  A natural explanation for the smallness of the neutrino mass requires them to
be Majorana particles violating lepton number by two units. Since lepton number
violation can have several interesting consequences in particle physics and
cosmology, it is of utmost importance to find out if there is lepton number
violation in nature and what is its magnitude. The neutrinoless double beta
decay experiment can answer these questions: if there is lepton number
violation and if neutrinos are Majorana particles. In addition, the magnitude
of neutrinoless double beta decay will constrain any other lepton number
violating processes. This lepton number violation may also be relatd to the
matter-antimatter asymmetry of the universe, dark matter and cosmological
constant.
