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 debate and development. With the 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.20 MoreReceived 2 March 2008DOI:https://doi.org/10.1103/PhysRevSTAB.11.041001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Highlights
The demand for high-energy circularly polarized photon beams has increased with the need to study gluon related features of the nucleon
The lower plot represents the increase in the asymmetry due to quasisymmetrical pairs together with the statistical error associated with this increase
It confirms the nonstatistical source of the asymmetry increase in the 70 –110 GeV range
Summary
The demand for high-energy circularly polarized photon beams has increased with the need to study gluon related features of the nucleon. The so-called ‘‘spin crisis of the † Deceased. X. at Northwestern University, Evanston, IL, USA. Nucleon’’ and its connection to the gluon polarization has attracted much attention [1]; for example, experiments to determine the gluon spin density of the nucleon [1,2,3] from polarized virtual photon-gluon fusion, and polarized virtual photoproduction of high transverse momentum mesons [4]. Future experiments will require intense highenergy photon beams with a high degree of circular polark. At University of Cape Town, Cape Town, South Africa. At University of the Witwatersrand, Johannesburg, South
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