A novel approach to determine photon polarization at collider experiments

The polarization of final-state photons is a critical observable for probing the fundamental mechanisms of particle and nuclear interactions, providing insights into spin and parity structure that are inaccessible through cross-section measurements alone. However, this observable remains largely unexplored in collider experiments, as general-purpose spectrometers traditionally lack the capability to measure it. This paper proposes a novel technique to integrate photon polarimeter function into such a spectrometer without compromising the spectrometer’s conventional performance. Key factors to enhance the polarimeter capability are investigated. This successful integration represents the first implementation of a photon polarimeter within a general-purpose spectrometer, establishing a valuable benchmark for the existing and future experiments. The ability to concurrently measure spin polarization and four-momentum data opens a new dimension for analysis, promising a more profound understanding of the underlying physics.

💡 Research Summary

The paper presents a novel method for measuring the linear polarization of final‑state photons within general‑purpose collider spectrometers, without the need for dedicated polarimeter hardware that would otherwise compromise the detector’s primary performance. The authors identify that photon polarization carries unique information on spin and parity dynamics, yet it has been largely inaccessible in high‑energy experiments because conventional spectrometers lack the capability to resolve the tiny azimuthal asymmetries generated in photon‑pair conversion.

The core idea is to exploit the natural γ‑conversion of photons in a thin carbon foil placed inside the tracking volume. This foil, already present in many detectors as a target for nucleon polarimetry, serves simultaneously as the conversion medium, avoiding any additional material budget. When a photon converts to an e⁺e⁻ pair, the pair’s azimuthal angle ϕ relative to the photon’s polarization plane follows the QED‑derived distribution dσ/dϕ ∝ 1 + A P cos