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arXiv · 2503.18350

Magneto-$ν$: Heavy neutral lepton search using $^{241}$Pu $β^-$ decays

C. Lee·X. Zhang·A. Kavner·T. Parsons-Davis·D. Lee·N. Hines·S. T. P. Boyd·M. Loidl·X. Mougeot·M. Rodrigues·M. K. Lee·J. Song·R. Wood·I. Jovanovic·G. B. Kim
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Abstract

The MAGNETO-$ν$ experiment searches for keV-scale heavy neutral leptons (HNLs) through precise measurements of the $β^-$-decay spectrum of $^{241}$Pu. 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 $^{241}$Pu $β^-$ decay to date. The $β$-endpoint energy was determined using $γ$ rays and X rays from an external $^{133}$Ba calibration source, yielding $Q_β= 22.273\,(33)$\,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, $|U_{e4}|^2 < 1.31 \times 10^{-3}$ at the 95\% confidence level.

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C. Lee, X. Zhang, A. Kavner, T. Parsons-Davis, D. Lee, N. Hines, S. T. P. Boyd, M. Loidl, X. Mougeot, M. Rodrigues, M. K. Lee, J. Song, R. Wood, I. Jovanovic, G. B. Kim. 2025-11-20. Magneto-$ν$: Heavy neutral lepton search using $^{241}$Pu $β^-$ decays. https://doi.org/10.1103/4xt7-mdyv

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