Progress and Problems in QCD - Report from the Hadronic Final States Working Group at DIS99

We present a summary of the Hadronic Final States parallel sessions of the DIS99 Workshop. Topics were presented over two days and included both theoretical and experimental talks. Recent progress in the understanding of QCD in deep inelastic scattering, e^+e^- collisions, and in gamma and p collisions was discussed.Comment: 15 pages, 22 figures, Latex, nbp.sty. To be published in the Proceedings of DIS'99, Zeuthen, Germany, 19-23 April 199

Hard diffractive scattering in photoproduction at HERA

Photoproduction events which have two or more jets have been studied in the $W sb{ gamma p}$ range 135 GeV $< W sb{ gamma p} <$280 GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals (${ Delta eta}$) of up to four units and have transverse energies greater than 6 GeV. A gap is define as the absence between the jets of particles with transverse energy greater than 300 MeV. The fraction of events containing a gap is measured as a function of ${ Delta eta}$. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of ${ Delta eta} sim3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard scattering via a strongly interacting colour singlet object