Transverse Tau Spin Correlations and the Weak Electric Dipole Moment at Future Z Factories
Abstract
Tau spin correlations at the Z pole probe CP violation with a sensitivity that depends strongly on the spin projection. For a fixed Z sample, transverse moments avoid the chirality suppression of longitudinal and triple-product observables, enhancing the response by a factor mZ/2mτ=26. Two transverse moments also select the axial tau charge, gaining |aτ/vτ|=13.5; the sign-weighted normal moment has a response $521$ times that of the triple product. These two transverse moments give single-parameter uncertainties no more than 14% above our calculated Cramer-Rao bound for independent single-pion measurements, approaching the best precision within this analysis. A five-moment fit with 5% off-peak luminosity yields statistical uncertainties of 4.5 (5.5)×10−7 and 2.7 (3.2)×10−6 on Red̃τW and Imd̃τW, respectively, at FCC-ee (CEPC), improving on ALEPH by three orders of magnitude. The electromagnetic-dipole uncertainty is 9.6 (12)×10−6 on Red̃τγ, two orders beyond Belle. These projections assume ideal angular reconstruction and constant form factors across the scan.
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BibTeX
@misc{du2026transverse,
title = {{Transverse Tau Spin Correlations and the Weak Electric Dipole Moment at Future $Z$ Factories}},
author = {Xin-Yu Du and Zi-Yue Zou and Xiao-Gang He and Keping Xie},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.18892},
eprint = {2609.18892},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.18892},
}
RIS
TY - UNPB
TI - Transverse Tau Spin Correlations and the Weak Electric Dipole Moment at Future Z Factories
AU - Xin-Yu Du
AU - Zi-Yue Zou
AU - Xiao-Gang He
AU - Keping Xie
PY - 2026
DA - 2026/09/16
PB - arXiv
DO - 10.48550/arxiv.2609.18892
UR - https://arxiv.org/abs/2609.18892
AB - Tau spin correlations at the Z pole probe CP violation with a sensitivity that depends strongly on the spin projection. For a fixed Z sample, transverse moments avoid the chirality suppression of longitudinal and triple-product observables, enhancing the response by a factor m_Z/2m_τ=26. Two transverse moments also select the axial tau charge, gaining |a_τ/v_τ|=13.5; the sign-weighted normal moment has a response $521$ times that of the triple product. These two transverse moments give single-parameter uncertainties no more than 14% above our calculated Cramer-Rao bound for independent single-pion measurements, approaching the best precision within this analysis. A five-moment fit with 5% off-peak luminosity yields statistical uncertainties of 4.5 (5.5)×10^−7 and 2.7 (3.2)×10^−6 on Red̃_τ^W and Imd̃_τ^W, respectively, at FCC-ee (CEPC), improving on ALEPH by three orders of magnitude. The electromagnetic-dipole uncertainty is 9.6 (12)×10^−6 on Red̃_τ^γ, two orders beyond Belle. These projections assume ideal angular reconstruction and constant form factors across the scan.
ER -