27510 2501.04801 10.1007/JHEP10(2025)217 217 217 Entanglement and Bell nonlocality in τ + τ − at the BEPC 02 Physical Sciences 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics Regular Article - Theoretical Physics 1 24 2025 10 27 2025 8 17 2025 9 27 2025 10 27 2025 10 27 U.S. Department of Energy DE-SC0007914 National Science Foundation PHY-2112829 The Author(s) 2025 Open Access. This article is distributed under the terms of the Creative Commons Attribution License ( CC-BY 4.0 ), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. 13130 2025 2025 10 10 Tao Han than@pitt.edu Matthew Low matthew.w.low@pitt.edu Youle Su ylsu21@m.fudan.edu.cn https://ror.org/01an3r305 grid.21925.3d 0000 0004 1936 9000 PITT PACC, Department of Physics and Astronomy University of Pittsburgh 3941 O’Hara St. Pittsburgh PA 15260 USA https://ror.org/013q1eq08 grid.8547.e 0000 0001 0125 2443 Department of Physics and Center for Field Theory and Particle Physics Fudan University Shanghai 200438 China Abstract Quantum entanglement and Bell nonlocality are two phenomena that occur only in quantum systems. In both cases, these are correlations between two subsystems that are classically absent. Traditionally, these phenomena have been measured in low-energy photon and electron experiments, but more recently they have also been measured in high-energy particle collider environments. In this work, we propose measuring the entanglement and Bell nonlocality in the τ + τ − state near and above its kinematic threshold at the Beijing Electron Positron Collider (BEPC). We find that in the existing dataset, entanglement is observable if systematic uncertainties are kept to 1%. In the upcoming run between 4.0 and 5.6 GeV, the entanglement is predicted to be measurable with a precision better than 4% and Bell nonlocality can be established at 5σ as long as systematic uncertainty can be controlled at level of 0.5%–2.0%, depending on the center-of-mass energy. Keywords Precision QED Electroweak Precision Physics ArXiv ePrint: 2501.04801