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Implications of a portal vectorlike lepton on associated Higgs boson production at a multi-TeV muon collider

Krishna Tewary ^1,*, Sanjoy Biswas ^1,†, and Shivam Verma ^2,‡

¹Department of Physics, Ramakrishna Mission Vivekananda Educational and Research Institute, Belur Math, Howrah 711202, India
²Theoretical Physics Division, Physical Research Laboratory, Shree Pannalal Patel Marg, Ahmedabad 380009, Gujarat, India

^*Contact author: krishna.tewary.phy22@gm.rkmvu.ac.in, krishnatewary3@gmail.com
^†Contact author: sanjoy.phy@gm.rkmvu.ac.in, sanjoyhri@gmail.com
^‡Contact author: shivamverma@prl.res.in, shivam.59910103@gmail.com

Phys. Rev. D 114, 035029 – Published 21 August, 2026

DOI: https://doi.org/10.1103/kpdt-4zm2

Abstract

We explore a portal vectorlike lepton (pVLL) extension of the Standard Model (SM) and study its implications for Higgs and vector-boson associated production ( $h V$ , with $V = Z$ -boson or dark photon) at a future muon collider facility. We show that while the $μ^{+} μ^{-} \to h Z$ production rate remains close to its SM prediction in a wide range of parameter space, the rate for $μ^{+} μ^{-} \to h γ_{d}$ can be substantially enhanced owing to the nondecoupling nature of the interaction involving the heavy lepton, the muon and the dark photon. We demonstrate that the $h γ_{d}$ production rate can exceed the corresponding $h Z$ rate by a factor of 1–100 within the perturbative unitarity limit, making it a promising channel for probing Higgs interactions and potential new physics effects. We also examine the role of the pVLL state in the context of dark matter (DM) phenomenology and identify regions of parameter space consistent with the observed relic abundance by extending the simplified setup with a viable DM candidate. The $h γ_{d}$ production can also be used to constrain the dark photon mass ( $m_{γ_{d}}$ ) and/or the dark gauge coupling ( $g_{d}$ ) consistent with various constraints including the current muon $g - 2$ measurements within the pVLL framework. We perform a detailed collider analysis of the $h γ_{d}$ process in the $b \bar{b} +$ missing energy final state. A $2 σ$ exclusion limit for $m_{γ_{d}}$ up to 80 GeV is obtained assuming $g_{d} = 0.05$ , $\sin θ_{L} = 4 \times 10^{- 5}$ , $\sin θ_{s} = 0.05$ , for a heavy lepton mass $\sim 3 TeV$ at a 3 TeV muon collider with an integrated luminosity of $1 {ab}^{- 1}$ .

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Phys. Rev. D 114, 035029– Published 21 August, 2026

Vol. 114, Iss. 3 — 1 August 2026

Received 7 December 2025
Accepted 13 July 2026

DOI: https://doi.org/10.1103/kpdt-4zm2

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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Implications of a portal vectorlike lepton on associated Higgs boson production at a multi-TeV muon collider

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