Speaker
Description
Several experiments --- HOLMES, ECHo, and NUMECS --- have begun investigating the electron-capture decay of $^{163}\mathrm{Ho}$ to determine the neutrino mass. They studied this process using low-temperature microcalorimetry, in which the decaying Holmium is embedded directly into the absorber of cryogenic detectors, typically a few hundred micrometers in size. This configuration enables high-resolution spectroscopy of the total energy released in the decay, with the only missing component being the energy carried away by the neutrino.
Recent measurements with HOLMES detectors have produced a high-statistics $^{163}\mathrm{Ho}$ spectrum [1,2]. To obtain an accurate phenomenological description of the EC spectrum, the data are first corrected for detector effects using Bayesian iterative unfolding, which accounts for the different energy resolutions across the array. The unfolded spectrum is then fitted with a phenomenological model including asymmetric Breit--Wigner resonances at the electron binding energies, shake-up and shake-off (SU/SOF) processes, and third order processes. The measured spectrum exhibits clear deviations from the simple single-hole theoretical description. The region near the endpoint appears smooth and featureless, with a signal rate larger than predicted by first-order models. These observations motivate updated studies of the neutrino-mass sensitivity for future Ho-based experiments.
These sensitivity studies are performed by generating simulated spectra, using the phenomenological model [1], under different experimental conditions, varying parameters such as energy resolution, total statistics, detector activity, pile-up fraction, and background. For each configuration, the sensitivity is evaluated by generating an ensemble of simulated spectra, each spectrum being Poisson-fluctuated and independently fitted. The simulated spectra are fitted in the endpoint region using a Bayesian framework, with posterior sampling performed via Hamiltonian Markov Chain Monte Carlo implemented in \texttt{Stan}. This procedure yields an ensemble of posterior distributions and upper limits on $m_\beta$; the final sensitivity is defined as the mean of these upper limits, while their standard deviation quantifies the associated uncertainty. Additional optimization studies investigate the impact of binning choices and region-of-interest selection on the inferred values of $m_\beta$, the $Q$ value, and the overall computational cost of the analysis pipeline. Further tests examine the influence of different prior choices for $m_\beta$, in particular comparing uniform priors in $m_\beta$ and in $m_\beta^2$.
The impact of several systematic effects is also assessed, including detector-to-detector resolution variations, non-Gaussian energy response, and non-linear energy calibration. hese results inform the design of next‑generation Ho‑based neutrino‑mass‑experiment detector modules and help define the experimental conditions required to achieve sub‑eV sensitivity to the neutrino mass.
[1] F. Ahrens, B. K. Alpert, D. T. Becker, D. A. Bennett, E. Bogoni, M. Borghesi, P. Campana, R. Carobene, A. Cattaneo,
A. Cian, H. H. Corti, N. Crescini, M. De Gerone, W. B. Doriese, M. Faverzani, L. Ferrari Barusso, E. Ferri, J. Fowler,
G. Gallucci, S. Gamba, J. D. Gard, H. Garrone, F. Gatti, A. Giachero, M. Gobbo, A. Irace, U. K¨oster, D. Labranca,
M. Lusignoli, F. Malnati, F. Mantegazzini, B. Margesin, J. A. B. Mates, E. Maugeri, E. Monticone, R. Moretti, A. Nucciotti,
G. C. O’Neil, L. Origo, G. Pessina, S. Ragazzi, M. Rajteri, C. D. Reintsema, D. R. Schmidt, D. S. Swetz, Z. Talip, J. N.
Ullom, and L. R. Vale. Phenomenological modeling of the 163ho calorimetric electron capture spectrum from the holmes
experiment, 2025.
[2] 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. K¨oster, 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,
and L. R. Vale. Most stringent bound on electron neutrino mass obtained with a scalable low-temperature microcalorimeter
array. Phys. Rev. Lett., 135:141801, Sep 2025.
| Collaboration or Other Affiliation | Holmes |
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