Trapped electron for neutrino mass measurement.

This proposal is focused on developing a trapped electron in the geonium chip as an ultra-sensitive detector of microwave radiation for fundamental physics applications. Specifically, in the two years duration of the project, our main goal is to provide a basic experimental proof-of concept of the capacity of our trapped electron (or 'geonium atom') for the planned future measurement of the neutrino's absolute mass scale, m_beta, from the energy endpoint E_0 = 18.6 keV of the Kurie spectrum of the beta-decay of tritium.

The currently ongoing KArslruhe TRItium Neutrino experiment (KATRIN) has recently reported an upper bound of m_beta<1.1 eV/c^2, and it aims at a final accuracy of 0.2 eV/c^2. Neutrino flavour oscillations together with cosmology observations set the ultimate boundary to m_beta>50 meV/c^2, or m_beta>9 meV/c^2 in the case of "inverted" or "normal" ordering of the neutrino mass hierarchy, respectively. These two values define the required accuracy for measuring the neutrino mass. Both are beyond the reach of KATRIN. In order to improve the latter, the novel Cyclotron Resonance Emission Spectroscopy (CRES) technique has been proposed for measuring m_beta. CRES allows for detecting (and counting) beta particles emitted from a radioactive source, such as tritium, and for measuring their kinetic energy. The US-based Project-8 collaboration has demonstrated experimentally the basic principle of CRES, by detecting the microwave radiation emitted by one single beta particle (decayed from gaseous Kripton) and measuring its kinetic energy. With this method, Project-8 aims at a future accuracy in the neutrino mass measurement of 40 meV/c^2. Meanwhile, in early 2021, the Quantum Technologies for Neutrino Mass Measurement (QTNM) consortium, lead by University College London (UCL), has been launched. QTNM is developing a CRES demonstrator apparatus (CRESDA), initially operated with a deuterium source. The ultimate goal is to measure m_beta (eventually within an international collaboration) in a tritium facility, possibly at Culham Centre for Fusion Energy.

While the basic working principle of CRES has been tested, the resolution achieved in the measurement of the kinetic energy of the beta particles amounts to 15 eV/c^2. This is around three orders of magnitude below the required sensitivity for a "guaranteed" measurement of m_beta. This proposal aims at delivering a microwave quantum sensor, the trapped electron, capable of measuring m_beta with the CRES technique, even in the most challenging scenario of "normal" ordering. Within the two years duration of the project we aim at demonstrating an energy resolution of 5 meV/c^2 or better with our geonium microwave sensor. The experiments will be performed at Sussex with an existing geonium chip facility. Our cooperation with the QTNM consortium will provide an external assessment of the suitability of the geonium sensor for measuring m_beta. It will also allow for planning its eventual subsequent integration into the future tritium-ready CRES facility.