Most Stringent Bound on Electron Neutrino Mass Obtained
with a Scalable Low-Temperature Microcalorimeter Array
B. K. Alpert ,
1
M. Balata ,
2
D. T. Becker ,
3
D. A. Bennett ,
1
M. Borghesi ,
4,5,*
P. Campana ,
4,5
R. Carobene ,
4,5
M. De Gerone ,
6
W. B. Doriese ,
1
M. Faverzani ,
4,5
L. Ferrari Barusso ,
7,6
E. Ferri ,
5
J. W. Fowler ,
1
G. Gallucci,
6
S. Gamba ,
4,5
J. D Gard ,
3
F. Gatti ,
7,6
A. Giachero ,
4,5
M. Gobbo ,
4,5
U. Köster ,
8
D. Labranca ,
4,5
M. Lusignoli ,
9,10
P. Manfrinetti ,
11
J. A. B. Mates ,
1
E. Maugeri ,
12
R. Moretti ,
4,5
S. Nisi ,
2
A. Nucciotti ,
4,5,†
G. C. O’Neil ,
1
L. Origo,
4,5
G. Pessina ,
5
S. Ragazzi ,
4,5
C. D. Reintsema ,
1
D. R. Schmidt ,
1,‡
D. Schumann ,
12
D. S Swetz ,
1
Z. Talip ,
12
J. N. Ullom ,
1,§
and L. R. Vale
1
1
National Institute of Standards and Technology (NIST), Boulder, Colorado, USA
2
Laboratori Nazionali del Gran Sasso (LNGS), INFN, Assergi (AQ), Italy
3
University of Colorado, Boulder, Colorado, USA
4
Dipartimento di Fisica, Universit`a di Milano-Bicocca, Milano, Italy
5
Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Milano-Bicocca, Milano, Italy
6
Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Genova, Genova, Italy
7
Dipartimento di Fisica, Universit`a di Genova, Genova, Italy
8
Institut Laue-Langevin (ILL), Grenoble, France
9
Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma 1, Roma, Italy
10
Dipartimento di Fisica, Sapienza, Universit `a di Roma, Roma, Italy
11
Dipartimento di Chimica, Universit`a di Genova, Genova, Italy
12
Paul Scherrer Institut (PSI), Villigen, Switzerland
(Received 17 March 2025; revised 3 July 2025; accepted 14 August 2025; published 29 September 2025)
The deter minat ion of the absolute neutrino mass scale re mains a fundamental op en questi on in particle
physics, with profound imp lications for both the standard model and cosmology. Direct kinematic
measureme nts, in dependent of model-dependent assumptions, provide t he most robust approach to
address this challenge. Here we presen t the most stringent up per bound on the effective electr on 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, achievin g
an average energy resolution of 6 eV FWHM with a scalable, multiplexed readout techniqu e. With a total
of 7 × 10
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-genera tion neutrino mass experiments and d emonstrate the poten tial of a scalable TES-based
microcalorimetric technique to push the sensitivity of direct neutrino mass measureme nts beyond the
current state of the art.
DOI: 10.1103/s9vl-7n24
Measuring the mass of neutrinos or antineutrinos is one
of the last critical tasks that need attention to complete the
understanding of the standard model of elementary par-
ticles and their interactions. While next-generation neutrino
experiments are expected to tackle the mass-ordering
problem [1,2], and the neutrinoless double beta decay
searches probe the Majorana nature of neutrinos [3–6], only
direct neutrino mass experiments can provide the definitive
answer on the absolute mass scale. Additionally, increasing
tensions with the results of oscillation experiments make
the neutrino mass derived from cosmological observations,
analyzed within the framework of the ΛCDM model and its
extensions, less reliable [7,8]. The strength of direct
neutrino mass experiments is that they rely solely on the
conservation of energy and momentum in weak nuclear
beta decays to determine the neutrino mass observable
which, for current instruments, is approximated by the
effective (anti)neutrino mass m
β
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
i¼1;2;3
jU
2
ei
jm
2
i
q
,
*
Contact author: matteo.borghesi@unimib.it
†
Contact author: angelo.nucciotti@unimib.it
‡
Contact author: dan.schmidt@nist.gov
§
Contact author: joel.ullom@nist.gov
Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribut ion to
the author(s) and the published article’s title, journal citation,
and DOI. Funded by SCOAP
3
.
PHYSICAL REVIEW LETTERS 135, 141801 (2025)
Editors ' Suggestion
0031-9007=25=135(14)=141801(9) 141801-1 Published by the American Physical Society
