Majorana neutrinos and lepton-number-violating signals in top-quark and W-boson rare decays

We discuss rare lepton-number-violating top-quark and W-boson four-body decays to final states containing a same-charge lepton pair, of the same or of different flavors: t→bW−ℓi+ℓj+ and W+→JJ¯′ℓi+ℓj+, where i≠j or i=j and JJ¯′ stands for two light jets originating from a u¯d or a c¯s pair. These ΔL=2 decays are forbidden in the Standard Model and may be mediated by exchanges of Majorana neutrinos. We adopt a model independent approach for the Majorana neutrinos mixing pattern and calculate the branching ratios (BR) for these decays. We find, for example, that for O(1) mixings between heavy and light Majorana neutrinos (not likely but not ruled out) and if at least one of the heavy Majorana neutrinos has a mass of ≲100GeV, then the BR's for these decays are: BR(t→bℓi+ℓj+W−)∼10−4 and BR(W+→ℓi+ℓj+JJ¯′)∼10−7 if mN∼100GeV and BR(t→bℓi+ℓj+JJ¯′)∼BR(W+→ℓi+ℓj+JJ¯′)∼0.01 if mN≲50GeV. Taking into account the present limits on the neutrino mixing parameters, we obtain more realistic values for these BR's: BR(t→bℓi+ℓj+W−)∼10−6 and BR(W+→ℓi+ℓj+JJ¯′)∼10−10 for mN∼100GeV and BR(t→bℓi+ℓj+JJ¯′)∼BR(W+→ℓi+ℓj+JJ¯′)∼10−6 for mN≲50GeV.