Eur. Phys. J. A (2007) 33: 147-155
https://doi.org/10.1140/epja/i2007-10447-9

Regular Article - Experimental Physics

Measurement of the beam asymmetry in η photoproduction off the proton

¹ Physikalisches Institut der Universität Bonn, Germany
² Helmholtz-Institut für Strahlen- und Kernphysik der Universität Bonn, Germany
³ Petersburg Nuclear Physics Institute, Gatchina, Russia
⁴ Physikalisches Institut, Universität Erlangen, Germany
⁵ KVI, University of Groningen, The Netherlands
⁶ Department of Physics, Florida State University, Tallahassee, FL, USA
⁷ Physikalisches Institut, Ruhr-Universität Bochum, Germany
⁸ Physikalisches Institut, Universität Basel, Switzerland
⁹ II. Physikalisches Institut, Universität Giessen, Germany

^* e-mail: schmieden@physik.uni-bonn.de

Received: 13 February 2007
Accepted: 20 July 2007
Published online: 22 August 2007

Abstract

The beam asymmetry, Σ , was measured at ELSA in the reaction p → ηp using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range E _γ = 800 to 1400MeV with the maximum polarisation of P _γ = 49 % obtained at 1305MeV. Both dominant decay modes of the η into two photons and 3π⁰ were used to extract the beam asymmetry from the azimuthal modulation of the cross-section. The measurements cover the angular range Θ_cm ≃ 50 -150 degrees. Large asymmetries up to 80% are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.