By measuring the very low rate of neutrinoless double beta decay (if it even occurs at all) in even-even nuclei, the neutrino mass can be estimated!

Double Beta Decay
Measuring the Mass of the Neutrino

The SEGA & MEGA Project

Enriched Xenon Observatory (EXO)

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One of the most elusive and exotic subatomic particles being investigated around the world today is the neutrino.   Understanding the family of neutrino particles and how they interact with other matter (and among themselves) has become one the most intensive physics research efforts ever attempted by mankind.  With a virtually undetectable mass, and without electric charge, these weakly interacting particles have been devilishly difficult to measure.  They can travel through a light year of solid lead without interacting, and they have been recently shown to "oscillate" from one neutrino flavor to another.  Most of the neutrino searches involve enormous detectors in deep underground caverns.

The Super-Kamiokande neutrino detector is in an active mine in the mountains of western Japan.  Filled with ultra-pure water and surrounded by 11,000 photomultiplier tubes, rings of Chenkov light emitted when a neutrino interacts with electrons in the chamber are recorded. The 50,000 ton Super-Kamiokande water detector in an active zinc mine in Japan recently provided the first conclusive evidence that neutrinos have mass and therefore oscillate among families (called "flavors").  Another detector that employs heavy water, called the Sudbury Neutrino Observatory (SNO) in the Creighton nickel mine in Ontario Canada, recently explained the previously observed apparent shortage of neutrinos from our own sun.  These and other detectors around the world are finding new evidence for physics beyond that of the Standard Model understood today (physicists' current theory of matter and the forces of nature).

At WIPP, planned neutrino research is limited to more modestly scaled experiments.  These involve measurement of what may be the rarest of all nuclear interactions - the occurrence of two simultaneous beta decays from a single nucleus. Researchers hope that detailed measurements of double beta decay events will provide the most sensitive estimate of the elusive neutrino's very small mass.  The other neutrino detectors around the world have conclusively shown neutrinos have mass, but they only measure the mass difference between the different members of the neutrino family.  Researchers at WIPP plan to make the first measurements of the absolute mass of the electron neutrino.

Double beta decay was observed for the first time in 1986.  This process, in which a nucleus emits two electrons and two antineutrinos, was observed in an isotope of selenium by Michael Moe and colleagues at the University of California, Irvine.  Later, double beta decay was seen in other nuclei, including forms of calcium, germanium, molybdenum and xenon.  The methods developed for these experiments are being used to search for another rare decay mode, neutrino-less double beta decay, in which only the two electrons are emitted. This process could occur only if an electron neutrino is its own antiparticle and if neutrinos have mass, in violation of the Standard Model.

Click here for a review of the SEGA & MEGA  Project in the WIPP underground
A cut-away view of Phase 2 of the Majorana Project.  The detector would employ 14-18 two-kilogram Ge-76 solid state crystals.

Two groups of researchers are fielding substantially different devices at WIPP that attempt to make the neutrino mass measurement.  Research collaborators led by Pacific Northwest National Laboratory (PNNL) are developing the Majorana Project which will use solid state semiconductors to make the measurement.  In contrast, collaborators lead by Giorgio Gratta of  Stanford University are designing and evaluating a detector called the Enriched Xenon Observatory (EXO) which will use a special pressurized chamber of the rare gas Xenon to make the measurement.

The EXO Collaboration web site is under construction, but you can see many pictures of the initial testing and download the proposal.
Click here to see the EXO Collaboration web site

Neutrinoless double beta decay is one of the exotic processes which is specifically not allowed in the standard model of particle physics today.  Its discovery (if possible) will illuminate many aspects of nonstandard physics, still in the shadows today.  From experimental lower limits on the lifetime of this process, important information about the effective electron Majorana neutrino mass, effective right handed weak interaction parameters, the Majorona coupling constant, R-parity violation SUSY parameters etc, can be extracted.  The e-e- scattering connected directly with the inverse process to 0nbb can also provide interesting physics beyond the Standard Model of particle physics.

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