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

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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.

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