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We study the prospects of probing neutrino mass models at the newly proposed antimuon collider μTRISTAN, involving μ+e− scattering at s√=346 GeV and μ+μ+ scattering at s√=2 TeV. We show that μTRISTAN is uniquely sensitive to leptophilic neutral and doubly-charged scalars naturally occurring in various neutrino mass models, such as Zee, Zee-Babu, cocktail, and type-II seesaw models, over a wide range of mass and coupling values, well beyond the current experimental constraints. It also allows for the possibility to correlate the collider signals with neutrino mixing parameters and charged lepton flavor violating observables. 

The nature of neutrino masses and the matter-antimatter asymmetry of our universe are two of the most important open problems in particle physics today and are notoriously difficult to test with current technology. Dirac neutrinos offer a solution through a leptogenesis mechanism that hinges on the smallness of neutrino masses and resultant non-thermalization of the right-handed neutrino partners in the early universe. We thoroughly explore possible realizations of this Dirac leptogenesis idea, revealing new windows for highly efficient asymmetry generation. In many of them, the number of relativistic degrees of freedom, Neff, is severely enhanced compared to standard cosmology and offers a novel handle to constrain Dirac leptogenesis with upcoming measurements of the cosmic microwave background. Realizations involving leptoquarks even allow for low-scale post-sphaleron baryogenesis and predict proton decay. These novel aspects render Dirac leptogenesis surprisingly testable.