Direct Experimental Constraints on the Spatial Extent of a Neutrino Wavepacket

Joseph Smolsky,Kyle G Leach， Louis Wagner,William K Warburton

Nature（2025）

Colorado School of Mines Department of Physics | Michigan State University Facility for Rare Isotope Beams | TRIUMF | NOVA School of Science and Technology Department of Physics | Université de Strasbourg Institut de Physique et Chimie des Matériaux de Strasbourg | Lawrence Livermore National Laboratory Department of Physics | STAR Cryoelectonics LLC | Colorado School of Mines Department of Mettalurgical and Materials Engineering | Lawrence Livermore National Laboratory | XIA LLC | LPC Caen | Laboratoire National Henri Becquerel (LNE-LNHB) | Pacific Northwest National Laboratory

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Abstract

Despite their high relative abundance in our Universe, neutrinos are the least understood fundamental particles of nature. In fact, the quantum properties of neutrinos emitted in experimentally relevant sources are theoretically contested1-4 and the spatial extent of the neutrino wavepacket is only loosely constrained by reactor neutrino oscillation data with a spread of 13 orders of magnitude5,6. Here we present a method to directly access this quantity by precisely measuring the energy width of the recoil daughter nucleus emitted in the radioactive decay of beryllium-7. The final state in the decay process contains a recoiling lithium-7 nucleus, which is entangled with an electron neutrino at creation. The lithium-7 energy spectrum is measured to high precision by directly embedding beryllium-7 radioisotopes into a high-resolution superconducting tunnel junction that is operated as a cryogenic sensor. Under this approach, we set a lower limit on the Heisenberg spatial uncertainty of the recoil daughter of 6.2 pm, which implies that the final-state system is localized at a scale more than a thousand times larger than the nucleus itself. From this measurement, the first, to our knowledge, direct lower limit on the spatial extent of a neutrino wavepacket is extracted. These results may have implications in several areas including the theoretical understanding of neutrino properties, the nature of localization in weak nuclear decays and the interpretation of neutrino physics data.

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Key words

Neutrino Detection,Neutrino Oscillations,Neutron Lifetime Measurement,Neutrino Interactions

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