Berlin 2005 – scientific programme

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HK: Physik der Hadronen und Kerne

HK 37: Elektromagnetische und Hadronische Proben

HK 37.4: Talk

Wednesday, March 9, 2005, 14:45–15:00, TU MA001

Linearly polarised photons at ELSA and Σ beam asymmetry for the η photoproduction off the proton* — •Daniel Elsner — Physikalisches Institut, Universität Bonn

Using coherent bremsstrahlung on a thin diamond crystal from the electron beam of the ELSA accelerator a tagged photon beam with a high degree of linear polarisation is produced. The required precise alignment of the crystal axis with respect to the beam direction is realised by a commercial goniometer. The coherent bremsstrahlung itself can be used for the calibration of the crystal orientation based upon the Stonehenge technique. From comparison of measured photon spectra with a calculation of bremsstrahlung production including collimation and other experimental constraints, we gain the degree of linear polarisation within a relativ uncertainty of 5%. During the CBELSA/TAPS beamtimes different data sets with maximum polarisation at photon energies between 1.2 and 2.0 GeV were measured. The reaction γ^→ + p → η + p was analysed in the two dominant neutral decay modes of the η. The status of the analysis of the photon beam asymmetry, Σ, and the unpolarized cross section will be reported. The comparison to existing data of Σ at photon energies around 1 GeV serves as an independent check of the polarisation determination.

* supported by DFG (SFB/TR 16).