CANDLES for the study of double beta decay of $^{48}$Ca and its future prospect

Study of neutrinoless double beta ($0 \nu \beta \beta$) decay becomes of particular importance after the confirmation of neutrino oscillation which shows that neutrinos have mass.  They could then be Majorana particles which violate lepton number conservation.  Once lepton number non-conservation is verified, we have a scenario to explain how our matter dominated universe is realized. It is known as a leptogenesys scenario. ($0 \nu \beta \beta$) decay is only a known process to verify Majorana nature of neutrino.

We have been studying ($0 \nu \beta \beta$) of $^{48}$Ca with the CANDLES detector system.  CaF$_2$ crystals are our central detector.  The $^{48}$Ca has the highest Q value (4.3 MeV) among double beta decay nuclei.  It means that the large decay rate for a given neutrino mass and the least background.  We constructed the detector at the Kamioka underground laboratory.

We have been running the experiment and obtained the best limit for the lifetime of $^{48}$Ca ($0 \nu \beta \beta$).

For the future progress of CANDLES, we are working on enrichment of 48Ca and bolometer system.  Small natural abundance of $^{48}$Ca (0.19%) is the shortcoming but indicates potential of 500 times improvement by enrichment paradoxically.  Bolometer technique can be applied to CaF$_2$ crystals by which we can expect substantial improvement of energy resolution. 

I would like to describe the current status of our experiment and its future prospects.