The GSI method for studying neutrino mass differences - For Pedestrians

A new experiment studying the behavior of a radioactive ion before its weak decay by K-capture suggests that neutrino masses and mixing can be investigated without detecting the neutrino. Every weak decay can be observed, thus avoiding the suppression by the low neutrino absorption cross section of the signal in conventional neutrino oscillation experiments. The normally unobservable long wave lengths are made observable by having the radioactive source move a long distance circulating around in a storage ring. A new oscillation phenomenon with nonexponential decay arises in this "watched pot" experiment where continous monitoring sets the decay clock back to zero while preserving oscillating phases in the initial state. The initial ion wave packet has a momentum spread required by Heisenberg and contains pairs of components with different momenta and energies. These can produce neutrino amplitudes in two mass eigenstates with different momenta which mix to produce a single $\nu_e$ state. In this typical quantum mechanics "two-slit" or "which path" experiment a transition between the same initial and final states can go via two paths in energy-momentum space. Their relative phases change with the propagation of the initial state between the points of entry and decay. The oscillations produced by these phase changes are shown to be consistent with quantum mechanics and causality.