Chasing Long-Lived Doubly Charged Scalars at Future Lepton Colliders

TL;DR

This paper investigates long-lived doubly charged scalars from a Type-II seesaw SU(2)_L triplet, focusing on $H^{\pm\pm}$ with macroscopic lifetimes that decay to like-sign $\mu$ pairs. The authors propose search strategies at future lepton colliders: Drell–Yan production at the ILC (500 GeV) and vector-boson fusion production at a 10 TeV muon collider, yielding a four-lepton plus missing-energy signature with displaced vertices as a direct lifetime probe. They identify viable benchmark points with $m_{H^{\pm\pm}}$ in the 100–180 GeV range and $v_t \sim 10^{-4}$–$10^{-3}$ GeV, and demonstrate that displaced-vertex cuts on track impact parameters enable discovery with modest integrated luminosities at the ILC and the muon collider, respectively. Mass reconstruction from same-sign dimuon pairs provides a powerful complementary observable. Overall, the work highlights the potential of future lepton colliders to probe LLPs in extended scalar sectors, particularly via clean multi-lepton final states and displaced-vertex signatures.

Abstract

We come up with a novel search strategy for long-lived doubly charged scalars at future proposed lepton colliders. The doubly charged scalar studied in this work belongs to an $SU(2)_L$ complex scalar triplet that accounts for tiny neutrino masses via the Type-II Seesaw mechanism. For scalar masses $\lesssim 200 $ GeV and appropriate values of the triplet vacuum expectation value, this state can be long-lived and decay predominantly into like-sign muon pairs (e.g. $μ^+μ^+ $ or $μ^-μ^-$), producing distinctive displaced-vertex signals. We investigate the pair production of these scalars at the International Linear Collider (ILC) and a prospective muon collider, considering their planned center-of-mass energies. Incorporating theoretical and experimental constraints, we study the resulting signature of four leptons accompanied by missing transverse energy. Displaced vertices offer direct evidence of the scalar's long lifetime, while we further show that the invariant mass distribution of same-sign dilepton pairs serves as a powerful complementary probe for discovering doubly charged Higgs bosons at both the ILC and muon collider.

Figures (7)

• Figure 1: Normalized transverse momentum distributions of the leading and sub-leading muons in signal events are shown for benchmark points BP1, BP2, and BP3. For comparison, the corresponding Standard Model background distributions are also included. Results are presented for the ILC at $\sqrt{s} = 500$ GeV with integrated luminosity, $\mathcal{L}_{\rm int}=4 ab^{-1}$.
• Figure 2: Normalized pseudo-rapidity distributions of the leading and sub-leading muons in signal events are shown for benchmark points BP1, BP2, and BP3. For comparison, the corresponding Standard Model background distributions are also included. Results are presented for the ILC at $\sqrt{s} = 500$ GeV with an integrated luminosity, $\mathcal{L}_{\rm int}=4 ab^{-1}$.
• Figure 3: Normalized impact parameter $\mid d_0 \mid$ distributions of the leading and sub-leading muons in signal events as well as the SM background. Results are presented for the ILC at $\sqrt{s} = 500$ GeV with an integrated luminosity, $\mathcal{L}_{\rm int}=4 {\rm ab}^{-1}$.
• Figure 4: Normalized invariant mass distribution for same-sign dimuon pair for 3 chosen benchmark points at the ILC with an integrated luminosity, $\mathcal{L}_{\rm int}=4 {\rm ab}^{-1}$.
• Figure 5: Normalized transverse momentum distributions of the leading and sub-leading muons in signal events are shown for benchmark points BP1, BP2, and BP3. Results are presented for the muon collider at $\sqrt{s} = 10$ TeV with $\mathcal{L}_{\rm int}=10 ~{\rm ab}^{-1}$.