Precise measurement of Sigma beam asymmetry for positive pion photoproduction on the proton from 550 to 1100 MeV

Sigma beam asymmetries for pi(+) photoproduction on the proton have been me asured in the energy range 550-1100 MeV over a wide angular range. The stat istical and systematic errors of the data are much lower than in previous m easurements, due to the high polarization of the photon beam and the cylind rical geometry of the 4 pi detector. Our results confirm the predictions W1 98 and SM95 of the VPI partial-wave analysis at the energies below 850 MeV and at 1050 MeV while, between these two regions, they differ at backward a ngles. It may indicate the smaller contribution of S-11,,(1650) resonance, than it was quoted previously by the VPI and PDG groups. A newly developed unitary isobar model reproduces our data up to 950 MeV. (C) 2000 Elsevier S cience B.V. All rights reserved.