Abstract
We present a measurement of the longitudinal spin asymmetry in photoproduction of pairs of hadrons with high transverse momentum . 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 pairs with and , the measured asymmetry is . 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.
- Received 15 July 1999
DOI:https://doi.org/10.1103/PhysRevLett.84.2584
©2000 American Physical Society