Gluon-spin effects in hadronic jet production.

After a brief overview on the baffling status of the proton spin content, we study the gluon-spin effects on the single-jet spin asymmetry ${\mathit{A}}_{\mathit{L}\mathit{L}}^{\mathrm{jet}}$ and on the polarized jet cross section \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}^{\mathrm{jet}}$ in proton-proton collisions. A measurement of ${\mathit{A}}_{\mathit{L}\mathit{L}}^{\mathrm{jet}}$ in the jet momentum region where the helicity asymmetry is dominated by gluon-quark hard scattering will furnish important information on the magnitude and even the sign of the gluon-spin component in a polarized proton. We find that \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}^{\mathrm{jet}}$ can be as large as \ensuremath{\sim}25 \ensuremath{\mu}b at \ensuremath{\surd}s =100 GeV, for example, if the European Muon Collaboration spin effect is explained in terms of large gluon polarization, while it is less than 1 \ensuremath{\mu}b if gluons are unpolarized.