Quantum entanglement and Bell inequality violation at colliders

The study of entanglement in particle physics has been gathering pace in the past few years. It is a new field that is providing important results about the possibility of detecting entanglement and testing Bell inequality at colliders for final states as diverse as top-quark or τ-lepton pairs, massive gauge bosons and vector mesons. In this review, after presenting definitions, tools and basic results that are necessary for understanding these developments, we summarize the main findings – as published up to the end of year 2023. These investigations have been mostly theoretical since the experiments are only now catching up, with the notable exception of the observation of entanglement in top-quark pair production at the Large Hadron Collider. We include a detailed discussion of the results for both qubit and qutrits systems, that is, final states containing spin one-half and spin one particles. Entanglement has also been proposed as a new tool to constrain new particles and fields beyond the Standard Model and we introduce the reader to this promising feature as well.