Search for a heavy neutral lepton with the MAGNETO- ν experiment using Pu 241 β − decays

The MAGNETO- ν experiment searches for keV-scale heavy neutral leptons (HNLs) through precise measurements of the β − -decay spectrum of Pu 241 . 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 Pu 241 β − decay to date. The β -endpoint energy was determined using γ rays and x-rays from an external Ba 133 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 e 4 | 2 < 1.31 × 10 − 3 at the 95% confidence level.