Three- and four-jet final states in photoproduction at HERA

Three- and four-jet final states have been measured in photoproduction at HERA using the ZEUS detector with an integrated luminosity of 121 pb−1121 pb−1. The results are presented for jets with transverse energy ETjet>6 GeV and pseudorapidity |ηjet|<2.4|ηjet|<2.4, in the kinematic region given by the virtuality of the photon Q2<1 GeV2Q2<1 GeV2 and the inelasticity 0.2⩽y⩽0.850.2⩽y⩽0.85 and in two mass regions defined as 25⩽Mnj<50 GeV25⩽Mnj<50 GeV and Mnj⩾50 GeVMnj⩾50 GeV, where MnjMnj is the invariant mass of the n  -jet system. The four-jet photoproduction cross section has been measured for the first time and represents the highest-order process studied at HERA. Both the three- and four-jet cross sections have been compared with leading-logarithmic parton-shower Monte Carlo models, with and without multi-parton interactions. The three-jet cross sections have been compared to an O(ααs2) perturbative QCD calculation.