Abstract
Experimental results from HERMES are presented on the longitudinal spin asymmetry in photoproduction of pairs of oppositely charged hadrons with high momentum transfer p T . For h + h − pairs with p T h 1 > 1.5 GeV/c and p T h 2 > 1.0 GeV/c, a negative spin asymmetry A ∥ = −0.28 ± 0.12 (stat.) ± 0.02 (syst.) is observed. Monte Carlo studies with the PYTHIA [1] event generator suggest that in the kinematical domain of the measurement, photon-gluon fusion (PGF) dominates over other underlying background subprocesses. Using this model, the measured asymmetry may be interpreted in terms of a positive gluon polarization in the nucleon.
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