Same-Sign Tetralepton Signature at $\mu$TRISTAN

Naturally tiny neutrino masses can be explained by the low scale seesaw with heavy neutral lepton $N$ coupling to the neutrinophilic Higgs doublet $\Phi_\nu$, which obtains a much smaller vacuum expectation value than the standard Higgs doublet $\Phi$. Within this model, the neutrino masses originate from the new Yukawa interaction $y \overline{L}\tilde{\Phi}_\nu N$. In this paper, we propose the novel same-sign tetralepton signature at the 2 TeV same-sign muon mode $\mu^+\mu^+$ of $\mu$TRISTAN. We investigate two distinct channels of this signature, which are both generated by the Yukawa interaction $y \overline{L}\tilde{\Phi}_\nu N$. One is from the pair production of charged Higgs $\mu^+\mu^+\to H^+ H^+\to \mu^+N +\mu^+ N\to \mu^+ \mu^+ jj + \mu^+ \mu^+ jj\to 4\mu^+ + 4j$, and the other one is from the single production of charged Higgs $\mu^+\mu^+ \to \mu^+ N H^+ \to \mu^+N +\mu^+ N\to \mu^+ \mu^+ jj + \mu^+ \mu^+ jj\to 4\mu^+ + 4j$. We then perform a detailed simulation of this same-sign tetralepton signature, and obtain the promising region at $\mu$TRISTAN.