The Project 8 collaboration aims to measure the absolute neutrino mass by combining the new technologies of Cyclotron Radiation Emission Spectroscopy and cold atomic tritium. Both molecular hydrogen and molecular tritium can be cracked into atoms by thermal dissociation. However, to trap these atoms in a magnetic field, they must first be cooled to millikelvin temperature. In Mainz, we are developing an atomic beam source that can provide atomic tritium for a next-generation neutrino mass experiment.

Despite being the most abundant particles in the universe, neutrinos are the only elements of the standard model with an unknown rest mass. Cosmological observations, in particular of the cosmic microwave background radiation and of the distribution of large-scale structures in the universe set limits on the sum of neutrino masses. In contrast, tritium endpoint spectroscopy – currently the most sensitive method to determine the neutrino mass in laboratory experiments – relies solely on energy and momentum conservation. When a tritium atom decays, an electron and an anti-electron neutrino is generated. The released energy from this decay is a combination of the electron’s maximum energy and the rest mass of the anti-electron neutrino which can be resolved through precision measurement at the endpoint of the beta decay energy spectrum.
In addition to the strict energy-resolution requirement, only about 10⁻¹³ of all tritium decays fall in the endpoint region of the spectrum. Thus, even the world leading experiment Karlsruhe Tritium Neutrino Experiment (KATRIN), which is set to finish its data taking campaign by the end of 2025, will be unable to probe neutrino masses under 200meV. New experimental techniques are needed to achieve sub-200meV sensitivities.

The Project 8 collaboration has pioneered a novel experimental technique – Cyclotron Radiation Emission Spectroscopy (CRES) – which promises a neutrino mass sensitivity of 40 meV. In the CRES approach, tritium is confined in a strong magnetic field, which forces the decay electron into a cyclotron orbit. The electron energy is determined by a precision measurement of the radio waves it emits due to its cyclotron motion. Due to relativistic effects, the radio waves’ frequency is inversely proportional to the electron energy. Even though the radiated power is as small as a femto-Watt, modern low-noise amplifier technology enable the detection of single electrons and the measurement of their energy with eV resolution. The figure on the right shows a spectrogram of such an event, where the electron is clearly identified as a high-power track above the noise, with a slope given by how fast energy is radiated away in the cyclotron motion. The intermediate jumps in frequency are caused by interactions with the rest gas. Only the frequency at the onset of the track is relevant to determine the initial electron energy.

In its molecular state, some of tritium’s decay energy will go into molecular exitations, setting a fundamental limit to the energy resolution that can be achieved. For the CRES approach, this limit can be overcome using a source of atomic tritium – which is comparatively easy to produce but difficult to retain in atomic state. We are working on building and characterizing an atomic source demonstrator that shows how atomic tritium can be produced and trapped in the quantities required to improve on the existing neutrino mass limits.

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