Entanglement and Bell Nonlocality in $\tau^+ \tau^-$ at the BEPC

TL;DR

This paper proposes measuring quantum entanglement and Bell nonlocality in tau lepton pairs at the BEPC collider, demonstrating potential for high-energy tests of fundamental quantum phenomena with achievable experimental precision.

Contribution

It introduces a novel approach to observe entanglement and Bell nonlocality in high-energy tau pairs at BEPC, extending quantum tests beyond traditional low-energy experiments.

Findings

Entanglement observable with 1% systematic uncertainty in existing data.

Predicted measurable entanglement with better than 4% precision in future runs.

Bell nonlocality can be established at 5σ with 0.5%-2.0% systematic control.

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 $\tau^+\tau^-$ 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 $5\sigma$ as long as systematic uncertainty can be controlled at level of 0.5% - 2.0%, depending on the center-of-mass energy.