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The gluon distributions of the proton can be determined from the scaling violations of the proton structure function (see poster on quark structure of the proton). The scaling violations are predicted by perturbative QCD in form of the DGLAP evolution equations. The figure shows the gluon distribution for two values of the photon virtuality. |
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Complementary information on the gluon of the proton comes from direct measurements that involve the photon-gluon-fusion process. With this process the gluon distribution can be determined at relatively large values of the gluon momentum x measured relative to the proton momentum. Two different signatures were analysed: |
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The production of D* mesons, these are particles with a heavy charm quark, is almost exclusively caused by the photon-gluon-fusion process. The figure shows the gluon distribution of the proton that was determined with D* events of deep inelastic positron-proton scattering as well as with photoproduction processes. Within errors the measurements agree well with the indirect extraction of the gluon distribution using the scaling violations of the proton structure function. |
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In a second, direct extraction of the gluon distribution, 2-jet events from deep inelastic positron-proton scattering were analysed. Apart from the photon-gluon-fusion process, the QCD Compton process contributes to this 2-jet production. In the latter process the scattered quark from the proton radiates a gluon that causes the second jet. In order to determine the gluon distribution of the proton, the contribution of the QCD Compton events is subtracted from the jet production using a simultaneous fit with the proton structure function data. The figure shows the resulting gluon distribution. The jet measurement complements the gluon extraction from the proton structure function data alone and provides a measurement up to gluon momenta of x = 0.01. |