Magneto-$\nu$: Heavy neutral lepton search using ${}^{241}$Pu ${\beta }^{-}$ decays

The MAGNETO-$\nu$ experiment searches for keV-scale heavy neutral leptons (HNLs) through precise measurements of the ${\beta }^{-}$-decay spectrum of ${}^{241}$Pu. We present spectra comprising a total of 194 million ${\beta }^{-}$ decays recorded using decay energy spectrometry with metallic magnetic calorimeters, representing the most statistically precise measurement of ${}^{241}$Pu ${\beta }^{-}$ decay to date. The $\beta$-endpoint energy was determined using $\gamma$ rays and X rays from an external ${}^{133}$Ba calibration source, yielding $Q_{\beta }=22.273(33)$,keV. The measured spectrum shows no statistically significant deviation from the allowed $\beta$-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, $\mathrm{\mid }{U}_{e4}{\mathrm{\mid }}^2<1.31\times {10}^{-3}$ at the 95% confidence level.