In this résumé we wish to provide an overview of the discussions of the photoproduction working group at the workshop. The first point we wish to make about the five articles which follow and summarize the discussions of the group is that they should be read in conjunction with this note and with the accounts of the three keynote talks, by Maxfield, Kramer and Sjöstrand, which also appear in these proceedings. These three talks, on `Photoproduction events' (Sjöstrand), `Photoproduction of jets' (Kramer) and `Experimental highlights' (Maxfield) provided the initial focus for the discussions of the group. The written versions serve the same purpose for the summaries and also incorporate, where appropriate, some of the results of our discussions. They also include some results from further work which arose out of these discussions. Much of our time was devoted to discussions of the parton distribution functions (PDFs) of the photon, both real and virtual, and how they can be obtained from HERA data. This had many ramifications. The report by Sjöstrand, Storrow and Vogt reviews the current state of knowledge of the photon PDFs and discusses the prospects of improving this knowledge by experiments at HERA. One conclusion that emerges is that a comparison of HERA jet data with NLO QCD calculations could provide tests of the PDFs of the real photon competitive with structure function measurements at LEP2, certainly in the large x region. Examples of the first signs of this are given in Kramer's article. Another conclusion is that the forward electron tagging capabilities at the HERA experiments provide a unique opportunity to study the structure of virtual photons. There have been some theoretical efforts here, which are discussed, but until now, no experimental information. The data that will be available from HERA (some were presented at the workshop) will be of much higher statistical accuracy than LEP2 data from double tag experiments on the virtual photon structure function. Also, as regards the gluon content of the photon, either real or virtual, structure function measurements provide essentially no information, whereas jet production does. One theoretical problem that arises in fixing the photon PDFs is the lack of a momentum sum rule comparable to the hadronic case, where it is invaluable in constraining the gluon density. There is a recent proposal by Frankfurt and Gurvich that there is a momentum sum rule, albeit of a different type, and that, even more interestingly, existing photon PDFs do not satisfy it. We had discussions on this, summarized in the article by Frankfurt and Gurvich. Another problem that arises when trying to obtain photon PDFs from data on the photoproduction of jets is that of multiple interactions, which are discussed in the article by Butterworth et al. As energies are increased, regions of low x and high parton density are explored and at some energy multiple parton interactions must occur in order that unitarity is not violated by the jet cross sections. This article discusses the underlying physics of the models used to incorporate multiple interactions and also discusses how these models are interfaced with Monte Carlo programs of hadronization. The effects on both the total cross sections and jet cross sections are discussed and it is suggested that both models and data are hinting that the effects of multiple interactions might be present at HERA energies. Turning to the physics of the soft pomeron, the article by Donnachie reviews the diffractive production of vector mesons and concludes that we have a consistent picture for the photoproduction of ρ,ω and ϕ, and also for electroproduction of these mesons at fixed target energies. However, cross section data on electroproduction of ρ0 and ϕ at HERA show a steeper energy dependence than that expected from a soft diffractive process, but consistent with QCD based calculations. We now turn to the photoproduction of heavy flavour vector mesons, and in particular the J/ψ. The steeper energy dependence than expected from the soft pomeron of the electroproduction cross section of ρ0 and ϕ is seen also in the photoproduction of J/ψ. While the hard scale in the case of the electroproduction is provided by the virtuality of the photon, Q2, it is the mass of the J/ψ that serves as a hard scale here, large enough to be considered as a hard interaction calculable in QCD. This is discussed in the article of Frankfurt and Levy. We would like to end this introduction by noting that photoproduction at HERA has turned out to be a source to study both soft and hard hadronic interactions and the interplay between them.
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Apr 16, 2009
G Calucci + 1
