Precise measurement of CP violating $τ$ EDM through $e^+ e^- \to γ^*, ψ(2s) \to τ^+ τ^-$

Abstract: A nonzero electric dipole moment of a tauon, $d_\tau$, signals CP violation and provides an important probe for new physics. We study methods to measure $d_\tau$ at low energy $e^+ e^-$ colliders through the processes $e^+e^- \to \gamma^*, \psi(2S) \to \tau^+\tau^-$ with $\tau^\pm$ decays into a charged hadron and a tau neutrino. We point out that, with measuring energies of the charged hadron, Im$(d_\tau)$ can be measured. On the other hand, selecting events of $\tau$ decays after traveling more than the detector resolution distance, Re$(d_\tau)$ can also be determined. We find that the precision at Super Tau-Charm Facility (STCF) running at the center energy of $m_{\psi (2S)}$ for 10 year data accumulation, the precision of Im$(d_\tau)$ and Re$(d_\tau)$ are found to be 1.8 and 11 in unit of $ 10^{-18}~e\,\text{cm}$, respectively. The sensitivity for $d_\tau$ measurement precision at the STCF can be reached its optimum at a central energy of $6.3~\text{GeV}$, achieving a precision of $0.7$ for Im$(d_\tau)$ and $2.8$ for Re$(d_\tau)$ in unit of $ 10^{-18}~e\,\text{cm}$.