Science Programme

The SNO+ Experiment is used to investigate a large suite of physics phenomena.
Find out more about our current active research interests here.

Neutrinoless Double Beta Decay

One of the most important open questions in physics is whether the neutrino is a Majorana fermion (as opposed to a Dirac fermion like other leptons). If so, the neutrino and anti-neutrino are identical particles. Many theories predict that neutrinos are Majorana particles, as this would allow for a mechanism that naturally explains the smallness of the neutrino mass.

The most promising way to discover Majorana neutrinos is through the detection of neutrinoless double beta decay (0νββ). The standard double beta decay process (2νββ) has been observed in a handful of nuclei, and occurs when a parent nucleus decays to a daugher nucleus via:

(A, Z) → (A, Z + 2) + 2e^- + 2̅ν_e In 0νββ, which is forbidden in the standard model, no neutrinos are emitted and the lepton number is violated by two units: (A, Z) → (A, Z + 2) + 2e^- In the standard interpretation, where 0νββ is mediated through light Majorana neutrino exchange, the neutrino can be absorbed at the vertex, as shown in figure 1.

Feynman diagram of neutrinoless double beta decay

Even if 0νββ occurs, its rate would be significantly less than the rate of 2νββ. One of the main challenges for 0νββ searches is to distinguish 0νββ decays from the 2νββ decays, which requires excellent energy resolution. In addition, given the rarity of the decay, the detectors require significant amounts of isotope.

The SNO+ experiment selected ¹³⁰Te as its 0νββ isotope. ¹³⁰Te is known to undergo 2νββ with a half-life of 8.2 x 1020 years and with a Q-value of 2.527 MeV. ¹³⁰Te was chosen because of its high natural abundance (34%), relatively high Q-value to avoid low energy radioactive backgrounds, and because it can be loaded into scintillator while maintaining excellent optical properties.

Assuming an m_ββ of 100 meV (which is around the level of the best current limit from KamLAND-Zen), the expected energy spectrum of the background and signal for SNO+ (after analysis cuts) is shown here: