Speaker
Description
The ECHo experiment aims to determine the electron neutrino mass from electron capture in $^{163}$Ho. This requires an accurate theoretical description of the differential decay rate as a function of how the total decay energy is shared between the neutrino and the electronic excitations.
We achieve this using multireference methods previously developed for core-level X-ray spectroscopy. Bound states are calculated with a state-selected restricted active-space configuration-interaction solver, yielding distinct open core–valence multiplets and bound-to-bound Auger–Meitner side peaks. The linewidth is treated ab initio through an energy-dependent self-energy that describes the coupling of bound states to unbound states with a continuous energy spectrum, including photons, charge-transfer excitations in the host material, and free electrons emitted in the Auger–Meitner process.
The dominant broadening mechanism is Coulomb scattering between electrons, which couples discrete atomic states to final states with free electrons. A key result for neutrino-mass determination is that the resonance line shapes are not Lorentzian but show an exponential decay at high energy. This behavior can be used to extract the neutrino mass from the endpoint region.
In addition, we discuss the deviations between theory and experiment and describe our plans to improve the theoretical calculation.
| Collaboration or Other Affiliation | ECHo |
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