Three-jet production in deep-inelastic scattering at HERA

Three-jet production is studied for the first time in deep-inelastic positron-proton scattering. The measurement carried out with the H1 detector at HERA covers a large range of four-momentum transfer squared 5 <Q(2) <5000 GeV2 and invariant three-jet masses 25 <M-3jet less than or similar to 140 GeV. Jets are defined by the inclusive k(perpendicular to) algorithm in the Breit frame. The size of the three-jet cross section and the ratio of the three-jet to the dijet cross section R-3/2 are described over the whole phase space by the predictions of perturbative QCD in next-to-leading order. The shapes of angular jet distributions deviate significantly from a uniform population of the available phase space but are well described by the QCD calculation. (C) 2001 Elsevier Science B.V. All rights reserved.