Observation of non-exponential orbital electron-capture decay of stored hydrogen-like ions

In experiments conducted recently at the ion storage-cooler ring ESR of the GSI Helmholtzzentrum, Darmstadt, the two-body orbital electron-capture (EC) decay of stored and cooled hydrogen-like 140Pr and 142Pm ions has been investigated. On top of the exponential decay, periodic modulations with a period T = 7 s and an amplitude A = 0.2 were found for both systems. Tentatively these observations were explained as a kind of “quantum beats”, due to the fact that the electron neutrino, generated in these strict two-body decays, is a flavor—but neither a mass- nor energy- nor momentum-eigenstate. This interpretation has raised many objections. In order to discuss them reasonably, the first section of this paper after the introduction aims for a precise description of the experimental technique and the observables revealed in our experiments. It will be proven that many of the objections are entirely irrelevant, because they are based on a misinterpretation of our experiments and, in particular, of the observables addressed there. In the last section, the actual experimental status will be summarized, including preliminary results of the EC decay of hydrogen-like 122I ions. An outlook is presented on forthcoming experiments which could corroborate or disprove our observations and their explanation.