Measurement of the spin asymmetry in the photoproduction of pairs of high-pT hadrons at HERMES

We present a measurement of the longitudinal spin asymmetry Ali in photoproduction of pairs of hadrons with high transverse momentum pi. Data were accumulated by the HERMES experiment using a 27.5 GeV polarized positron beam and a polarized hydrogen target internal to the HERA storage ring. For h(+)h(-) pairs with p(T)(h1) > 1.5 GeV/c and p(T)(h2) > 1.0 GeV/c, the measured asymmetry is A(parallel to) = -0.28 +/- 0.12(stat) +/- 0.02(syst). This negative value is in contrast to the positive asymmetries typically measured in deep inelastic scattering: from protons, and is interpreted to arise from a positive gluon polarization.