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

Maxime Defurne, Arthur Muhulet

TL;DR

This work introduces a MAPS-based pair polarimeter for multi-GeV photons that combines monolithic active pixel sensors with an extended low-density converter to enhance conversion efficiency while controlling multiple scattering. A full reconstruction pipeline based on Kalman filtering, cluster-shape modeling, and depth-aware azimuthal-angle extraction is developed to recover the conversion geometry and polarization with a high intrinsic analyzing power. Systematic optimization across cell length, MAPS spacing, and converter density demonstrates that a compact 1 m device can achieve a Figure-of-Merit substantially above prior designs, enabling polarization measurements at energies relevant to deeply virtual Compton scattering and potentially enabling tests of fundamental physics. The results point to practical DVCS polarization studies and broaden the impact of gamma-polarimetry in astrophysics and fundamental interactions.

Abstract

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.

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

TL;DR

This work introduces a MAPS-based pair polarimeter for multi-GeV photons that combines monolithic active pixel sensors with an extended low-density converter to enhance conversion efficiency while controlling multiple scattering. A full reconstruction pipeline based on Kalman filtering, cluster-shape modeling, and depth-aware azimuthal-angle extraction is developed to recover the conversion geometry and polarization with a high intrinsic analyzing power. Systematic optimization across cell length, MAPS spacing, and converter density demonstrates that a compact 1 m device can achieve a Figure-of-Merit substantially above prior designs, enabling polarization measurements at energies relevant to deeply virtual Compton scattering and potentially enabling tests of fundamental physics. The results point to practical DVCS polarization studies and broaden the impact of gamma-polarimetry in astrophysics and fundamental interactions.

Abstract

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 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 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.

Paper Structure

This paper contains 20 sections, 11 equations, 14 figures.

Table of Contents

  1. Introduction
  2. Principle of multi-GeV $\gamma$-polarimetry
  3. State-of-the-art in $\gamma$-polarimetry via pair production
  4. A revisited concept of pair polarimeter for DVCS
  5. A realistic MAPS description
  6. MAPS response to single particle
  7. MAPS response to two close particles
  8. Cluster shape analysis and modeling
  9. Shape analysis
  10. Modeling the position distributions
  11. Reconstruction algorithm for a multi-GeV photon polarimeter
  12. Reconstructing the two tracks and the azimuthal angle of the conversion
  13. Extracting the effective analyzing power
  14. Optimization of the polarimeter design
  15. Cell length
  16. ...and 5 more sections

Figures (14)

  • Figure 1: Left: Schematic of the pair polarimeter. Right: Optimization of the Figure-of-merit (black line) with the converter thickness. The red line is the conversion rate, the purple line the analyzing power of the polarimeter $A_{_{I}}$ and the blue line the resolution on the azimuthal angle. Figures are from bogdan.
  • Figure 2: Left: Schematics of the cell proposed for GluToN$\gamma$. Right: ALPIDE MAPS detectors used by ALICE muon forward tracker with a 5$\mu m$-resolution.
  • Figure 3: Top: Efficiency as function of the threshold for a true MAGER2016434 (top) and a toy-MC (bottom) ALPIDE sensor. Bottom: Resolutions and average cluster size as function of the threshold for a true MAGER2016434 (top) and a toy-MC (bottom) ALPIDE sensor.
  • Figure 4: Average cluster size as function of the particle position within a pixel for a true MAGER2016434 (left) and a toy-MC (right) ALPIDE sensor.
  • Figure 5: 1-cluster/2-cluster event configuration as function of the distance between the two particles in the simulation.
  • ...and 9 more figures