Project 8

The experiment
Despite being the most abundant particles in the universe, neutrinos are the only elements of the standard model who’s rest mass remains unknown. 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 – the currently most sensitive method to determine the neutrino mass in laboratory experiments – relies solely on energy and momentum conservation. In the decay of a tritium atom, an electron and an anti-electron neutrino are generated. The maximum energy of the electron is given by the energy liberated in the decay minus the rest mass of the neutrino, and can thus be inferred by precision measurement at the endpoint of the spectrum. However, next to the issue of determining the electron energy precisely enough, the fraction of electrons in the relevant part of the spectrum is only 10^{−13} of all tritium decays. Thus, even the world leading laboratory experiment KATRIN which is currently being commissioned in Karlsruhe will not be able to probe the range below 200meV.

To advance further, the Project 8 collaboration has pioneered a novel experimental technique, Cyclotron Radiation Emission Spectroscopy (CRES), which holds the promise of 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, which due to relativistic effects is inversely proportional to the electron energy. Despite the fact that 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 below shows a spectrogram of such an event, where the electron is clearly identified 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.

Mainz involvment
Our efforts in Mainz focus on two aspects of the experiment: since the frequencies of interest are in the GHz regime, the signal must be digitized at a very high speed causing an enormous data volume. At the same time, the signature lies in the frequency domain, so that intelligent and fast trigger algorithms are required to identify the presence of a track in real-time. At the same time, if bound in a molecule some energy of the decay can go into molecular excitations, setting a fundamental limit to the energy resolution that can be achieved. In 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 have therefore begun to build and design a demonstrator that shows how atomic tritium can be produced and trapped in the quantities required to improve on the existing neutrino mass limits.

Please have a look here for Bachelor, Master and PhD theses in the ETAP group.