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Probing Lepton Flavor Violation at the ILC and CLIC

Pankaj Munbodh

TL;DR

Probing Lepton Flavor Violation in the tau-mu sector using SMEFT, the paper analyzes the process e+e- to tau mu at the ILC and CLIC with Initial State Radiation and beam polarization. It shows that three SMEFT operator classes contribute at tree level and that the cross section depends on SMEFT coefficients and helicities, with a key result that four-fermion operators yield a cross section that scales linearly with $s$. A Monte Carlo approach including beam energy spread and detector effects yields projected sensitivities that, for polarized beams, can compete with or surpass Belle-II tau decays in certain channels. Overall, the study demonstrates strong potential of future linear colliders to probe LFV within SMEFT.

Abstract

Lepton flavor violation in the $τμ$ sector would be a clear sign of Beyond Standard Model physics. We employ the SMEFT framework to study the process $e^+e^-\toτμ$ at the ILC and CLIC. We find that the $e^+e^-$ beam polarizations achievable at these machines allow us to probe the chirality structure of the SMEFT operators. In addition, the high center of mass energy leads to a substantial increase in sensitivity to the four-fermion operators that rivals, and in some cases, surpasses tau decay projections from Belle-II.

Probing Lepton Flavor Violation at the ILC and CLIC

TL;DR

Probing Lepton Flavor Violation in the tau-mu sector using SMEFT, the paper analyzes the process e+e- to tau mu at the ILC and CLIC with Initial State Radiation and beam polarization. It shows that three SMEFT operator classes contribute at tree level and that the cross section depends on SMEFT coefficients and helicities, with a key result that four-fermion operators yield a cross section that scales linearly with . A Monte Carlo approach including beam energy spread and detector effects yields projected sensitivities that, for polarized beams, can compete with or surpass Belle-II tau decays in certain channels. Overall, the study demonstrates strong potential of future linear colliders to probe LFV within SMEFT.

Abstract

Lepton flavor violation in the sector would be a clear sign of Beyond Standard Model physics. We employ the SMEFT framework to study the process at the ILC and CLIC. We find that the beam polarizations achievable at these machines allow us to probe the chirality structure of the SMEFT operators. In addition, the high center of mass energy leads to a substantial increase in sensitivity to the four-fermion operators that rivals, and in some cases, surpasses tau decay projections from Belle-II.
Paper Structure (5 sections, 3 equations, 2 figures)

This paper contains 5 sections, 3 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Observables
  3. Expected sensitivities at ILC and CLIC
  4. Conclusion
  5. Acknowledgements

Figures (2)

  • Figure 1: Adapted from Altmannshofer:2025nbp. Sensitivity to pairs of SMEFT coefficients at the $\sqrt{s} = 3$ TeV run at CLIC. The various shades of blue represent our $2\sigma$ sensitivity projections for different $e^+/e^-$ beam polarizations in the process $e^+e^- \to \tau\mu$. The lighter and darker red regions correspond to the combined $2\sigma$ and $1\sigma$ constraints, respectively. The yellow region indicates the current $2\sigma$ bounds from searches for the $\tau\to\mu e^+e^-$ decay at BaBar and Belle Hayasaka:2010npBaBar:2010axs, while the solid dashed line shows the expected sensitivity projection from Belle II Belle-II:2018jsg. The UV scale is fixed to $\Lambda = 5$ TeV.
  • Figure 2: Adapted from Altmannshofer:2025nbp. Sensitivity to the new physics scale $\Lambda$ from $\tau\to\mu e^+ e^-$, and from the process $e^+e^-\to\tau\mu$ at future colliders. Each SMEFT Wilson coefficient is set to a value of unity at the scale $\Lambda$ ($C_i(\Lambda)=1$), with all others set to zero. Only projected sensitivities are shown.