Measuring the parton content of the photon at the DESYepcollider HERA

We discuss the possibility to measure the parton, and especially gluon, content of the photon at the forthcoming DESY ep collider HERA. We first reemphasize that the production of two jets with high transverse momentum $p_T$ is dominated by events where the photon is resolved into quarks and gluons for $p_T$≲40 GeV. We then show that these ‘‘resolved’’ contributions, and in particular those that are initiated by a gluon from the photon, are especially important at large jet rapidities. Rapidity distributions also allow us to isolate the direct photon-gluon fusion contributions to bb¯ production. This makes it possible to use this process to determine the gluon content of the proton. However, a direct measurement of the gluon content of the photon will only be possible if the spectator jet from the photon can be detected. We find that this jet typically has an energy of 10–12 GeV and estimate its opening angle to be around 15°, which should allow for its detection.