Precise measurement of CP violating τ EDM through e+e− → γ*, ψ(2s) → τ+τ−

Abstract

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