The Breit-Wheeler process is the simplest process for creating matter and antimatter from the collision of two photons. This conversion of light into matter is a direct corollary of Einstein’s mass-energy equivalence, E = mc^2. The process was theorized in 1934, but went nearly a century without experimental confirmation until it was discovered by the STAR Collaboration at RHIC in 2021. This discovery was made at top RHIC energy in gold-gold collisions. While the process is now well established at high energies, there is a peculiar feature predicted by Quantum Electrodynamics (QED) that the interaction cross section will diverge (infrared divergence) at low photon energy. The flexibility of RHIC, allowing heavy-ion collisions at lower energies, allows this prediction to be tested and other characteristics of the newfound process to be uncovered.
In this presentation, we will present measurements of the energy dependence of Breit-Wheeler process in gold-gold collisions at STAR. The corresponding results computed by QED are consistent with STAR measurements and found to be sensitive to the nuclear charge distribution. Following this approach we demonstrate that the experimental measurements of the Breit-Wheeler process in heavy-ion collisions can be used to quantitatively constrain the nuclear charge radius of high energy atomic nuclei.
