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

TL;DR

MAGNETO-$\nu$ employs decay-energy spectrometry with metallic magnetic calorimeters to measure the $^{241}$Pu $\beta^-$ spectrum at unprecedented statistics, enabling a keV-scale HNL search via spectral distortions. The analysis integrates a precise $Q_\beta$ calibration, atomic-correction physics, and a likelihood-based HNL search across $m_4$ in the keV range, finding no significant HNL signal. From 194 million decays, it sets a competitive upper limit $|U_{e4}|^2 < 1.31\times10^{-3}$ at $m_4\approx 11.5$ keV and projects substantial gains with larger datasets and improved control of ADC nonlinearity. The work demonstrates the viability of MMC-based DES for precision beta-spectrometry and warm-dark-matter searches, with a clear path toward deeper sensitivity (e.g., $|U_{e4}|^2 \sim 4\times10^{-4}$ at $\sim13$ keV) in future phases.

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.

Figures (17)

• Figure 1: HNL mixing signature in the $^{241}\text{Pu}$$\beta$ decay spectrum. Normal decays emitting active neutrinos terminate at $Q_\beta$ (dashed), whereas decays emitting hypothetical HNLs with mass $m_4$ = 10 keV end at $Q_\beta - m_4$ (gray solid), near 12 keV. The total spectrum (black solid), given by the sum of both decay modes, exhibits a characteristic “kink” at the endpoint of the HNL branch. An unusually large admixture, $|U_{e4}|^2 = 0.2$, is used here for illustration.
• Figure 2: Decay scheme of $^{241}$Pu ENDF-VII. Most $^{241}$Pu nuclei undergo $\beta$ decay to $^{241}$Am with $Q_\beta$ = 20.78 (17) keV Wang_2021. A small fraction undergoes $\alpha$ decay producing $^{237}$U, which subsequently $\beta$-decays.
• Figure 3: (Top) Principle of MMC-based DES. A $^{241}$Pu nucleus embedded in a gold absorber decays, and the decay energy is transferred to the Au:Er MMC sensor, altering its magnetization. (Left) Representative $\alpha$-decay signals from Run 122; Inset shows a zoomed view of pulse rise. Channel 1 pulses are faster due to smaller heat capacity of its MMC. The $y$-axis is in arbitrary units. (Right) Photograph of the MAGNETO-$\nu$ setup.
• Figure 4: (Top) Raw Channel 1 pulse (gray) with corresponding short- (green) and long- (red) trapezoid-shaped pulses. Pulse amplitude is measured at $t_a$ (blue dashed) on the long pulse. (Inset) Zoomed-in view of a smaller pulse showing that the raw pulse begins (red dashed) 0.08 ms before the short pulse crosses threshold (green dashed); $t = 0$ is defined by linear fits (white dot-dashed) to the long pulse. (Bottom) A sensor-hit signal rises and decays faster than a typical absorber signal in the top panel. All $y$-axes are in arbitrary units.
• Figure 5: Run 122 trigger efficiencies with best-fit sigmoid curves. Trigger dead time limited the asymptotic efficiency.