New limits on effective Majorana neutrino masses from rare kaon decays

The rare kaon decay K+→π−μ+μ+ is violating lepton number by two units and is investigated within the context of Majorana neutrino masses. Using a new upper bound from E865 an upper limit on the effective Majorana mass term 〈mμμ〉≲500 GeV could be obtained. This is improving existing bounds by about one order of magnitude. Implications on heavy neutrinos are discussed as well as future possibilities to improve limits on this as well as other elements of the Majorana mass matrix are investigated.