GDH Integral on the Proton from Cross Section Differences

We studied the inclusive reaction (rvec e) (rvec p) → e(prime)X with a 2.5 GeV and a 4.2 GeV incident beam of polarized electrons. The virtual photon four momentum squared, Q2, varied from 0.2 GeV2/c2 to 1 GeV2/c2 and the center of mass total energy W ranged up to 2 GeV. The measurement was performed at the CEBAF Large Acceptance Spectrometer (CLAS) of the Thomas Jefferson National Accelerator Facility during the Fall of 1998. A cryogenic NH3 target was used as a source of protons polarized in the direction of the incident electron beam. For the first time, proton spin structure functions were studied with precision in the resonance region. The data indicate that a dramatic change in the spin structure occurs between the deep inelastic high Q2 region and the region of low Q2 where photons become nearly real