Measurement of the Proton Spin Structure Function g1p with a Pure Hydrogen target.

Abstract A measurement of the proton spin structure function g 1 p ( x , Q 2 ) in deep-inelastic scattering is presented. The data were taken with the 27.6 GeV longitudinally polarised positron beam at HERA incident on a longitudinally polarised pure hydrogen gas target internal to the storage ring. The kinematic range is 0.021 x 2 Q 2 2 . The integral ∫ 0.021 0.85 g 1 p (x) dx evaluated at Q 0 2 of 2.5 GeV 2 is 0.122±0.003(stat.)±0.010(syst.).