In recent years, the physics of neutrinos has been in the spotlight. Neutrinos are among the oddest of the Standard Model's menagerie of elementary particles: electrically neutral, and interacting only via the weak nuclear force, they are extremely difficult to detect. Originally assumed to be massless, the observation in the 1990s of the spontaneous transformation between the three known types of neutrinos (electron, muon and tau) – called "neutrino oscillations" – proved that neutrinos have a tiny mass, on the order of 100,000 times smaller than the next lightest Standard Model particle, the electron.
This finite mass opened up the possibility – an option only available to neutrinos – that neutrinos might exist as "Majorana" particles, meaning that they are their own antiparticles, rather than the "Dirac" type, in which particle and antiparticle are distinct species, a category that includes all other matter particles in the Standard Model. If true, this long-sought after property might help explain the large and mysterious disparity in the neutrino masses and the masses of all other particles, and may even help provide a mechanism for the observed large ratio of matter to antimatter throughout the universe.
The best and perhaps only way to determine if neutrinos are Majorana particles or not is to search for an as-yet-unseen nuclear process known as "neutrinoless double beta decay,” a specific radioactive decay that, if neutrinos are their own antiparticles, would occur without the emission of any neutrinos. Observation of this process would prove the Majorana nature of neutrinos and would be sensitive to the still-unknown absolute value of the neutrino mass, a quantity not determined in oscillation experiments.
The Enriched Xenon Observatory (EXO) collaboration, centered at Stanford and SLAC, has constructed and operated an apparatus to search for neutrinoless double beta decay. The "EXO-200" detector consists of what’s called a Time Projection Chamber (TPC), filled with over 100 kilograms of liquid xenon enriched in the 136 isotope. The TPC is used to precisely determine the total energy and the spatial location of particle decays, and to thereby distinguish an interesting process from the background “noise” that results from natural radioactivity and cosmic rays. The experiment is constructed of ultra-low radioactivity materials and since 2011 has operated about half a mile underground in order to attenuate cosmic rays in the DOE-operated WIPP facility near Carlsbad, New Mexico.
The collaboration has so far published the most precise measurement of the half-life for the two-neutrino double beta decay process of about 2 x 10^21 years, and a limit at greater than 10^25 years for the neutrinoless double beta decay process – a limit that constrains the neutrino's mass to less than nearly one millionth of the electron's mass. Plans are now underway for a ton-scale version of the experiment ("nEXO") with much greater sensitivity.