Measurement of the Spin Asymmetry in the Photoproduction of
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High-{\bf $p_T$} Hadron Pairs at HERMES
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Abstract:
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A measurement of the longitudinel spin asymmetry $A_{||}$ in the
quasi-real photoproduction of hadron pairs with high transverse
momentum $p_T$ is presented.
Data were accumulated by the HERMES experiment using
a polarized atomic hydrogen target internal to the 27.6 GeV polarized
HERA positron beam.
For $h^+h^-$ pairs with $p_T^{h_1}>1.5$ GeV/c and $p_T^{h_2}>1.0$
GeV/c, the measured asymmetry is $A_{||}=-0.28\pm 0.12~{\rm (stat.)}\pm
0.02~{\rm (syst.)}$.  This negative value is in contrast to the positive
asymmetries typically measured in deep inelastic scattering from
protons and can be interpreted to arise from a positive gluon
polarization.