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(-1). The results are presented for jets with transverse energy E-T(jet)> 6 GeV and pseudorapidity vertical bar eta(jet)vertical bar < 2.4, in the kinematic region given by the virtuality of the photon Q(2)<1 GeV2 and the inelasticity 0.2 <= y <= 0.85 and in two mass regions defined as 25 <= M-nj < 50 GeV and M-nj >= 50 GeV, where M-nj 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(alpha alpha(2)(s)) perturbative QCD calculation.