Quantum electrodynamics (QED), the relativistic quantum field theory of electromagnetism, is the best tested theory in physics. Only a handful of fundamental interactions between light and matter are allowed by QED, almost all of which have been observed in the 80 or so years since their prediction. Only the Breit-Wheeler process, the simplest mechanism for converting 'light quanta' into matter and antimatter, has eluded observation for decades.
In this talk I will present STAR measurements of e+e- pair production in ultra-peripheral and peripheral Au+Au collisions at sqrt(sNN) = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the gamma gamma to e+ e- process from other possible production mechanisms and to definitively identify the Breit-Wheeler process for the first time. Furthermore, the measured distribution of electron-positron pairs reveals a striking fourth-order angular modulation which demonstrates the linear polarization of the colliding photons and provides the first laboratory evidence for vacuum birefringence.
I'll briefly discuss applications of these discoveries demonstrating that linearly polarized photons, useful for testing the limits of QED, can also be a powerful tool for studying nuclear matter in novel ways and for addressing timely issues in the field. Finally, I'll end the talk by discussing the application of these discoveries to the future decades of physics to come at the LHC and how they allow new avenues for investigating some of the “most intellectually pressing questions" [1] that the future electron-ion collider will address.
[1] EIC White Paper https://www.bnl.gov/npp/docs/EIC_White_Paper_Final.pdf
