ar X iv : h ep - p h / 99 05 54 6 v 1 3 0 M ay 1 99

The production of photons in hadronic collisions is quite interesting under two different aspects. Since the photon couples directly only to quarks, photon signals can be used to study the hard dynamics in a cleaner way with respect to processes where only hadrons are involved; the obvious drawback is that promptphoton cross sections are much smaller than say jet cross sections. Secondly, photon data constitute an unique tool for pinning down the gluon density at intermediate and large x, since the number of partonic processes involved is smaller with respect to other processes which are equally or more sensitive to the gluon density. Unfortunately, the real situation is much worse than that described above. The main reason is that in a complicated environment, like that arising in high-energy hadronic collisions, there are lots of photons around, mainly coming from the decay π → γγ. This is a huge background, since the two photons are often detected as a single one. However, the signature of photons coming from hadron decays is rather different from that of prompt photons, the mother hadron being usually surrounded by other hadrons. Therefore, photons originating from decays have the same direction of a relatively large number of hadrons. It follows that an efficient way of selecting prompt-photon events is that of requiring the photon to be isolated from hadron tracks in the detector. There is also a further complication: photons can originate from fluctuations of hadrons with the same quantum numbers. Perturbative QCD is not able