Performance of the Pair Spectrometer in Hall D at Jefferson Lab

Abstract: This article describes the performance of the pair spectrometer installed in experimental Hall D at Jefferson Lab and its operation in multiple experiments with the GlueX detector. The primary purpose of the pair spectrometer is the precise determination of the flux of beam photons incident on the GlueX target, a critical input for physics analyses such as absolute cross-section measurements. The photon energy spectrum is determined by reconstructing electron-positron pairs produced in a thin converter inserted into the photon beam. The spectrometer is integrated into the GlueX trigger system, enabling continuous real-time monitoring of the photon flux and the recording of $e<sup>+e<sup>-$ pair candidates for offline analysis. In addition, the pair spectrometer provides a versatile test facility for evaluating calorimeter prototypes using leptons with well-defined energies produced via electromagnetic pair production.