Measurement of three-jet distributions in photoproduction at HERA

Abstract The cross section for the photoproduction of events containing three jets with a three-jet invariant mass of M 3J >50 GeV has been measured with the ZEUS detector at HERA. The three-jet angular distributions are inconsistent with a uniform population of the available phase space but are well described by parton shower models and O (αα s 2 ) pQCD calculations. Comparisons with the parton shower model indicate a strong contribution from initial state radiation as well as a sensitivity to the effects of colour coherence.