Linearly polarised photon beams at ELSA and measurement of the beam asymmetry in pi^0-photoproduction off the proton

TL;DR

This paper describes the production of linearly polarised photon beams at ELSA using coherent bremsstrahlung off a diamond crystal, and measures the beam asymmetry in pi^0 photoproduction to validate the polarisation degree.

Contribution

It introduces a method for producing and precisely measuring linearly polarised photon beams at ELSA and demonstrates its application in measuring beam asymmetry in pi^0 photoproduction.

Findings

Achieved polarisation degrees of about 40%.

Good agreement between measured and expected polarisation.

Validated polarisation measurement through beam asymmetry in pi^0 photoproduction.

Abstract

At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of P = 40% are typically achieved at half of the primary electron energy. The determination of P is confirmed by measuring the beam asymmetry, \Sigma, in pi^0 photoproduction and a comparison of the results to independent measurements using laser backscattering.