Project 8 research description
The discovery of neutrino mass (implied by neutrino flavor oscillations) is often viewed as the first hint of the presence of BSM physics. The absolute neutrino mass is a parameter that is essential to know both for particle physics and also for astrophysics, through its influence on structure formation in the early universe. The quantum mechanical interference phenomenon of neutrino oscillations implies meV-scale neutrino mass differences, and the seesaw mechanism for neutrino mass generation naturally connects this very small mass scale to new physics at high energy scales. Although the absolute scale for neutrino masses has yet to be identified, continued experimental work to probe the neutrino mass is of fundamental importance.
Tritium beta decay remains the most sensitive system in which to directly measure or constrain the absolute neutrino mass. The process of tritium beta decay, T --> 3He e- n, is practically identical to that of free neutron beta decay; one of the neutrons inside the tritium nucleus undergoes weak decay, transforming into a proton to form 3He, with an emission of an electron and a neutrino. The nucleon few-body interactions modify the energy levels, and the emitted electrons have energies no larger than the end-point energy of 18.6 keV. Compared to the free neutron decay (782 keV), the energies of the electron from tritium beta decay are more susceptible to the absolute mass of the neutrino, which shares the total energy released.
The absolute mass of the neutrino can be extracted through the kinematics of the beta decay, a direct approach that is nearly model independent and does not rely on whether the neutrino is a Dirac or a Majorana particle. As such, the science program with tritium beta decay has been primarily focused on the extraction of the absolute neutrino mass. The best tritium beta decay experiment, KArlsruhe TRItium Neutrino experiment (KATRIN), has reported a new upper limit of 1.1 eV/c2 (90\% CL) after its first science run.
It employs a high-intensity gaseous molecular tritium source and a high-resolution electrostatic filter with magnetic adiabatic collimation to target a neutrino-mass sensitivity of 0.2 eV/c2.
The energy of the beta emitted from tritium atoms near its endpoint energy, through simple kinematics, reveals the absolute mass of the electron neutrinos. The Project 8 experiment aims to measure the mass of neutrinos using a novel technique of Cyclotron Radiation Emission Spectroscopy (CRES) to reach an unparalleled sensitivity of 40 meV. This mass sensitivity covers the range of inverted neutrino mass hierarchy and reaches into the prediction for the normal hierarchy. To achieve this goal, we need to trap atomic tritium. A magneto-gravitational trap using a Halbach array of permanent magnets, similar to the UCN trapping in the UCNτ experiment, could be readily applied to trap cold tritium atoms.