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

TL;DR

This work demonstrates that reconstructed angular distributions of Beyond-Standard-Model states predicted by inverse Type-I, Type-II, and inverse Type-III Seesaw models can effectively distinguish among these scenarios at future leptonic colliders. Using a PYTHIA8-based framework, the authors analyze μ^+ μ^- , μ^+ μ^+ , μ^+ γ, and μ^+ e^- colliders, showing characteristic bowl-, dome-, and flat-like angular patterns tied to the spins and couplings of heavy neutrinos, scalar triplets, and fermionic triplets. Across channels and collider configurations, Type-II signatures (notably Δ^{++}) often yield clean, reconstructible angular distributions, while iType-I and iType-III show distinct, model-specific patterns in both neutral and charged heavy-lepton sectors, including asymmetries in iType-III. The findings indicate that angular observables provide complementary, information-rich guidance for disentangling Seesaw realizations, with muon-based facilities offering particularly promising reach for TeV-scale states and off-diagonal Yukawa structures. The study underscores the practical potential of angular analysis as a tool for future collider program planning and model discrimination in neutrino-mass generating mechanisms.

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 $μ^+μ^-$, $μ^+μ^+$, $μ^+γ$, and $μ^+e^-$ 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 $μe$ colliders in probing off-diagonal Yukawa structures.

Figures (21)

• Figure 1: Schematic diagram of a generic lepton collision. The polar angle $\theta$ of one of the final-state particles with respect to the beam axis (the $Z$-axis) is defined in the CM frame.
• Figure 2: Angular distributions for the BSM particles in iType-I (a), Type-II (b) and iType-III (c) Seesaw with $Y=Y_\Delta =0.2$, $M_n=M_\Delta=1.2$ TeV and $\sqrt{s}=3.0$ TeV in the $\mu^+ \mu^-$ collider.
• Figure 3: Contours of the total cross-sections (fb) for $\mu^+ \mu^-$ collider in $M_n$ or $M_{\Delta}$ versus the centre-of-mass energy ($\sqrt{\hat{s}}$) plane for $Y_\Delta$ and $Y =0.2$, $\mu_\Delta$ and $\mu_n=10$ eV.
• Figure 4: Di-jet-mono-lepton invariant mass distribution ($M_{jjl}$) for (a) BP1 and (b) BP2 at the centre-of-mass energies of 3.0, 6.0 TeV, respectively with the integrated luminosity of 1000 fb$^{-1}$. The total (signal $+$ SM background) signature is depicted in brown and the SM background is in olive green.
• Figure 5: Same sign di-lepton invariant mass distribution ($M_{\rm SSD}$) for (a) BP1 and (b) BP2 at the centre-of-mass energies of 3.0, 6.0 TeV, respectively with the integrated luminosity of 1000 fb$^{-1}$. The total (signal $+$ SM background scaled by 2) signature is depicted in brown and the SM background (scaled by 2) is in olive green.