Description of the calorimetrically measured Ho-163 spectrum

The study of the end-point region of calorimetrically measured Ho-163 electron capture spectrum will allow, in the next future, to achieve sub-eV sensitivity on the electron neutrino mass. Within the ECHo experiment, first high statistics Ho-163 spectra have been measured with very high energy resolution. Such precise measurements allowed the identification of structures in the Ho-163 spectrum which are not described if only first order excited states, consisting in a hole in the electronic shells of the daughter atom left by the captured electron, are considered. Second order excited states in the daughter dysprosium atoms characterized by two holes in the atomic shells could be the reason of the additional structure in the spectrum. We present the comparison of the high statistics Ho-163 spectra acquired in a low background environment with theories including second order excited states. At the present status there is not a clear agreement between theory and data. We discuss how new high statistic measurements of the Ho-163 spectrum and new experiments could provide inputs to better understand the role of higher order excited states occurring after electron capture processes.