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HK: Fachverband Hadronen und Kerne

HK 46: Hadron Structure and Spectroscopy I

HK 46.7: Talk

Wednesday, March 18, 2009, 15:45–16:00, H-ZO 20

Use of the coherent ¹²C(γ,π⁰) reaction to measure photon polarisation — •Ian James Douglas MacGregor, Jamie Robinson, and Ken Livingston — University of Glasgow, Glasgow, UK

Photonuclear cross sections depend on numerous factors, some of which are masked in the unpolarized case, where sums and/or averages are performed over spin states. The use of linearly polarized photons allows investigation into the difference between parallel and perpendicular responses through photon asymmetry Σ measurements. Σ is particularly sensitive to angular momentum contributions to nuclear currents and to interference between contributions thus providing a more sensitive observable to test theories of nuclear interactions.

Accurate Σ measurements rely on an accurate determination of the degree of photon linear polarization. We present measurements of photon polarization performed using the highly segmented  4π Crystal Ball detector at Mainz. Coherent photoproduction of π⁰ mesons from carbon is used as a polarimeter reaction while investigating the physics of two-nucleon emission in the ¹²C(γ,pp) reaction. Since both the π⁰ and the nucleus are spin zero, all information regarding the linear polarization of the photon is passed to the π⁰ azimuthal distribution. The photon asymmetry Σ(γ,π⁰) equals 1 over the energies and angles of the experiment and thus photon polarization can be extracted directly from the azimuthal distribution of the emitted pion. This technique is used to determine photon polarization as a function of photon energy on a run by run basis.