One of the most important open questions in neutrino physics is the question of whether neutrinos are Majorana or Dirac particles. For Dirac particles, which include most familiar kind, particles are distinct from anti-particles. For Majorana particles, on the other hand, particles and anti-particles are identical except for their helicities. Many theorists believe that neutrinos should be Majorana particles, as the theory of Majorana masses admits a mechanism that naturally explains the smallness of the neutrino mass [1] .

Experimentally, the most interesting aspect of a Majorana mass is that a Majorana particle can act as it own anti-particle. This is because helicity can appear reversed in a frame of reference that “overtakes” the particle (so that the particle seems to move ‘backwards’ in that frame). Since Majorana particles and anti-particles are differentiated only by their helicities, switching the helicity in this way allows a particle in one frame of reference to be an anti-particle in another.

Attempts to detect the (possible) Majorana nature of neutrinos focus around the double beta decay process:

(A, Z) → (A, Z + 2) + 2e- + 2ν̅e

If neutrinos are Majorana particles, the anti-neutrino emitted by one of the neutrons can be absorbed as a neutrino by the other. The resulting process, in which no neutrinos are emitted, is neutrinoless double beta decay:

(A, Z) → (A, Z + 2) + 2e-

If neutrinoless double beta decay were observed, it would not only prove that neutrinos are Majorana particles, but it would also provide a measurement of the neutrino mass, since the rate of neutrinoless double beta decay is related to the square of the neutrino mass [2]. Therefore, the results of neutrinoless double beta decay searches are usually given in terms of limits on the possible Majorana mass of the neutrino (i.e. the limit on the mass of the neutrino provided that it is a Majorana-type particle).

One experiment [3]   claims to have detected neutrinoless double beta decay in 76Ge with a neutrino mass in the range of about 0.15eV-0.44eV. Other experiments are needed to confirm or refute this claim.

Unfortunately, even if neutrinoless double beta decay is allowed, it occurs at only a small fraction of the rate of two neutrino double beta decay. One of the main challenges of double beta decay searches, then, is to distinguish the small number of mono-energetic electron pairs produced by neutrinoless double beta decay from the large continuum of electron pairs produced through two neutrino double beta decay. In order to ensure that these signals can be separated, it is necessary to have an experiment with very good energy resolution. Figure 1, below, shows how the two neutrino and zero neutrino double beta decay energy spectra appear with a typical liquid scintillator detector's energy resolution.

The other main requirement of a double beta decay search is that the experiment must be large. The neutrinoless double beta decay process is so rare that to get enough statistics to make an interesting measurement, a large amount of candidate isotope is required. The largest current double beta decay experiments use on the order of 100kg of candidate isotope, with some 1000kg experiments under discussion.

Loading the double beta decay isotope 130Te into the SNO+ liquid scintillator has the potential to allow for an extremely powerful double beta decay search. We have developed a brand new technique to do this, and to remove other contaminants that might otherwise interfere with the measurement. Although the energy resolution of the detector will not be as good as that of other existing experiments, the amount of isotope that could be suspended in the scintillator is very large. This means that SNO+ can hope to see a large enough number of neutrinoless double beta decay events that we can fit to the energy spectra of the 2 neutrino and 0 neutrino signals (and those of the radioactive backgrounds), making us much less dependent on energy resolution than competing experiments. In fact, based on some preliminary simulations, if we loaded the scintillator with 0.3% natural Te (which would contain 800kg of 130Te isotope) we would be able to detect neutrinoless double beta decay at neutrino masses approaching the range of the “inverted hierarchy,” a particularly interesting regime for theoretical predictions related to one of two possible ways for the 3 neutrino masses to be ordered. A 3% loading, correspond to 8 tons of 130Te isotope in the detector, would give us the potential to probe the majority of this interesting range with high sensitivity.

          Figure 1: Comparison of the predicted sensitivity and timescale for SNO+

           compared with current neutrinoless double beta decay limits. The left

           y-axis depicts the lowest effective Majorana neutrino mass values probed

           by a given experiment, which we would like to be as small as possible.

[1] Mohapatra, R.N., in Current Aspects of Neutrino Physics (Springer-Verlag) hep-ph/9910365 (1999).

[2] Elliott, S.R., and Vogel, P., Annu. Rev. Nucl. Part. Sci. 52 115 (2002).

[3] Kalpdor-Kleingrothaus, H.V. et al, Phys. Lett. B586 198 (2004)


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