Most Stringent Bound on Electron Neutrino Mass Obtained with a Scalable Low-Temperature Microcalorimeter Array

The determination of the absolute neutrino mass scale remains a fundamental open question in particle physics, with profound implications for both the standard model and cosmology. Direct kinematic measurements, independent of model-dependent assumptions, provide the most robust approach to address this challenge. Here we present the most stringent upper bound on the effective electron neutrino mass ever obtained with a calorimetric measurement of the electron capture decay of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Ho</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>163</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math>. The HOLMES experiment employs an array of ion-implanted transition-edge sensor (TES) microcalorimeters, achieving an average energy resolution of 6 eV FWHM with a scalable, multiplexed readout technique. With a total of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>7</c:mn><c:mo>×</c:mo><c:msup><c:mn>10</c:mn><c:mn>7</c:mn></c:msup></c:math> decay events recorded over two months and a Bayesian statistical analysis, we derive an upper limit of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msub><e:mrow><e:mi>m</e:mi></e:mrow><e:mrow><e:mi>β</e:mi></e:mrow></e:msub><e:mo><</e:mo><e:mn>27</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mrow><e:mi>eV</e:mi><e:mo>/</e:mo><e:msup><e:mrow><e:mi>c</e:mi></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msup></e:mrow></e:mrow></e:math> at 90% credibility. These results validate the feasibility of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mmultiscripts><g:mrow><g:mi>Ho</g:mi></g:mrow><g:mprescripts/><g:none/><g:mrow><g:mn>163</g:mn></g:mrow></g:mmultiscripts></g:mrow></g:math> calorimetry for next-generation neutrino mass experiments and demonstrate the potential of a scalable TES-based microcalorimetric technique to push the sensitivity of direct neutrino mass measurements beyond the current state of the art.