Tribute to Henry Primakoff: Chiral Perturbation Theory Tests via Primakoff Reactions

Abstract

Consider high energy (GeV) beam particles scattering from the Coulomb field of a target nucleus (Z, A). The Coulomb field acts as a target of $γ^*$ virtual photons, with the target density proportional to Z$^2$. Henry Primakoff was the first to propose determining the lifetime of the $π^0$ meson by measuring the production cross section for the reaction $γγ^* \rightarrow π^0$. This process occurs when a high-energy gamma-ray interacts with the Coulomb field. Quasi real exchanged photons ($γ^*$) are identified by isolating the sharp Coulomb peak at very low values of the squared four momentum transfer t to the target nucleus. The scattering cross section via one-photon exchange is proportional to the fine-structure constant $α$ and inversely proportional to t$^2$. Since t is inversely related to the squared center-of-mass energy (s), it decreases rapidly as s increases. Consequently, despite the weakness of the electromagnetic interaction, the interaction amplitude can still be significant. We will first discuss Primakoff's scientific career and personal life. Next, we will review the Primakoff scattering experiments that measured the pion polarizability and the $γ\rightarrowπππ$ chiral anomaly amplitude at CERN COMPASS and the $π^0$ lifetime at Jefferson Laboratory (JLab). The data from these experiments are in good agreement with two-flavor (u, d) Chiral Perturbation Theory (ChPT) predictions. We explain that additional Primakoff measurements with kaons and $η$ mesons are needed to test how well three flavor (u, d, s) ChPT captures strange quark effects.