Bell Tests and Entanglement in Collider Experiments (Christopher Timpson)

There has recently been considerable interest in experimental measurement of Bell inequality violation, and tests for the presence of entanglement, in high energy colliders. The Bell inequality was originally introduced to test for the presence of nonlocality (action-at-a-distance) in the world, and there have been various high-quality experimental tests demonstrating its violation for separation of systems of the order of 10s to 1000s of metres. The collider context allows exploration of the question of whether the violation persists for femtometre length-scales. Focus experiments include top/anti-top pair decay and Higgs to WW or ZZ decays. There has been some controversy about what the significance of Bell inequality violation could be in collider experiments, however, for prima facie, these experiments do not allow what is a crucial part of a standard Bell test, namely, the possibility of freely varying the settings which determine what spin measurements are performed on each sub-system. It is important to distinguish a dual role for Bell inequality violation however: in addition to (under certain circumstances) indicating the presence of nonlocality, Bell inequality violation also serves as an entanglement witness: it demonstrates that one genuinely has an entangled quantum state. This role as an entanglement witness is of interest and does not require measurement-setting variation. Beyond this, however, I shall also explore the prospects for Bell-type tests in colliders which do genuinely amount to tests for nonlocality.