Neutrino mass from dimension-9 lepton number violating operators

One of the open questions of particle physics is the origin of neutrino masses; among the possibilities is a Majorana neutrino mass which is only possible if neutrinos are their own antiparticles and hence lepton-number is violated in nature.

Based on the Schechter-Valle theorem any lepton-number-violating process by two units due to a dimension-9 operator induces a Majorana neutrino mass. We calculate the maximally induced neutrino mass from bounds on lepton number violating processes like neutrinoless double beta decay and lepton number violating meson and tau decays. We find that these mass terms are many orders

of magnitude too small to explain neutrino masses. Therefore, other operators must give the leading contributions to neutrino masses, which could be of Dirac or Majorana nature.