Search for a heavy neutral lepton with the MAGNETO- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ν</mml:mi> </mml:math> experiment using <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mm

The MAGNETO- <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>ν</a:mi> </a:math> experiment searches for keV-scale heavy neutral leptons (HNLs) through precise measurements of the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msup> <b:mi>β</b:mi> <b:mo>−</b:mo> </b:msup> </b:math> -decay spectrum of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mmultiscripts> <c:mi>Pu</c:mi> <c:mprescripts/> <c:none/> <c:mn>241</c:mn> </c:mmultiscripts> </c:math> . We present spectra comprising a total of 194 million <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mi>β</d:mi> <d:mo>−</d:mo> </d:msup> </d:math> decays recorded using decay energy spectrometry with metallic magnetic calorimeters, representing the most statistically precise measurement of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mmultiscripts> <e:mi>Pu</e:mi> <e:mprescripts/> <e:none/> <e:mn>241</e:mn> </e:mmultiscripts> <e:mspace width="0.16em"/> <e:msup> <e:mi>β</e:mi> <e:mo>−</e:mo> </e:msup> </e:mrow> </e:math> decay to date. The <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mi>β</g:mi> </g:math> -endpoint energy was determined using <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mi>γ</h:mi> </h:math> rays and x-rays from an external <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mmultiscripts> <i:mi>Ba</i:mi> <i:mprescripts/> <i:none/> <i:mn>133</i:mn> </i:mmultiscripts> </i:math> calibration source, yielding <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:msub> <j:mi>Q</j:mi> <j:mi>β</j:mi> </j:msub> <j:mo>=</j:mo> <j:mn>22.273</j:mn> <j:mspace width="0.16em"/> <j:mrow> <j:mo>(</j:mo> <j:mn>33</j:mn> <j:mo>)</j:mo> </j:mrow> <j:mspace width="0.28em"/> <j:mi>keV</j:mi> </j:mrow> </j:math> . The measured spectrum shows no statistically significant deviation from the allowed <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mi>β</m:mi> </m:math> -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, <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:mrow> <n:mo>|</n:mo> </n:mrow> <n:msub> <n:mi>U</n:mi> <n:mrow> <n:mi>e</n:mi> <n:mn>4</n:mn> </n:mrow> </n:msub> <n:msup> <n:mrow> <n:mo>|</n:mo> </n:mrow> <n:mn>2</n:mn> </n:msup> <n:mo>&lt;</n:mo> <n:mn>1.31</n:mn> <n:mo>×</n:mo> <n:msup> <n:mn>10</n:mn> <n:mrow> <n:mo>−</n:mo> <n:mn>3</n:mn> </n:mrow> </n:msup> </n:mrow> </n:math> at the 95% confidence level.