Skip to main content

The Majorana Collaboration
For more information, contact Majorana Collaboration Spokesperson: Steve Elliott (

Neutrinos are elusive particles that are created in nuclear reactions such as in the sun, nuclear reactors, and when cosmic rays crash into the Earth?s atmosphere. Since the first discovery of the neutrino in 1956, scientists have learned little about them, except that they generally stream through the Earth more easily than light through a window.

The absolute mass of the neutrino plays an important role in several areas of physics. In cosmology, neutrino mass affects large-scale structure of galaxy clusters and neutrino characteristics might provide clues to the particle-antiparticle asymmetry of the Universe. Neutrinos are created in very large numbers during supernova explosions, and their mass and mixing parameters can strongly affect these events. Numerous fundamental nuclear physics processes are affected by the mass of the neutrino. Understanding the mass of the neutrino could also help scientists to understand how particles obtain a mass in nature.

Where does double-beta decay come in?

Several experiments (including one performed at the Homestake mine!) have already demonstrated that neutrinos have some mass. In fact, they have determined the differences between the masses of the three types of neutrinos (electron, muon, and tau neutrinos). This demonstration is itself evidence that our understanding of neutrinos in the Standard Model of particle physics is incomplete. A sensitive method is now needed to determine the mass of one of the neutrino types to understand them all. Neutrinoless double-beta decay can determine the absolute scale of the electron neutrino mass.

How can a “neutrinoless” reaction tell you about neutrinos?

To answer that, we have to look at a cartoon of what happens during a neutrinoless double-beta decay:
The two black points in the middle of the diagram are where a virtual
neutrino (shown as the red arc) is emitted and then reabsorbed. The
virtual neutrino is emitted with a right-handed spin (like a football thrown
by a right-handed quarterback), and must be absorbed with a lefthanded
spin. This condition requires the neutrino to have two new

  • The neutrino must be indistinguishable from the anti neutrino
  • The neutrino must have a nonzero mass

The second property is what gives us fundamental information about the
neutrino. If the neutrino has a comparatively heavy mass, it will be
easier for the virtual neutrino to make the switch from being righthanded
to left handed. If that switch is easier to make, then
neutrinoless double-beta decay will happen more often. Therefore, if we
can measure the rate of this reaction, we can extract the absolute mass
of the electron neutrino.

How do you measure the rate of this reaction?

There are around ten isotopes known to undergo TWO-neutrino doublebeta
decay (which is allowed by the Standard Model). These are the ones
on which people focus the search for neutrinoLESS double-beta decay.
The MAJORANA experiment is going to focus on 76Ge, a particular form of
the atom germanium with 32 protons and 44 neutrons.