Coherent bremsstrahlung, coherent pair production, birefringence and polarimetry in the 20-170 GeV energy range using aligned crystals

TL;DR

This study investigates coherent bremsstrahlung and pair production in aligned crystals at 20-170 GeV, demonstrating new polarimetry techniques and confirming theoretical predictions of these processes' properties and polarization dependences.

Contribution

It provides experimental validation of theoretical models for CB and CPP at high energies and introduces advanced polarimetry methods using birefringence in aligned crystals.

Findings

Theoretical predictions match measured cross sections and polarization observables.

Aligned crystals enhance hard photon production in SOS orientation.

Low polarization levels observed for high-energy SOS photons.

Abstract

The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20-170 GeV. The experimental arrangement allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active theoretical debate and development. With the theoretical approach used in this paper both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high energy SOS photons.