Spin structure of the proton and the Q² evolution of the Gerasimov-Drell-Hearn integral

Abstract

We studied the inclusive reaction e[left arrow above] p[left arrow above] [left arrow] e'X with a 2.5GeV and a 4.2GeV incident beam of polarized electrons. The virtual photon four momentum squared, Q2, varied from 0.2GeV2/c2 to 1GeV2/c2 and the center of mass total energy W ranged up to 2GeV. 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.