Invisible Decays of the Z in Portal Matter Models at Tera-Z
Abstract
Searches for new contributions to the invisible width of the Z, ΔΓinv, can be used to provide model-dependent constraints on new physics that may arise from an unknown dark sector. In the vector portal setup considered here it is shown that loops of portal matter (PM) fields, having both Standard Model (SM) and dark sector quantum numbers, can lead to new contributions to ΔΓinv that satisfy current experimental constraints while being simultaneously accessible to measurements at the future Tera-Z. Such PM fields, which here take the form of vector-like fermions, are well known to be able to induce kinetic mixing (KM) between the SM and dark photons (DP) at low energies. This analysis is performed within a simplified version of a previously examined `E6−like' setup that predicts a calculable value for the strength of KM in a phenomenologically interesting range. On the other hand, owing to the constraint on the invisible branching fraction of the Higgs, the tree-level mixing of the SM and a weak isosinglet dark Higgs fields yields a value of ΔΓinv which is far too small to be observable. A similar result holds for the shift in the Z's invisible width induced by the mass and/or KM of the Z with the light DP which then subsequently decays to dark matter. On the other hand, PM models wherein the light dark Higgs carries a significant admixture of a T3≠ 0 weak isospin component are shown to already violate existing ΔΓinv constraints in the absence of parameter tuning. Future measurements of ΔΓinv can thus play an important role in providing indirect constraints on dark sector interactions and PM model building, in particular, on PM loops not necessarily associated with the generation of KM of the DP with the corresponding SM field.
Related
- Lifetime-Resolved Triple-Dimuon Resonances from Rare Z Decays at Tera-Z
- Signatures of Invisible Fermions in Λb0 → Λc+ ℓ- X̅inv Decays
- Probing Massive Invisible Dark Photons in Higgs Decays via Gluon--Gluon Fusion at ATLAS Run 3
- Probing the dark axion portal via J/ψ decays at BESIII and STCF
- Kaon Portal to Freeze-in Dark Matter
BibTeX
@misc{rizzo2026invisible,
title = {{Invisible Decays of the $Z$ in Portal Matter Models at Tera-Z}},
author = {Thomas G. Rizzo},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.16843},
eprint = {2610.16843},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.16843},
}
RIS
TY - UNPB
TI - Invisible Decays of the Z in Portal Matter Models at Tera-Z
AU - Thomas G. Rizzo
PY - 2026
DA - 2026/10/12
PB - arXiv
DO - 10.48550/arxiv.2610.16843
UR - https://arxiv.org/abs/2610.16843
AB - Searches for new contributions to the invisible width of the Z, ΔΓ_inv, can be used to provide model-dependent constraints on new physics that may arise from an unknown dark sector. In the vector portal setup considered here it is shown that loops of portal matter (PM) fields, having both Standard Model (SM) and dark sector quantum numbers, can lead to new contributions to ΔΓ_inv that satisfy current experimental constraints while being simultaneously accessible to measurements at the future Tera-Z. Such PM fields, which here take the form of vector-like fermions, are well known to be able to induce kinetic mixing (KM) between the SM and dark photons (DP) at low energies. This analysis is performed within a simplified version of a previously examined `E_6−like' setup that predicts a calculable value for the strength of KM in a phenomenologically interesting range. On the other hand, owing to the constraint on the invisible branching fraction of the Higgs, the tree-level mixing of the SM and a weak isosinglet dark Higgs fields yields a value of ΔΓ_inv which is far too small to be observable. A similar result holds for the shift in the Z's invisible width induced by the mass and/or KM of the Z with the light DP which then subsequently decays to dark matter. On the other hand, PM models wherein the light dark Higgs carries a significant admixture of a T_3≠ 0 weak isospin component are shown to already violate existing ΔΓ_inv constraints in the absence of parameter tuning. Future measurements of ΔΓ_inv can thus play an important role in providing indirect constraints on dark sector interactions and PM model building, in particular, on PM loops not necessarily associated with the generation of KM of the DP with the corresponding SM field.
ER -