Measurement of Δ σ γN ( k ) and the High Energy Contribution to the Gerasimov - Drell - Hearn Sum Rule

We propose to measure the spin-dependent total cross section for circularly polarized photons absorbed on longitudinally polarized protons and neutrons in the photon energy range 2.5 < k < 5.5 GeV. This will be the first measurement of the difference between leftand right-handed polarized photoproduction above the resonance region. These data are needed to learn about the high energy convergence of the fundamental Gerasimov-Drell-Hearn sum rule for the proton and neutron. Substantial contributions are expected in this energy range, especially for the isovector combination. Measurements of the magnitude and energy-dependence of polarized photoabsorption at high energy will provide the baseline for understanding of soft Regge physics, essential to the interpretation of data taken with virtual photons. The results can be compared with the variety of models based on Reggeon and Pomeron exchange. We propose to use the frozen spin target, circularly polarized tagged photon beam and CLAS detector in Hall B. The proposed experiment will be an extension of an already approved experiment [35] at beam energies up to 2.3 GeV. We request 15 days of running time, with 5 days of data taking with the polarized proton target and 10 days of data taking with the polarized deuteron target.