Characterize the spatial extent and other quantum properties of neutrinos from realistic sources

Characterize the quantum properties of neutrinos emitted from experimentally relevant sources, with particular emphasis on determining the spatial extent (position-space width) of the neutrino wavepacket, which remains virtually unknown and is only loosely constrained by reactor antineutrino oscillation data across 13 orders of magnitude.

Background

Neutrino oscillations depend on the coherence properties of the neutrino state, which are encoded in the wavepacket’s spatial and momentum distributions. Because neutrinos interact only weakly, direct measurements of these quantum properties are challenging and have typically relied on indirect constraints from large-scale oscillation experiments.

The paper introduces a new approach that probes these properties via precision measurements of the recoil spectrum in electron-capture decay, exploiting entanglement between the recoiling nucleus and the emitted neutrino. While the authors report the first direct limits on the wavepacket width from an electron-capture source, they note that, in general, the quantum properties—especially the spatial extent—remain only loosely constrained and largely unknown across many experimentally relevant sources.