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Searches for the leptophilic Z' boson at the International Linear Collider and Linear Collider Facility

Aleksander Filip Żarnecki

Abstract

Production of the leptophilic Z' boson at future colliders was proposed as one of the benchmark Beyond the Standard Model (BSM) scenarios for the Physics Briefing Book prepared for the 2026 update of the European Strategy for Particle Physics. Presented in this contributions are results on the sensitivity of the experiments at the International Linear Collider (ILC) and at the Linear Collider Facility (LCF) at CERN to the considered scenario. Studied as a function of the Z' boson mass is the expected signal in the di-muon decay channel, which is expected to have the highest sensitivity. Signal and background simulation in WHIZARD includes effects of multi-photon emission and initial state radiation (ISR), with proper matching between ISR and matrix element domains. Expected limits on the Z' couplings to SM leptons are presented for ILC and LCF running at 250 GeV and LCF running at 550 GeV.

Searches for the leptophilic Z' boson at the International Linear Collider and Linear Collider Facility

Abstract

Production of the leptophilic Z' boson at future colliders was proposed as one of the benchmark Beyond the Standard Model (BSM) scenarios for the Physics Briefing Book prepared for the 2026 update of the European Strategy for Particle Physics. Presented in this contributions are results on the sensitivity of the experiments at the International Linear Collider (ILC) and at the Linear Collider Facility (LCF) at CERN to the considered scenario. Studied as a function of the Z' boson mass is the expected signal in the di-muon decay channel, which is expected to have the highest sensitivity. Signal and background simulation in WHIZARD includes effects of multi-photon emission and initial state radiation (ISR), with proper matching between ISR and matrix element domains. Expected limits on the Z' couplings to SM leptons are presented for ILC and LCF running at 250 GeV and LCF running at 550 GeV.
Paper Structure (6 sections, 2 equations, 4 figures)

This paper contains 6 sections, 2 equations, 4 figures.

Table of Contents

  1. Motivation
  2. Model
  3. Event simulation
  4. Event selection
  5. Results
  6. Conclusions

Figures (4)

  • Figure 1: Distribution of the energy for the muon pair and for the highest energy photon in Z' production events simulated with Whizard. Left: for ILC running at 250 GeV and Z' mass of 50 GeV (red), 100 GeV (green) and 150 GeV (blue). Right: for LCF running at 550 GeV and Z' mass of 100 GeV (blue), 200 GeV (green), 300 GeV (red) and 400 GeV (magenta).
  • Figure 2: Distribution of the sum of the reconstructed muon pair energy and the absolute value of its longitudinal momentum for SM background events (red histogram) and the Z' production events (blue histogram). Left: for ILC running at 250 GeV and Z' mass of 50 GeV. Right: for LCF running at 550 GeV and Z' mass of 200 GeV. Signal samples are normalised to Z' coupling of $g_{Z'}=0.001$.
  • Figure 3: Distribution of the reconstructed muon pair invariant mass for SM background events (red histogram) and the Z' production events, after radiative event selection cuts (blue histograms). Left: for ILC running at 250 GeV and Z' mass of 50 GeV, 100 GeV, 150 GeV and 200 GeV. Right: for LCF running at 550 GeV and Z' mass of 100 GeV, 200 GeV, 300 GeV, 400 GeV and 500 GeV. Signal samples are normalised to Z' coupling of $g_{Z'}=0.001$.
  • Figure 4: Expected 95% CL exclusion limits on the leptophilic Z' boson coupling. Left: limits expected from the di-muon invariant mass distribution for $\textrm{e}^{_+}\!\textrm{e}^{_-}\xspace \to \mu^{_+}\!\mu^{_-}\xspace (\gamma)$ events, based on fast detector simulation for for the ILC and LCF running at 250GeV, and LCF running at 550GeV. Right: comparison of LCF limits at 250GeV with existing limits from LEP2 and projected exclusion curves for HL-LHC and other future facilities (figure taken from deBlas:2025gyz).