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Discovery Prospects for a Leptophilic Gauge Boson $Z_\ell$ at CEPC and ILC

Seyit Okan Kara

TL;DR

The paper investigates the discovery potential for a leptophilic gauge boson $Z_\ell$ arising from a minimal $U(1)'_\ell$ extension, coupling exclusively to leptons. By focusing on the clean $e^+e^- \to \mu^+\mu^-$ channel and incorporating realistic ISR, BS, and detector effects, the study provides a side-by-side projection for CEPC and ILC: CEPC at $\sqrt{s}=240$ GeV probes $g_\ell$ down to ~$10^{-3}$ for $M_{Z_\ell}\lesssim 220$ GeV, while ILC at higher energies extends sensitivity to heavier resonances into the multi-hundred GeV–TeV range. The results demonstrate the complementarity of circular and linear lepton colliders in testing purely leptophilic interactions and offer realistic benchmarks for future collider programs. The analysis also highlights that the di-muon invariant-mass spectrum would reveal clear $Z_\ell$ resonance peaks even after beam effects, reinforcing the testability of leptophilic $Z_\ell$ scenarios at next-generation facilities.

Abstract

We investigate the discovery prospects of a leptophilic gauge boson Z_l at future $e^+e^-$ colliders, focusing on a comparative study of the Circular Electron-Positron Collider (CEPC) and the International Linear Collider (ILC). This state can arise from an additional U(1)'_l gauge symmetry under which quarks are neutral and all leptons have a universal charge, motivated by neutrino oscillations and physics beyond the Standard Model. As a clean benchmark we study $e^+e^- \to μ^+μ^-$ including realistic initial state radiation (ISR) and beamstrahlung. We find that CEPC at sqrt(s)=240 GeV can probe couplings down to $g_l \sim 10^{-3}$ for $Z_l$ masses up to about 220 GeV, while the ILC extends the sensitivity to heavier states in the multi-hundred GeV range through its higher sqrt(s) stages. These results highlight the complementarity of circular and linear colliders in testing purely leptophilic interactions. Published in Chinese Physics C 50 (2026) 033104. DOI: 10.1088/1674-1137/ae1de4

Discovery Prospects for a Leptophilic Gauge Boson $Z_\ell$ at CEPC and ILC

TL;DR

The paper investigates the discovery potential for a leptophilic gauge boson arising from a minimal extension, coupling exclusively to leptons. By focusing on the clean channel and incorporating realistic ISR, BS, and detector effects, the study provides a side-by-side projection for CEPC and ILC: CEPC at GeV probes down to ~ for GeV, while ILC at higher energies extends sensitivity to heavier resonances into the multi-hundred GeV–TeV range. The results demonstrate the complementarity of circular and linear lepton colliders in testing purely leptophilic interactions and offer realistic benchmarks for future collider programs. The analysis also highlights that the di-muon invariant-mass spectrum would reveal clear resonance peaks even after beam effects, reinforcing the testability of leptophilic scenarios at next-generation facilities.

Abstract

We investigate the discovery prospects of a leptophilic gauge boson Z_l at future colliders, focusing on a comparative study of the Circular Electron-Positron Collider (CEPC) and the International Linear Collider (ILC). This state can arise from an additional U(1)'_l gauge symmetry under which quarks are neutral and all leptons have a universal charge, motivated by neutrino oscillations and physics beyond the Standard Model. As a clean benchmark we study including realistic initial state radiation (ISR) and beamstrahlung. We find that CEPC at sqrt(s)=240 GeV can probe couplings down to for masses up to about 220 GeV, while the ILC extends the sensitivity to heavier states in the multi-hundred GeV range through its higher sqrt(s) stages. These results highlight the complementarity of circular and linear colliders in testing purely leptophilic interactions. Published in Chinese Physics C 50 (2026) 033104. DOI: 10.1088/1674-1137/ae1de4

Paper Structure

This paper contains 7 sections, 6 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Theoretical Framework
  3. Collider Setup
  4. Signal and Background Analysis
  5. Results and Discussion
  6. Conclusions
  7. Data Availability

Figures (8)

  • Figure 1: Representative Feynman diagrams for the signal process $e^+e^- \to Z_\ell \to \mu^+\mu^-$ (left) and the dominant Standard Model background $e^+e^- \to \gamma^*/Z \to \mu^+\mu^-$ (right).
  • Figure 2: Total cross section $\sigma(e^+e^-\!\to\mu^+\mu^-)$ as a function of the center-of-mass energy $\sqrt{s}$ for CEPC and ILC benchmark configurations. The CEPC baseline at $\sqrt{s}=240~\mathrm{GeV}$ with $20~\mathrm{ab}^{-1}$ integrated luminosity provides excellent sensitivity to light leptophilic gauge bosons, while the higher-energy ILC stages (250 and 500 GeV) extend the accessible mass range to heavier $Z_\ell$ states.
  • Figure 3: Total cross section $\sigma(e^+e^-\!\to\mu^+\mu^-)$ and its relative deviation $(\sigma_{\rm NP}/\sigma_{\rm SM}-1)$ as functions of $M_{Z_\ell}$ for representative couplings $g_\ell$. (a) CEPC at $\sqrt{s}=240~\text{GeV}$ with $20~\text{ab}^{-1}$ integrated luminosity (TDR baseline). (b) ILC at $\sqrt{s}=250~\text{GeV}$ with $2~\text{ab}^{-1}$ polarized beams. (c) ILC at $\sqrt{s}=500~\text{GeV}$, where higher energies extend the sensitivity to heavier $Z_\ell$ states. Right-hand panels show the fractional deviation from the SM prediction, highlighting the discovery potential even for small couplings.
  • Figure 4: Energy scans of the total cross section $\sigma(e^+e^-\!\to\mu^+\mu^-)$ for representative couplings $g_\ell$ at future $e^+e^-$ colliders. (a) CEPC at $\sqrt{s}=240~\mathrm{GeV}$ (20 $\mathrm{ab}^{-1}$, TDR baseline). (b) ILC at $\sqrt{s}=250~\mathrm{GeV}$ (2 $\mathrm{ab}^{-1}$, polarized). (c) ILC at $\sqrt{s}=500~\mathrm{GeV}$. Line styles distinguish the benchmark scenarios; the SM expectation is shown in gray.
  • Figure 5: Total cross section $\sigma(e^+e^-\!\to\mu^+\mu^-)$ as a function of the leptophilic coupling $g_\ell$ for CEPC and ILC setups, illustrating the impact of initial-state radiation (ISR) and beamstrahlung (BS). (a) CEPC at $\sqrt{s}=240~\mathrm{GeV}$, (b) ILC at $\sqrt{s}=250~\mathrm{GeV}$, and (c) ILC at $\sqrt{s}=500~\mathrm{GeV}$. Solid, dotted, and dash–dot lines correspond to the full ISR+BS simulation, ISR only, and the idealized No ISR+BS case, respectively. Both axes are displayed on logarithmic scales.
  • ...and 3 more figures