmstadt.

Laser spectroscopy of the ground-state hyperfine structure in H-like and Li-like bismuth

The LIBELLE experiment performed at the experimental storage ring (ESR) at the GSI Helmholtz Center in Darmstadt aims for the determination of the ground state hyperfine (HFS) transitions and lifetimes in hydrogen-like (209Bi82+) and lithium-like (209Bi80+) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. While the HFS transition in H-like bismuth was already observed in earlier experiments at the ESR, the LIBELLE experiment succeeded for the first time to measure the HFS transition in Li-like bismuth in a laser spectroscopy experiment.

Hyperfine transition in 209Bi80+—one step forward

The hyperfine transitions in lithium-like and hydrogen-like bismuth were remeasured by direct laser spectroscopy at the experimental storage ring. For this we have now employed a voltage divider which enabled us to monitor the electron cooler voltage in situ. This will improve the experimental accuracy by about one order of magnitude with respect to our previous measurement using the same technique.

Improved accuracy of in-ring laser spectroscopy by in-situ electron cooler voltage measurement

Laser spectroscopy experiments of highly charged, heavy ions at the experimental storage ring ESR have been performed for more than twenty years by now [1, 2, 3], aiming at tests of fundamental theories. A conclusive test of bound-state quantum electrodynamics (BS-QED) in strong fields, however, has not been reached so far, because of the large uncertainty arising from the unknown spatial distribution of the nuclear magnetization (Bohr-Weisskop f effect). The method formulated by Shabaev and coworkers [4], defining a specific difference between the hyperfine splittings in hydrogenand lithium-like ions of the same species removes these uncertainties and provides the possibility to test BS-QED without nuclear uncertainties. Although the attempt in 2011 to measure the hyperfine splitting energies of the ground states in hydrogenand lithiumlike bismuth ions was for the first time successful in detecting both resonances, it yielded a large uncertainty caused by an inaccurate knowledge of the ion velocity [5]. Hence, a second attempt was performed in March 2014 with an improved setup.