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Direct experimental constraints on the spatial extent of a neutrino wavepacket
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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, 2, 3–4 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.
Descrição
Funding Information: We thank D. Moore, D. Carney, B. Jones, C. Argüelles-Delgado, J. Formaggio and F. Sarazin for useful discussions. The BeEST experiment is funded by the Gordon and Betty Moore Foundation (no. 10.37807/GBMF11571); the US Department of Energy, Office of Science, Office of Nuclear Physics, under award nos. DE-SC0021245 and SCW1758; the LLNL Laboratory Directed Research and Development program through grant nos. 19-FS-027 and 20-LW-006; the European Metrology Programme for Innovation and Research (EMPIR) project nos. 17FUN02 MetroMMC and 20FUN09 PrimA-LTD; and the FCT—Fundação para a Ciência e Tecnologia (Portugal)—through national funds in the framework of the project no. UID/04559/2020 (LIBPhys). TRIUMF receives federal funding through a contribution agreement with the National Research Council of Canada. This work was performed under the auspices of the US Department of Energy by LLNL under contract no. DE-AC52-07NA27344. F.P. is funded as part of the Open Call Initiative at Pacific Northwest National Laboratory and conducted under the Laboratory Directed Research and Development Program. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the US Department of Energy. K.G.L. acknowledges support from the Facility for Rare Isotope Beams (FRIB) while on sabbatical. FRIB is a US Department of Energy, Office of Science User Facility, under award no. DE-SC0000661. Publisher Copyright: © The Author(s) 2025.
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