A novel approach to determine photon polarization at collider experiments

TL;DR

This paper tackles measuring final-state photon polarization at collider experiments by embedding a photon polarimeter into a general-purpose spectrometer. The approach relies on exploiting $\gamma$-conversion to extract the azimuthal polarization asymmetry via a calibration table that maps observed $\mathcal{A}P$ to the true value, accounting for photon energy and detector angular resolution. Realistic Geant4 simulations with an H-NS-like layout demonstrate that $0.5$ mm carbon foil conversion and angular resolution of about $2$–$3$ mrad enable polarization extraction, and that optimizing converter thickness, foil location, and vertex resolution improves precision. The framework provides a practical benchmark for existing and future experiments and could empower concurrent measurements of polarization and four-momentum in a single instrument.

Abstract

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.

Figures (8)

• Figure 1: Illustration of gamma conversion to an $e^+e^-$ pair. The directions of positron (electron) momentum are defined by the angles $\theta_{+(-)}$ and $\phi_{+(-)}$. $\phi_0$ is the polarization angle of the incident photon. $\phi$ is the azimuthal angle of the event.
• Figure 2: The impact of detector angular resolution. The black line shows the original asymmetry of 0.1 at an opening angle of 1 mrad. The red dashed line indicates the smeared asymmetry with detector angular resolution of 1 mrad, and the blue dot-dashed line is with resolution of 5 mrad.
• Figure 3: Concept of H-NS detector(Left) and illustration of $\gamma$-conversion to $e^+e^-$ pair on carbon layer in the H-NS (Right). This study focuses on the photons from the primary vertex..
• Figure 4: Angular momentum resolution $\Delta\theta$ (Left) and $\Delta\phi$ (Right) for positron from photon energy of 1 GeV. The dashed blue line represents the first fit using three silicon layers after the carbon foil. The solid red line shows the second iteration with a common vertex with electron track applied.
• Figure 5: The smear effect on azimuthal distributions under different detector configurations, including an ideal detector (black lines), H-NS (red lines) and another detector configuration (DetX). For clarity in comparison, the H-NS and DetX distributions have been scaled.