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.

References

In fact, the quantum properties of neutrinos emitted in experimentally relevant sources are virtually unknown and the spatial extent of the neutrino wavepacket is only loosely constrained by reactor neutrino oscillation data with a spread of 13 orders of magnitude.

— Direct Experimental Constraints on the Spatial Extent of a Neutrino Wavepacket  (2404.03102 - Smolsky et al., 2024) in Abstract

So, CFI, which is a function of probability and directly deals with the detection of the experiment, is more reliable for our analysis. Moreover, we have previously shown the classical Fisher Information (CFI) is proportional to the ∂²χ²/∂σₓ² as proved in our previous work, establishing CFI as a statistically grounded sensitivity measure rather than a purely heuristic diagnostic.

— Probing Quantum Foundations in Long-Baseline Neutrino Oscillations: Wave Packet Approach from MINOS Data to DUNE Predictions  (2609.02975 - Farooq et al., 2 Sep 2026) in Section 5, “Classical Fisher Information : Sensitivity of σₓ”