Tritium beta decay
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. Furthermore, a confirmation that the neutrino masses and the dark energy scale are nearly the same may well lend support to certain BSM physics speculations. 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-+ νe, is practicaly 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 14.95 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. It has a half-lifetime of 12.32 years. The absolute mass of the neutrino can be extracted through the kinematics of the β-decay [Fermi1934], 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 [KATRIN:2019yun]. 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 Project 8 uses a new method of electron spectroscopy, Cyclotron Radiation Emission Spectroscopy (CRES), to measure the energies of the β-decayed electrons, by measuring the frequencies of the cyclotron motion of the electrons trapped in a strong, uniform magnetic field [Monreal:2009za]. This technique was demonstrated by the collaboration in 2014 [Project8:2014ivu] and showed its promise to push the neutrino mass measurement, in the final Phase IV configuration using atomic tritium, to a sensitivity of ~ 40 meV [Project8:2017nal]. This will cover the full range of the inverted mass hierarchy of the neutrinos and reach into the challenging realm of the normal hierarchy.
The Project 8 collaboration has finished phase II, successfully demonstrating the novel technique of Cyclotron Radiation Emission Spectroscopy (CRES) to measure the energy of the beta emission from tritium molecules. In the current phase III, the two major goals are (1) to demonstrate CRES using antenna arrays and (2) to scale up the trapping of atomic tritium. To achieve the latter goal, a cryogenic Halbach array, in lieu of the more costly superconducting Ioffe trap, could be a viable technique. Based on the successful application of the Halbach array in the UCNτ experiment, we were consulted by the Project 8 collaboration on the effort to trap tritium atoms. It turns out that the magnetic field requirements to trap UCN and cold tritium atoms are quite similar. If tritium atoms could be cooled to 0.3 K, the trapping field strength is no larger than 1 T. Currently, we are discussing the fabrication of a large-scale magnetic array with several vendors, using magnets fabricated from the uncommonly-used compound PrFeB (or SmCo) for operation at 3 K. (The more common NdFeB magnets used in UCNτ would lose magnetism below 140 K.)
Project 8’s first atom-trap prototype will be produced at Illinois. Eventually, this trap (or a redesigned one) will be delivered to Mainz to be integrated with the atomic beam source and demonstrate the trapping of atomic hydrogens/tritiums. The Illinois group has extensive experience with trapping ultracold neutrons (with the UCNtau experiment) using Halbach arrays, but still, there are key issues to explore for Project 8. Atomic hydrogen/tritiums can be confined by Halbach array fields, which decay exponentially from the surface of the array. To be compatible with the magnetic field and its uniformity in implementing CRES for the desired energy resolution, we plan an array of permanent magnets lining the inner wall of a long cylinder; the CRES field is applied along the symmetry axis of the cylinder and the Halbach array field are confined to the perpendicular planes. Cares need to be taken to avoid field zeros due to accidental cancellation between the two sources of field, especially in the regions near the end caps of this cylindrical trap. Finally, the residual non-uniformity in the region of the electron trap needs to be mitigated to be compatible with CRES. Although this first prototype need not meet the ultimate demands of CRES experiments (only those of the ATD), all design elements must be consistent with future scaling for those experiments. Methods for diagnosing the temperature and density of atoms in the trap, either with simple filament resistance measurements in-situ or using a high-resolution residual gas analyzer ex-situ, must be developed.