Searching for the LFV $γγeμ$ interaction at future $e^- e^+$ colliders

TL;DR

This work investigates charged lepton-flavor violation in the γγ eμ channel within an EFT framework featuring dimension-7 diphoton operators. By analyzing the process $e^+e^-\to e^+e^-e\mu$ via photon fusion at CEPC ($\sqrt{s}=240$ GeV) and FCC-ee ($\sqrt{s}=240,350$ GeV), the authors show that a neural-network classifier using kinematic variables of the scattered electrons and central muons can substantially improve signal discrimination over cut-based methods. They derive current constraints on the effective couplings from related LFV processes, map the viable parameter space, and project discovery reach: a 5σ observation is feasible for $|G_S^{e\mu}|$, $|G_P^{e\mu}|$ around a few $\times10^{-10}$ GeV$^{-3}$ given projected luminosities, with EFT validity requiring $\Lambda \gtrsim 2\sqrt{s}$. The results highlight the strong LFV sensitivity of future circular $e^+e^-$ colliders in photon-fusion channels, offering a clean, complementary probe to low-energy LFV searches and LHC studies.

Abstract

We investigate the Lepton Flavor Violating (LFV) process $e^{+}e^{-}\to e^{+}e^{-}eμ$ ($e=e^-,\,e^+,\,μ=μ^-,\,μ^+ $) at future circular colliders, probing the effective $γγeμ$ interaction through photon fusion. Within an Effective Field Theory (EFT) framework compatible with theoretical and experimental constraints, we identify regions in the parameter space of the effective couplings where the signal could be observed at the Circular Electron-Positron Collider (CEPC) at $\sqrt{s}=240\ \text{GeV}$ and the Future Circular Collider (FCC-ee) at $\sqrt{s}=240$ and $350\ \text{GeV}$. Our analysis leverages distinctive kinematic distributions -- particularly the transverse momentum of the scattered electron $p_{T}^{(e^{-}\ \text{scattered})}$ and the central muon $p_{T}^μ$ -- to achieve efficient signal-background separation. By employing a neural network classifier, we enhance the sensitivity beyond traditional cut-based methods, demonstrating the discovery potential of these facilities for LFV searches in the clean environment of $e^{+}e^{-}$ collisions.

Figures (8)

• Figure 1: Feynman diagrams that contribute to the $\ell_i\to\ell_j\gamma$ decay from the effective dimension-7 diphoton operators represented by the black circles.
• Figure 2: Feynman diagrams that induce the $\ell_i\to\ell_j\gamma\gamma$ decays from the effective dimension-7 diphoton operators (black circle). The black square represents higher order contributions.
• Figure 3: Feynman diagram that contribute to the $e\to\mu$ conversion from the effective dimension-7 diphoton operators represented by the black circle.
• Figure 4: Parameter space allowed by the upper limit on $\rm {BR}(\mu\to e\gamma)$ in the $|G_S^{e\mu}|-|G_P^{e\mu}|$ plane. The green, blue and red regions correspond to cut-off scales of $\Lambda=480,\,700,\,800$ GeV, respectively.
• Figure 5: Feynman diagram of the signal $e^-e^+\to e^- e^+ e\,\mu$. The black circle represents the effective interaction.