First measurement of the helicity-dependent $\mth{\arrowvec{\gamma} \arrowvec{p} \to p \eta}$ differential cross-section

The helicity dependence of the $\arrowvec{\gamma} \arrowvec{p} \to p \eta$ reaction has been measured for the first time at a center-of-mass angle $\theta_\eta^* =70^\circ$ in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4$\pi$-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis