Report on Progress in Physics: Observation of the Breit-Wheeler Process and Vacuum Birefringence in Heavy-Ion Collisions

This Report reviews the effort over several decades to observe the linear Breit-Wheeler process ($\gamma \gamma \to e^{+}{e}^{-}$) and vacuum birefringence in high-energy particle and heavy-ion collider experiment. This Report, motivated by STAR’s recent observations, attempts to summarize the key issues related to the interpretation of polarized $\gamma \gamma \to l^{+}{l}^{-}$ measurements in high-energy experiments. To that end, we start by reviewing the historical context and essential theoretical developments, before focusing on the decades of progress made in high-energy collider experiments. Special attention is given to the evolution in experimental approaches in response to various challenges and the demanding detector capabilities required to unambiguously identify the linear Breit-Wheeler process and to detect the signatures of vacuum birefringence. We close the report with a discussion, followed by a look at near-future opportunities for utilizing these discoveries and for testing QED in previously unexplored regimes.