Nuclear Double Beta Decay, Fundamental Particle Physics, Hot Dark
  Matter, And Dark Energy
  Nuclear double beta decay, an extremely rare radioactive decay process, is -
in one of its variants - one of the most exciting means of research into
particle physics beyond the standard model. The large progress in sensitivity
of experiments searching for neutrinoless double beta decay in the last two
decades - based largely on the use of large amounts of enriched source material
in "active source experiments" - has lead to the observation of the occurrence
of this process in nature (on a 6.4 sigma level), with the largest half-life
ever observed for a nuclear decay process (2.2 x 10^{25} y). This has
fundamental consequences for particle physics - violation of lepton number,
Majorana nature of the neutrino. These results are independent of any
information on nuclear matrix elements (NME)*. It further leads to sharp
restrictions for SUSY theories, sneutrino mass, right-handed W-boson mass,
superheavy neutrino masses, compositeness, leptoquarks, violation of Lorentz
invariance and equivalence principle in the neutrino sector. The masses of
light-neutrinos are found to be degenerate, and to be at least 0.22 +- 0.02 eV.
This fixes the contribution of neutrinos as hot dark matter to >=4.7% of the
total observed dark matter. The neutrino mass determined might solve also the
dark energy puzzle. *(It is briefly discussed how important NME for 0nubb decay
really are.)
