Three-jet production in the reaction ep --> eXp has been studied with the Z
EUS detector at HERA using an integrated luminosity of 42.74 pb(-1).The dat
a were measured in the kinematic region 5 < Q(2) < 100 GeV2, 200 < W < 250
GeV and 23 < M-X < 40 GeV. The diffractive signal was selected by requiring
a large rapidity gap in the outgoing proton direction. Jets were reconstru
cted in the centre-of-mass system of X using the exclusive k(T)-algorithm.
A sample of three-jet events in diffraction has been identified. Differenti
al cross sections were measured as a function of the jet pseudorapidity and
jet transverse momentum with respect to the virtual photon-pomeron axis. T
he jets going in the pomeron direction are broader than those going in the
virtual-photon direction. This is consistent with models predicting that gl
uons a-re predominantly produced in the pomeron direction and quarks in the
virtual-photon direction. (C) 2001 Elsevier Science B.V. All rights reserv
ed.