Sensitivity Study of the Tau Lepton Electric Dipole Moment at the Super Tau-Charm Facility

TL;DR

This paper presents a simulation-based study optimizing methods to measure the tau lepton's electric dipole moment at the Super Tau-Charm Facility, achieving a sensitivity of approximately 3.89×10⁻¹⁸ e·cm.

Contribution

It introduces machine learning-enhanced event selection and an analytical framework for tau momentum reconstruction to improve EDM sensitivity at STCF.

Findings

Achieved 80% signal purity with 6.3% efficiency.

Developed optimal observables for EDM sensitivity.

Estimated EDM measurement sensitivity of 3.89×10⁻¹⁸ e·cm.

Abstract

This study investigates the intrinsic electric dipole moment (EDM) of the $\tau$ lepton, which is an important quantity in the search for physics beyond the Standard Model (BSM). In preparation for future measurements at the Super Tau-Charm Facility (STCF), we employ Monte Carlo simulations of the $e^+e^- \rightarrow \tau^+\tau^-$ process and optimize the analysis methodology for EDM extraction. Machine learning techniques are implemented to efficiently identify signal events ($\tau^\pm\rightarrow\pi^\pm\pi^0\nu_\tau$), which result in a significant improvement in signal-to-noise ratio. Our optimized event selection algorithm achieves $80.0\%$ signal purity with $6.3\%$ efficiency. We develop an analytical approach for $\tau$ lepton momentum reconstruction and derive the squared spin density matrix along with optimal observables, which maximize the sensitivity to $d_\tau$. The relationship between these observables and the EDM is established with the estimated sensitivity of $|d_\tau| < 3.89\times 10^{-18}\,e\cdot\mathrm{cm}$ at a $68\%$ confidence level. These results provide a foundation for future experimental measurements of the $\tau$ lepton EDM in STCF experiments.