Magneto-$\hat{I}½$: Heavy neutral lepton search using ${}^{241}$Pu $β^-$ decays

Abstract

The MAGNETO-$\hat{I}½$ 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 ${\hat{I}}^2$-endpoint energy was determined using ${\hat{I}}^3$ rays and X rays from an external ${}^{133}$Ba calibration source, yielding $Q_{\hat{I}}^2=22.273(33)$\,keV. The measured spectrum shows no statistically significant deviation from the allowed ${\hat{I}}^2$-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.