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Nuclear and Particle Physics

Neutrino Mass (Project 8)

Neutrinos produced in the instant of the Big Bang are predicted by cosmological models to outnumber particles of normal matter in our universe, mostly protons and neutrons, by a factor of 109. Neutrinos were long assumed to be strictly massless, but recent experiments indicate that they do indeed have a nonzero mass, albeit without determining what that mass is apart from upper and lower limits. Even with a tiny mass consistent with the upper limit, neutrinos, by their sheer abundance, could outweigh all of the normal matter in the universe by as much as a factor of five. Therefore, a precise measurement of neutrino mass is critical to our understanding of the gravitational evolution of the universe.

A method has recently been proposed for ultra-precise and virtually background-free spectroscopy of medium-energy electrons as a means to determine directly the mass of the neutrino in tritium beta decay, the basis for the proposed Project 8 experiment. If gaseous tritium beta decays in a region with a strong uniform magnetic field, the daughter electron will undergo a spiraling cyclotron motion, radiating at the cyclotron frequency, which is directly related to the electron energy. A measurement of this radiation should have sufficient frequency precision to see the tiny reduction in the maximum electron energy due to the non-zero mass of the co-produced daughter neutrino. The Project 8 collaboration is developing a proof of concept apparatus, demonstrating that the cyclotron radiation is detectable with sufficient frequency precision.

drawing of Project 8

Monreal and Formaggio, Phys. Rev.D80, 051301 (2009)

The Project 8 collaboration, involving the University of Washington, Massachusetts Institute of Technology, University of California Santa Barbara, California Institute of Technology, National Radio Astronomy Observation, and PNNL, was formed to develop the technique, and to use it to measure the vanishingly small mass of the neutrino. PNNL is contributing the radioactive gas sources, gas handling system, and RF detection expertise. The technique could also be used to observe hyperfine structure due to nuclear effects in atomic spectra, to observe "new" physics in neutron decay, and ultimately to make a direct observation of the relic neutrinos created in the instant of the Big Bang.

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