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Search for a heavy neutral lepton with the MAGNETO-ν experiment using Pu241β− decays

C. Lee1,*, X. Zhang1, A. Kavner1,†, T. Parsons-Davis1, D. Lee1, N. Hines1, S. T. P. Boyd2, M. Loidl3, X. Mougeot3 et al.

M. Rodrigues3, M. K. Lee4, J. W. Song4, R. Wood5,‡, I. Jovanovic5,‡, and G. B. Kim1,§

  • *Contact author: lee1118@llnl.gov
  • †Present address: University of Zurich, Rämistrasse 71, 8006 Zürich, Switzerland.
  • ‡Present address: University of California, Berkeley; 4163 Etcheverry Hall, Berkeley, CA 94720.
  • §Contact author: kim90@llnl.gov
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Phys. Rev. C 113, 024620 – Published 26 February, 2026

DOI: https://doi.org/10.1103/4xt7-mdyv

Abstract

The MAGNETO-ν experiment searches for keV-scale heavy neutral leptons (HNLs) through precise measurements of the β−-decay spectrum of Pu241. We present spectra comprising a total of 194 million β− decays recorded using decay energy spectrometry with metallic magnetic calorimeters, representing the most statistically precise measurement of Pu241β− decay to date. The β-endpoint energy was determined using γ rays and x-rays from an external Ba133 calibration source, yielding Qβ=22.273(33)keV. The measured spectrum shows no statistically significant deviation from the allowed β-decay model. From a subset of the high-statistics data, we set an upper limit on the mixing of an 11.5-keV HNL with the electron neutrino, |Ue4|2<1.31×10−3 at the 95% confidence level.

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Phys. Rev. C 113, 024620– Published 26 February, 2026
  • Received 31 August 2025
  • Revised 20 November 2025
  • Accepted 13 January 2026
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