  Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)sigma0
reactions are measured for the first time for Egamma=1.5-2.4 GeV and
0.6<cos(theta_cm(K+))<1.0 by using linearly polarized photons at the
Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are
positive and gradually increase with rising photon energy. The data are not
consistent with theoretical predictions based on tree-level effective
Lagrangian approaches. Including the new results in the development of the
models is, therefore, crucial for understanding the reaction mechanism and to
test the presence of baryon resonances which are predicted in quark models but
are sofar undiscovered.

Nuclear Experiment