Eberhard Widmann erklärt das HBAR-Experiment.
Steven D Bass (Innsbruck)
Aus Anlass der Aktualisierung der Europäischen Strategie der Teilchenphysik, die Ende Mai beschlossen werden soll, wurde vom FAKT (Fachausschuss für Kern- und Teilchenphysik der Österreichischen Physikalischen Gesellschaft) ein...
We report on studies of electron capture decays (EC) of highly ionized nuclei with one or two electrons in their K-orbit (H- and He-like ions) in the Experimental Storage Ring (ESR) of GSI, Darmstadt. These experiments allow for the first time investigating two-body weak decays with a pair of entangled lepton states with quasi mono-energetic massive neutrinos, the properties of which can thus be studied without the inefficient neutrino detection.
In recent experiments we found that the H-like 140Pr58+ and 142Pm60+ ions decay 1.49(8) and 1.44(6) times faster than the He-like 140Pr57+ and 142Pm59+ ions. This anomaly is explained by spin effects due to the hyperfine structure of the H-like ions. The EC/ß+ branching ratios are consistent with expectations from standard weak decay theories within 3%.
Even more surprising is the recently observed time dependence of the electron capture rate of H-like 140Pr58+ and 142Pm60+ ions. It is not exponential but time modulated with amplitudes of a = 0.18(3) and 0.22(3), and periods of T = 7.06(8) s and 7.10(22) s, respectively. It corresponds to an energy difference of 8.6×10-16 eV for a quantum beat type phenomenon. We attribute it to the small recoil energy difference DE of the detected entangled daughter ions induced by the kick of mixed massive neutrinos with masses m1 and m2. Using the cm relation DE = Dm²/2M, derived from energy and momentum conservation in the decay of a nucleus with mass M and neutrinos with a mass difference Dm = m2 – m1, a squared mass difference Dm² = 2.22(3)×10–4 eV² is derived. It is 2.9 times larger than reported recently by the KamLAND neutrino oscillation experiment. We have discussed this difference in terms of neutrino mass modification by lepton-W boson loops in the high Coulomb field of the heavy daughter nucleus, but the large difference in Dm² is not understood. A further problem is the small modulation amplitude with an average value <a> = 0.20(2), which would lead to a small neutrino mixing angle q ~ 6o compared with a value of about 34o from sun neutrino oscillations. The modulation amplitude may be reduced by the preceding photon decay in H-like ions. In a preliminary analysis of the ß+ branch of 142Pm we found no modulation with a < 0.03 of this three-body decay with a broad neutrino spectrum in accordance with our theoretical prediction. If verified, this is direct evidence that the modulation is caused by mixed massive neutrinos from quasi-free two-body decays.