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Lepton number violation from Higgs/Z decays into a pair of right-handed neutrinos

Arindam Das1,2,*, Takaaki Nomura3,†, and Kei Yagyu4,‡

  • *Contact author: adas@particle.sci.hokudai.ac.jp
  • †Contact author: nomura@scu.edu.cn
  • ‡Contact author: yagyu@rs.tus.ac.jp
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Phys. Rev. D 114, 015001 – Published 1 July, 2026

DOI: https://doi.org/10.1103/ynd4-dh1b

Abstract

We explore signatures of lepton number violation (LNV) from decays of the discovered Higgs (h) and Z bosons into a pair of right-handed neutrinos (RHNs). Due to the Majorana nature of RHNs, the final state can be the same-sign dilepton plus jets leading to two units of LNV. As a simple but plausible scenario, we investigate such a signal in models with a new U(1)X gauge symmetry, which naturally introduces three RHNs for gauge anomaly cancellation and is spontaneously broken down by a vacuum expectation value of an isospin singlet scalar field (ϕ). In this scenario, h and the Z boson can decay into a pair of RHNs via the h−ϕ mixing and the Z−Z′ mixing with Z′ being a new massive gauge boson, respectively. Estimating the LNV signal and corresponding backgrounds for the ℓ±ℓ±4j final states, we find bounds on the h−ϕ mixing and the Z−Z′ mixing as a function of a mass of RHNs at Higgs factories, e.g., at the High-Luminosity LHC with s=14  TeV, future e−e+ colliders at s=250  GeV, muon colliders at s=125  GeV, as well as at Z factories, e.g., CEPC and Future Circular Collider (FCC-ee) at s=91.2  GeV. We also discuss limits on the scalar mixing at e−μ+ and μ+μ+ collisions (μTRISTAN) with s=346  GeV/775  GeV and 2 TeV, respectively.

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Phys. Rev. D 114, 015001– Published 1 July, 2026
  • Received 10 December 2025
  • Accepted 20 May 2026
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