Measurement of multijet events at low $x_{Bj}$ and low $Q^2$ with the ZEUS detector at HERA

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In this thesis, cross sections of inclusive and differential diand trijet production in deep inelastic electron-proton scattering at low Bjorken-x have been determined and compared to perturbative QCD calculations at nextto-leading order. The data were taken during the years 1998 − 2000 with the ZEUS detector at the HERA collider and had an integrated luminosity of 82 pb. The center-of-mass energy was √ s = 318GeV. The phase space was defined by 10 < xBj < 10 , 10 GeV < Q < 100 GeV, and 0.1 < y < 0.6. The jets were identified in the hadronic center-of-mass frame using the inclusive kT -clustering algorithm. The cross sections were measured differentially in virtuality, Q, inelasticity of the photon, y, Bjorken-x, xBj, transverse energy of the jets in the hadronic center-of-mass frame, E T,HCM, and the pseudorapidity of the jets in the laboratory frame, η LAB. Multidifferential cross sections were measured as functions of xBj and correlations of the jet momenta in the transverse plane. The 6m Tagger, which was reinstalled in 2003, was calibrated using the measurement of the photon energies by the Spectrometer. First measurements of the acceptance of the luminosity photon detectors where carried out.