{"ID":23012308,"CreatedAt":"2026-09-17T04:38:43.538340206Z","UpdatedAt":"2026-09-17T04:38:43.538340206Z","DeletedAt":null,"paper_url":"https://arxiv.org/abs/2609.18892","arxiv_id":"2609.18892","title":"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 $m_Z/2m_τ=26$. 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 $4.5 (5.5)\\times10^{-7}$ and $2.7 (3.2)\\times10^{-6}$ on $\\operatorname{Re}\\widetilde{d}_τ^W$ and $\\operatorname{Im}\\widetilde{d}_τ^W$, respectively, at FCC-ee (CEPC), improving on ALEPH by three orders of magnitude. The electromagnetic-dipole uncertainty is $9.6 (12)\\times10^{-6}$ on $\\operatorname{Re}\\widetilde{d}_τ^γ$, two orders beyond Belle. These projections assume ideal angular reconstruction and constant form factors across the scan.","short_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 $m_Z/2m_τ=26$. Two transverse moments al...","url_abs":"https://arxiv.org/abs/2609.18892","url_pdf":"https://arxiv.org/pdf/2609.18892v1","authors":"[\"Xin-Yu Du\",\"Zi-Yue Zou\",\"Xiao-Gang He\",\"Keping Xie\"]","published":"2026-09-16T16:25:03Z","proceeding":"hep-ph","tasks":"[\"hep-ph\",\"hep-ex\"]","methods":"[]","has_code":false}