A measurement of the beam asymmetry parameter Σ for neutral pion photoproduction in the energy range 1.2–2.8 GeV

An experiment has been completed at the Daresbury synchrotron to measure the asymmetry in the photoproduction cross section of neutral pions on hydrogen, for photons polarised normal to and in the production plane. The source of polarised photons was coherent bremsstrahlung of electrons traversing the lattice structure of diamond and the polarisation P of the γ beam was calculated from the measured intensity of the coherent spike. The asymmetry parameter Σ, defined as Σ = (σ⊥ ? σ|)/(σ⊥ + σ|) where σ⊥(σ|) are the cross sections for photons polarised perpendicular (parallel) to the production plane, has been measured over a range of photon energies from 1.2 to 2.8 GeV and over a range of ?t (the square of the four-momentum transfer) from 0.13 (GeV/c)² to 1.4 (GeV/c)². A marked energy variation in the value of Σ is found over the energy region 1.6–1.8 GeV.