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Chasing long-lived doubly charged scalars at future lepton colliders

Nandini Das1,2,*, Dilip Kumar Ghosh3,†, Nivedita Ghosh4,‡, and Ritesh K. Singh5,§

  • *Contact author: nandinidas.rs@gmail.com
  • †Contact author: tpdkg@iacs.res.in
  • ‡Contact author: nivedita.ghosh@ipmu.jp
  • §Contact author: ritesh.singh@iiserkol.ac.in
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Phys. Rev. D 114, 055022 – Published 14 September, 2026

DOI: https://doi.org/10.1103/9d93-x783

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 ≲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.

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Phys. Rev. D 114, 055022– Published 14 September, 2026
  • Received 24 February 2026
  • Accepted 21 August 2026
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