A pair polarimeter for multi-GeV $γ$-rays

Accessing the polarization of photon allows to understand the mechanisms behind its emission or scattering, revealing much about a peculiar environment or a probed object. For energy above $\sim$10~MeV, the pair production dominates the photon-matter interaction and the photon polarization is accessible via the azimuthal angle of the conversion. Unfortunately pair polarimeters have a low figure-of-merit for multi-GeV photons and are mostly used for beam characterization. In this paper, we report a new concept of a compact pair polarimeter associating monolithic active pixel sensors to low-density extended solid converters to reach simultaneously a high efficiency of $\sim$7% and intrinsic analyzing power ranging from 0.2 to 0.5. This new concept will add a new obersvable to the multi-messenger physics, isolate the intrinsic strong force in nucleons and possibly reveal violations of Lorentz invariance.

💡 Research Summary

The study presents a groundbreaking approach to measuring the polarization of multi-GeV gamma rays, a task that has long been limited by the low figure-of-merit of existing pair polarimeters. Photon polarization is a vital observable for deciphering the physical mechanisms of emission and scattering in extreme astrophysical environments. At energies exceeding approximately 10 MeV, the interaction between photons and matter is dominated by pair production, where the polarization information is encoded in the azimuthal angle of the resulting electron-positron pair. Historically, pair polarimeters have struggled with efficiency at higher energies, restricting their utility primarily to beam characterization in laboratory settings rather than wide-field astrophysical observations.

To overcome these limitations, the authors propose a novel concept for a compact pair polarimeter. The design integrates Monolithic Active Pixel Sensors (MAPS) with low-density extended solid converters. The MAPS technology provides the high spatial resolution necessary to precisely reconstruct the trajectories and azimuthal angles of the produced pairs, while the use of low-density converters minimizes the detrimental effects of multiple scattering, which typically degrades polarization information. This innovative architecture achieves a simultaneous high efficiency of approximately 7% and an intrinsic analyzing power ranging from 0.2 to 0.5.

The implications of this development are profound across multiple domains of physics. In the realm of multi-messenger astronomy, this polarimeter introduces a new observable, allowing scientists to complement data from gravitational waves and neutrinos with polarization-sensitive gamma-ray observations. Furthermore, the device offers the precision required to isolate the intrinsic strong force within nucleons, providing deeper insights into nuclear physics. Finally, the ability to measure high-energy polarization opens a new window for testing the fundamental principles of relativity, specifically searching for potential violations of Lorentz invariance. This advancement represents a significant leap forward in our ability to probe the most energetic and mysterious phenomena in the universe.