Probing the absolute neutrino mass scale with 163 Ho: The HOLMES project

The HOLMES project aims to directly measure the [Formula presented] mass down to the eV scale using the electron capture decay (EC) of 163 Ho. It will perform a precise calorimetric measurement of the end-point of the Ho energy spectrum looking for the deformation caused by a finite [Formula presented] mass. The choice of 163 Ho as source is driven by the very low Q-value of the EC reaction, which allows for high sensitivity with low activities (O(10[Formula presented])Bq/detector), thus reducing the pile-up probability. A large array made by thousands of transition edge sensors based micro-calorimeters will be used. The calorimetric approach eliminates systematic uncertainties arising from the use of an external [Formula presented] source, and minimizes the effect of the atomic de-excitation process. The commissioning of the first implanted sub-array is scheduled for the end of 2018. It will provide useful data about the EC decay of 163 Ho together with a first limit on [Formula presented] mass.