Speaker
Description
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, $\mathrm{Im}(d_{\tau})$ can be measured. On the other hand, selecting events of $\tau$ decays after traveling more than the detector resolution distance, $\mathrm{Re}(d_{\tau})$ 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 $\mathrm{Im}(d_{\tau})$ and $\mathrm{Re}(d_{\tau})$ are found to be $1.8$ and $11$ in unit of $10^{-18} \, e \, \mathrm{cm}$, respectively. The sensitivity for $d_{\tau}$ measurement precision at the STCF can be reached its optimum at a central energy of $6.3 \, \mathrm{GeV}$, achieving a precision of $0.7$ for $\mathrm{Im}(d_{\tau})$ and $2.8$ for $\mathrm{Re}(d_{\tau})$ in unit of $10^{-18} \, e \, \mathrm{cm}$.
