GDH Integral on the Proton from Cross Section Differences

We studied the inclusive reaction {rvec e} {rvec p} {yields} e{prime}X with a 2.5 GeV and a 4.2 GeV incident beam of polarized electrons. The virtual photon four momentumsquared, Q{sup 2}, varied from 0.2 GeV{sup 2}/c{sup 2} to 1 GeV{sup 2}/c{sup 2} 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 NH{sub 3} target was used as a source of protons polarized in the direction of the incident electron beam. For the first time, proton spin structurefunctions were studied with precision in the resonance region. The data indicate that a dramatic change in the spin structureoccurs between the deep inelastic high Q{sup 2} region and the region of low Q{sup 2} where photons become nearly real.