The helicity dependence of the single pion photoproduction on the proton ha
s been measured in the energy range from 200 to 450 MeV for the first time.
The experiment, performed at the Mainz microtron MAMI, used a 4 pi-detecto
r system, a circularly polarized, tagged photon beam, and a frozen-spin tar
get. The data obtained provide new information for multipole analyses of pi
on photoproduction and determine the main contributions to the Gerasimov-Dr
ell-Hearn sum rule and the forward spin polarizability gamma(0).