Spin-Zero Polarimetry for Linearly Polarised Photons at MAMI

Abstract

Background: The tagged Coherent Bremsstrahlung technique is used to provide beams of linearly polarised photons at several of the world’s leading photonuclear facilities, including MAMI in Mainz, ELSA at Bonn, and GlueX at Jefferson Laboratory. The degree of linear polarisation in real photon experiments has direct impact on measurements of polarisation observables. Measurement of the linear polarisation is often the biggest source of systematic error in these experiments. Continuous, in-beam measurement will also allow the beam position to be adjusted to optimise the polarisation. Purpose: To determine whether it is possible to make an improved measurement of the degree of linear polarisation using a polarimeter based on a 12C target. Methods: Coherent π0 meson production from a spin-zero nucleus has a photon asymmetry of 1 and hence provides a direct method of measuring the beam polarisation. Results: Current methods of measurement are being employed as a comparison. Simulated statistics of π0 mesons produced off the polarimeter suggest the measurement is viable. Early results from production data support the viability of live, event-by-event measurements of the polarisation using this method. Conclusions: A spin-zero nucleus such as carbon will be developed as a polarimeter for measuring the degree of linear polarisation in hadron physics experiments with tagged photons of up to 1557 MeV.