  It is often said that neutrino mass is a window to a new physics beyond the
standard model (SM). This is certainly true if neutrinos are Majorana particles
since the SM with Majorana neutrino mass is not a complete theory. The
classical text-book test of neutrino Majorana mass, the neutrino-less double
beta decay depends on the completion, and thus cannot probe neutrino mass. As
pointed out already more than twenty five years ago, the colliders such as
Tevatron or LHC offer a hope of probing directly the origin of neutrino
Majorana mass through lepton number violating production of like sign lepton
pairs. I discuss this in the context of all three types of seesaw mechanism. I
then discuss in detail the situation in $L-R$ symmetric theories, which led
originally to the seesaw and which incorporates naturally both type I and type
II. A $W_R$ gauge boson with a mass in a few TeV region could easily dominate
the neutrino-less double beta decay, and its discovery at LHC would have
spectacular signatures of parity restoration and lepton number violation. At
the end I give an example of a predictive SU(5) grand unified theory that
results in a hybrid type I and III seesaw with a light fermion triplet below
TeV scale.
