Measuring the neutrino mass with Holmium: the HOLMES experiment

by Dr Matteo De Gerone (INFN Genova)

Monday 27 Oct 2025, 11:30 → 13:30 Europe/Rome

C. Villi meeting room

Measuring the absolute neutrino mass remains one of the central challenges in particle physics and astrophysics. The only model-independent approach is direct measurement, which relies on analyzing the kinematics of electrons emitted in beta decays. Two experimental strategies exist for this purpose: spectrometers and calorimeters. Spectrometers, such as KATRIN, have set the most stringent limits to date, but their design — with the source external to the detector — introduces possible systematic uncertainties. Calorimetric experiments, in contrast, embed the source directly into low-temperature detectors, capturing all of the decay energy except the portion carried away by the neutrino.

 

This seminar will focus on calorimetric approaches, with particular attention to the HOLMES experiment. HOLMES aims to demonstrate the feasibility of probing the neutrino mass with sub-eV sensitivity by measuring the electron capture decay of 163Ho. Achieving this sensitivity requires carefully balancing statistical precision with the unresolved pile-up fraction — a key challenge for calorimeters. This balance is addressed by distributing the total activity over a large array of detectors, which in turn necessitates an advanced multiplexed readout system.

 

HOLMES utilizes superconducting micro-calorimeter arrays with microwave multiplexed readout. The 163Ho isotope is produced through neutron irradiation of 162Er and implanted into the detectors using a custom-built ion implanter. After extensive commissioning and optimization of both detectors and readout systems, the implanter has successfully carried out low-dose implantations of about 0.5 Bq per pixel.

 

I will present the first results from HOLMES’ initial high-statistics data-taking run. These include the most stringent bound to date on the electron neutrino mass obtained with a scalable low-temperature micro-calorimeter array, along with the first comprehensive phenomenological description of the full calorimetric spectrum.

 

I will also outline the upcoming challenges and opportunities as the project moves forward to a new phase, HOLMES+, which will feature larger arrays with higher per-pixel activity. This next stage aims to further improve statistical sensitivity and represents an important step toward achieving sub-eV neutrino mass measurements within the coming decade.