NEUTRINOLESS DOUBLE-BETA DECAY
One of the most important open questions in neutrino physics is whether the neutrino is a Majorana fermion. In the case where the neutrino is a Majorana particle the neutrino and anti-neutrino are identical particles. Many beyond the standard model theories predict that neutrinos are Majorana particles as this allows 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- + 2ν̅e
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.
Figure 1: Feynman diagram of 0νββ decay driven by a light Majorana neutrino exchange.
Assuming 0νββ occurs, its rate is significantly smaller 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 abundnace (34%), its relatively high Q-value to avoid low energy radioactive backgrounds, and because it can be loaded into scintillator while maintaining excellent optical properties.
The expected energy spectrum of the background and signal for SNO+ (after analysis cuts) is shown in Figure 2, assuming an mββ of 100 meV (which is around the level of the best current limit from KamLAND-Zen ). The breakdown of the backgrounds in a small energy region around the Q-value of the decay is shown in Figure 3. Interestingly, the dominant background for the SNO+ 0νββ search will be 8B solar neutrino elastic scatters.
Figure 2: The expected SNO+ spectrum plot, showing a hypothetical signal at mββ of 100 meV.
Figure 3: A breakdown of SNO+'s expected backgrounds in the first year of data-taking.
 Mohapatra, R.N., in Current Aspects of Neutrino Physics (Springer-Verlag) hep-ph/9910365 (1999)
 CUORE Collaboration, Eur. Phys. J 77, no. 1, 13 (2017)
 KamLAND-Zen Collaboration, Phys. Rev. Lett. 117, 082503 (2016)