Multijets in Photoproduction at HERA

Cross-sections for the photoproduction of events containing four high - ET jets have been measured with the ZEUS detector using an integrated luminosity of 38.7 pb-1. At low invariant mass of the four-jet system, m4J, the observed event distributions are sensitive to the underlying event model. The inclusion of multi-parton interactions in Monte Carlo simulations lead to a significantly better description of the data. For m4J > 50 GeV the data is adequately described by models with no simulation of an underlying event, suggesting that this is a kinematic region suitable for comparison with perturbative QCD calculations. (author)