Workshop on the Physics of Excited Nucleon – NSTAR2009, Beijing, April 19 – 22, 2009

Invited plenary talk

Invited parallel talk

Oral contribution

Authors: Daria Sokhan, Daniel Watts, on behalf of CLAS Collaboration.

Keywords: Pion, Photoproduction, Beam Asymmetry, CLAS

Abstract:

The new generation of pion photoproduction measurements from the nucleon is expected to give completely new, precise information on the poorly established nucleon excitation spectrum. Measuring a total of eight single- and double- polarisation observables from experiments employing polarised photon beam and target and recoil baryon polarimetry has the potential to give the first model independent extraction of the production amplitudes for single pseudoscalar meson production. This will greatly enhance our capabilities to determine the nucleon excitation spectrum in a nearly model-independent way [1].

Measurements of pion photoproduction on the neutron are particularly sparse despite such data being crucial to achieving a determination of the resonance isospin and the reliable extraction of the electromagnetic couplings of the excited states.

I present a preliminary analysis of the photon beam asymmetry observable (Sigma) from the n(gamma,pi-)p channel in the invariant mass range 1.6 - 2.3 GeV. The measurement was obtained using the near-4pi CLAS detector at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1 - 2.3 GeV, incident on a liquid deuterium target. The measurement will greatly expand the world data-set both in energy and angle, which currently consists of only three experiments limited to the 1.70 - 2.05 GeV range in energy and 35deg - 90deg in polar angle [2][3][4].

[1] I. Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975)

[2] J.Alspector et al., Phys. Rev. Lett. 28, 1403 (1972)

[3] L. Abrahamian et al., Sov. J. Nucl. Phys. 32, 69 (1980)

[4] F. Adamyan et al., J. Phys. G 15, 1797 (1989)