Multiboson Signatures of Doubly Charged Scalars at a Same-Sign Muon Collider

Abstract

We investigate the sensitivity of a same-sign ${\mu }^{+}{\mu }^{+}$ collider to doubly charged scalars in the Type-II seesaw framework, focusing on the regime in which the doubly charged scalar decays dominantly into same-sign $W$-boson pairs. Motivated by the $\mu$TRISTAN proposal, for a benchmark we consider a degenerate triplet spectrum at a center-of-mass energy of $2\mathrm{TeV}$ and an integrated luminosity of $1{\mathrm{ab}}^{-1}$. The signal process is studied in the fully hadronic $W$-decay mode, leading to a characteristic ${\mu }^{+}{\mu }^{+}+8j$ final state. We develop a cut-based analysis based on high jet multiplicity and global hadronic $W$ reconstruction, and then improve its sensitivity with a multivariate strategy exploiting reconstructed $W$ observables, global event kinematics, multi-boson variables and spectator-muon information. The best-performing setup reaches a $2\sigma$ sensitivity level for doubly charged scalar masses up to $425-430\mathrm{GeV}$, extending the cut-based reach by a few tens of GeV. This indicates an improvement over the current LHC coverage, while also providing an independent probe based on a qualitatively different production mode and collider environment.

Comments: 20 pages, 6 figures