Inverse neutrinoless double beta decay revisited: Neutrinos, Higgs triplets, and a muon collider

We revisit the process of inverse neutrinoless double beta decay (e^-e^-W^-W^-) at future linear colliders. The cases of Majorana neutrino and Higgs triplet exchange are considered. We also discuss the processes e^-^-W^-W^- and ^-^-W^-W^-, which are motivated by the possibility of muon colliders. For heavy neutrino exchange, we show that masses up to 10^6 (10^5) GeV could be probed for ee and e machines, respectively. The stringent limits for mixing of heavy neutrinos with muons render ^-^-W^-W^- less promising, even though this process is not constrained by limits from neutrinoless double beta decay. If Higgs triplets are responsible for inverse neutrinoless double beta decay, observable signals are only possible if a very narrow resonance is met. We also consider unitarity aspects of the process in case both Higgs triplets and neutrinos are exchanged. An exact seesaw relation connecting low energy data with heavy neutrino and triplet parameters is found.