Unraveling the Scotogenic Model at Muon Collider

Abstract: The Scotogenic model extends the standard model with three singlet fermion $N_i$ and one inert doublet scalar $\eta$ to address the common origin of tiny neutrino mass and dark matter. For fermion dark matter $N_1$, a hierarchical Yukawa structure $$|y_{1e}|\ll|y_{1\mu}|\sim|y_{1\tau}|\sim\mathcal{O}(1)$$ is usually favored to satisfy constraints from lepton flavor violation and relic density. Such large $\mu$-related Yukawa coupling would greatly enhance the pair production of charged scalar $\eta<sup>\pm$ at the muon collider. In this paper, we investigate the dilepton signature of the Scotogenic model at a 14 TeV muon collider. For the dimuon signature $$\mu^{+\mu^-+/} \hspace{-0.65em} E_T$$, we find that most viable samples can be probed with $200~\text{fb}<sup>{-1}$ data. The ditau signature $$\tau^{+\tau^-+/} \hspace{-0.65em}E_T$$ is usually less promising but is important to probe the small $|y_{1\mu}|$ region. The mono-photon signature $\gamma+/ \hspace{-0.65em} E_T$ could also probe the compressed mass region $M_1\lesssim M_{\eta<sup>\pm}$. Masses of charged scalar $\eta<sup>\pm$ and dark matter $N_1$ can be further extracted by a binned likelihood fit of the dilepton energy.