The electron beam at HERA is a copious source of (quasireal) photons which are radiated off. These photons have a high cross section to interact with the protons at high center of mass energies of about 200 GeV. A recent study thesis O. Kaufmann '98 has used hard scattering processes which lead to at least two hard jets in the final state to measure the parton content of the photon especially the gluons. The results are shown in the following figures:
The paper is now published: dijet production in gamma-p interactions and photon structure
effective parton distribution in the photon
effective parton distribution is shown as a function of the momentum fraction
of the photon x which is carried by the parton. At large x the distribution
is dominated by quarks and antiquarks whereas at low x gluons dominate.
The quark distribution shown as black curve is extracted from two photon
measuerements at LEP.
If this known fraction is subtracted then we can determine the gluon momentum disribution in the photon as shown below.
new measurement ( red points) shows that the gluon distribution is significant
a small x. The new measurement is in good agreement with less precise earlier
measurements. The black curves show different proposed parametrisations
of the gluon distribution based on QCD fits to e+e- data.