Heavy Majorana neutrinos at a future muon collider

In this thesis work we have explored the production of heavy Majorana neutrinos at a future muon collider. At variance with the Standard Model which predicts neutrinos to be massless, the observation of neutrino oscillations implies that they are massive particles. This has opened a new era in particle physics, where the origin of neutrino masses and their nature, Dirac or Majorana, are being explored. The Majorana nature of neutrinos can be experimentally probed only via lepton-number violating processes involving charged leptons. We have focused our attention on the production of a heavy neutrino at multi-TeV hadron and lepton colliders (specifically at a muon collider). We have considered the vector boson fusion channel W+W+ → l+l′+, mediated by Majorana neutrinos and verified the unitarity properties of this process in a Type I See-Saw. We have then studied same-sign lepton pair production, at both hadron and lepton machines in simplified Heavy Neutrino models, showing that the latter have the potential to either discover or constrain the existence of heavy Majorana neutrinos.