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Most stringent bound on electron neutrino mass obtained with a scalable low temperature microcalorimeter array

B. K. Alpert, M. Balata, D. T. Becker, D. A. Bennett, M. Borghesi, P. Campana, R. Carobene, M. De Gerone, W. B. Doriese, M. Faverzani, L. Ferrari Barusso, E. Ferri, J. W. Fowler, G. Gallucci, S. Gamba, J. D. Gard, F. Gatti, A. Giachero, M. Gobbo, U. Köster, D. Labranca, M. Lusignoli, P. Manfrinetti, J. A. B. Mates, E. Maugeri, R. Moretti, S. Nisi, A. Nucciotti, G. C. O'Neil, L. Origo, G. Pessina, S. Ragazzi, C. D. Reintsema, D. R. Schmidt, D. Schumann, D. S. Swetz, Z. Talip, J. N. Ullom, L. R. Vale

Abstract

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. In this Letter, 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 $^{163}$Ho. 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 $7\times10^7$ decay events recorded over two months and a Bayesian statistical analysis, we derive an upper limit of $m_β<27$ eV/c$^2$ at 90% credibility. These results validate the feasibility of $^{163}$Ho 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.

Most stringent bound on electron neutrino mass obtained with a scalable low temperature microcalorimeter array

Abstract

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. In this Letter, 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 Ho. 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 decay events recorded over two months and a Bayesian statistical analysis, we derive an upper limit of eV/c at 90% credibility. These results validate the feasibility of Ho 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.

Paper Structure

This paper contains 6 sections, 8 equations, 7 figures, 1 table.

Table of Contents

  1. Appendix A: $^{163}$Ho sample preparation and embedding
  2. Appendix B: Detector array multiplexed readout
  3. Appendix C: Data analysis
  4. Appendix D: Bayesian parameter estimation
  5. Appendix E: Monte Carlo investigation of the relevant systematic effects.
  6. Appendix F: Scaled sensitivity of future holmium-based experiments

Figures (7)

  • Figure 1: The total recorded $^{163}$Ho calorimetric spectrum obtained summing about 1000 partial calibrated spectra measured with the HOLMES microcalorimeters. The spectrum contains about $6\times10^7$ events above the 300 eV threshold. The top-right inset shows the distribution of the energy resolution (FWHM) of the individual partial spectra, evaluated from the noise equivalent power (NEP). The observed double-peaked structure in this distribution reflects an improvement in detector performance between the two physics runs.
  • Figure 2: Left: Copper box containing the 64 TES array in the middle. The two chips on either sides of the array are the bias network and the microwave multiplexer, respectively. The array dimensions are approximately ($20\times10$) mm$^2$. The multiplexer has the feedline aligned with the SMA connectors used for feeding the readout tones. For readout, two SMAs on one side are connected via a short semirigid coaxial cable. Right: Schematic, not to scale, representation of the HOLMES TES microcalorimeter used in the experiment.
  • Figure 3: Top: Results of the Bayesian analysis of the calorimetric $^{163}$Ho spectrum in the ROI with dashed lines showing the various components in Eq. (\ref{['eq:SpecSum']}) and Eq. (\ref{['eq:HoEffect']}). Each individual spectral component of Eq. (\ref{['eq:HoEffect']}) is multiplied by $\mathcal{F}_\mathrm{PS}$ and convolved with $\mathcal{R}_{\rm eff}$. The red line and the reddish band represent the mean and standard deviation of the distribution of the generated data, following the posteriors. The bottom part shows the residuals $r$ between the experimental data and the mean of the generated data, normalized by the standard deviation of the latter.
  • Figure 4: Detail of the posteriors for $m_\beta$ and $E_0$ with their correlation as a result of the Bayesian analysis of the $^{163}$Ho calorimetric spectrum.
  • Figure 5: Stability of the corrected energy gain over multiple days as shown by the events in the M and N peaks. The corrected gain drift remains well within the detector’s energy resolution minimizing systematic uncertainties in the energy scale. The dashed lines in the insets on the right delimit a $\pm\sigma$ region around the mean of the highlighted peak. The inset on the left shows an impulse from a $^{163}$Ho decay event.
  • ...and 2 more figures