Seesaw Models under the Lens of Angular Distributions at $\mu^+\mu^-,\, \mu^+ \mu^+, \, \mu^+ \gamma$ and $\mu^+ e^-$ Colliders

Abstract

The Seesaw extensions of the Standard Model not only provide a natural explanation for tiny neutrino masses, they also predict additional heavy states such as Majorana neutrinos, charged leptons, and scalar triplets. Distinguishing between these scenarios at future colliders will be essential if such particles are discovered. In this work, we investigate how leptonic colliders offer complementary avenues for this task, focusing on the role of reconstructed angular distributions, which encode information about the underlying matrix elements. One only needs to reconstruct the new particle (or SM charged lepton) in the final state and examine its angular distribution relative to the beam axis, without bothering about the other particles in the final state, and this is sufficient to reveal the underlying seesaw scenario. We perform a detailed PYTHIA8-based simulation for different Seesaw realizations: inverse Type-I, Type-II, and inverse Type-III at $\mu^+\mu^-$, $\mu^+\mu^+$, $\mu^+\gamma$, and $\mu^+e^-$ colliders. Characteristic angular patterns emerge that enable discrimination among the models, with muon colliders providing particularly promising reach. We also comment on the prospects of asymmetric $\mu e$ colliders in probing off-diagonal Yukawa structures.