Two-body weak decay studies in an ion storage ring

We have studied in a heavy ion storage ring at GSI Darmstadt, Germany, the time dependence of the orbital electron capture decays of H-like 140Pr, 142Pm, and 122I ions and found that the electron capture rate is not purely exponential but in addition time modulated with periods of T7.o6(8) s, 7.10(22) s and 6.11(3) s for 140Pr, 142Pm and 122I, respectively, measured in the laboratory system of the ions moving with 71% of speed of light (Lorentz factor γ= 1.43). The modulation amplitude is a= 0.20(3) in average for all three ions. Such modulation periods correspond to a small energy difference of 8.6x10-16 eV, and 7.5x10-16 eV, respectively, for a quantum beat type phenomenon. We attribute it to the mixing of massive electron neutrinos emitted in the decays with squared mass difference Δm2 = 2.20(3) x10-4 eV2. It is about 2.9 times larger than latest value reported by the KamLAND antineutrino oscillation experiment.