One of the experimental facilities at the German laboratory GSI Helmholtz Centre for Heavy Ion Research in Darmstadt is an Experimental Storage Ring (ESR) with electron cooling in which large numbers of highly charged radioactive ions can be stored for extended periods of time. This facility provides the means to make precise measurements of their decay modes. The absence of most or all of the electrons in the ions simplifies theoretical treatments of their influence on the decay. Also, such a high degree of ionization is typical in stellar environments where such decays play an important role in nucleosynthesis.
In 2007 an ESR experiment reported the observation of unexpected modulation in time of the rate of electron capture decays of highly ionized heavy atoms — 140Pr58+, which have a lifetime of 3.39 min. Such findings were soon repeated by the same group, and extended to include the decay of 142Pm60+ (lifetime 40.5 s). The oscillations in decay rate had time periods near to 7 s and amplitudes of about 20%. Such a phenomenon had not been previously observed, and was difficult to understand. The experimental group considered it very improbable that the appearance of the phenomenon is due to a technical artefact because they report that their detection technique provides—during the whole observation time—complete and uninterrupted information upon the status of each stored ion.
As this type of weak decay involves the production of an electron neutrino, attempts have been made to relate the observed oscillations to neutrino oscillations, but this proposal was highly controversial.
In 2013, a similar experimental group at the ESR now called the Two-Body-Weak-Decays Collaboration reported further observations of the phenomenon with measurements on 142Pm60+ with much higher precision in period and amplitude. The same period was observed, but the amplitude was only about a half of that previously seen.
Based on the assumption that the measurements have no serious flaws, as of 2017 on the order of fifty articles have been published offering and debating various possible theoretical explanations for the oscillating modulations. No consensus has emerged. An example of a proposed explanation relates to the difference in mass of neutrino mass eigenstates, but not through the well-known neutrino flavour oscillations. It is proposed that the quantum state of the daughter ion is "prematurely" disentangled from that of the co-produced neutrino by the ion's motion in its circular orbit guided by the magnetic fields of the storage ring magnets, well before detection of the ion. (Such disentanglement is usually caused by detection of the daughter in experiments not using a storage ring.) A probability amplitude for the ion circulating in the ring for a given time before detection is calculated to depend on the neutrino mass, so that the corresponding probability including all neutrino mass eigenstates would include an interference term involving the neutrino mass difference. The oscillation period estimated in this scheme is within an order of magnitude of that observed.
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