Direct experimental constraints on the spatial extent of a neutrino wavepacket

Joseph Smolsky (joseph.smolsky@mines.edu), Kyle G. Leach (kleach@mines.edu), Ryan Abells, Pedro Amaro, Adrien Andoche, Keith Borbridge, Connor Bray, Robin Cantor, David Diercks, Spencer Fretwell, Stephan Friedrich, Abigail Gillespie, Mauro Guerra, Ad Hall, Cameron N. Harris, Jackson T. Harris, Leendert M. Hayen, Paul-Antoine Hervieux, Calvin Hinkle, Geon-Bo Kim, Inwook Kim, Amii Lamm, Annika Lennarz, Vincenzo Lordi, Jorge Machado, Andrew Marino, David McKeen, Xavier Mougeot, Francisco Ponce, Chris Ruiz, Amit Samanta, José Paulo Santos, Caitlyn Stone-Whitehead, John Taylor, Joseph Templet, Sriteja Upadhyayula, Louis Wagner and William K. Warburton
Additional contact information
Joseph Smolsky: Colorado School of Mines
Kyle G. Leach: Colorado School of Mines
Ryan Abells: TRIUMF
Pedro Amaro: Universidade Nova de Lisboa
Adrien Andoche: UMR 7504
Keith Borbridge: Colorado School of Mines
Connor Bray: Colorado School of Mines
Robin Cantor: STAR Cryoelectonics LLC
David Diercks: Colorado School of Mines
Spencer Fretwell: Colorado School of Mines
Stephan Friedrich: Lawrence Livermore National Laboratory
Abigail Gillespie: Colorado School of Mines
Mauro Guerra: Universidade Nova de Lisboa
Ad Hall: STAR Cryoelectonics LLC
Cameron N. Harris: Colorado School of Mines
Jackson T. Harris: XIA LLC
Leendert M. Hayen: Université de Caen
Paul-Antoine Hervieux: UMR 7504
Calvin Hinkle: Colorado School of Mines
Geon-Bo Kim: Lawrence Livermore National Laboratory
Inwook Kim: Lawrence Livermore National Laboratory
Amii Lamm: Colorado School of Mines
Annika Lennarz: TRIUMF
Vincenzo Lordi: Lawrence Livermore National Laboratory
Jorge Machado: Universidade Nova de Lisboa
Andrew Marino: Colorado School of Mines
David McKeen: TRIUMF
Xavier Mougeot: Laboratoire National Henri Becquerel (LNE-LNHB)
Francisco Ponce: Pacific Northwest National Laboratory
Chris Ruiz: TRIUMF
Amit Samanta: Lawrence Livermore National Laboratory
José Paulo Santos: Universidade Nova de Lisboa
Caitlyn Stone-Whitehead: Colorado School of Mines
John Taylor: Colorado School of Mines
Joseph Templet: Colorado School of Mines
Sriteja Upadhyayula: TRIUMF
Louis Wagner: Michigan State University
William K. Warburton: XIA LLC

Nature, 2025, vol. 638, issue 8051, 640-644

Abstract: 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.

Date: 2025
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DOI: 10.1038/s41586-024-08479-6

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