Eur. Phys. J. C (2007) 51: 549-568
https://doi.org/10.1140/epjc/s10052-007-0325-4

Regular Article - Experimental Physics

Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

¹ I. Physikalisches Institut der RWTH, Aachen, Germany
² Vinca Institute of Nuclear Sciences, Belgrade, Serbia
³ School of Physics and Astronomy, University of Birmingham, Birmingham, UK
⁴ Inter-University Institute for High Energies ULB-VUB, Universiteit Antwerpen, Brussels, Antwerpen, Belgium
⁵ Rutherford Appleton Laboratory, Chilton, Didcot, UK
⁶ Institute for Nuclear Physics, Cracow, Poland
⁷ Institut für Physik, Universität Dortmund, Dortmund, Germany
⁸ Joint Institute for Nuclear Research, Dubna, Russia
⁹ CE-Saclay, CEA, DSM/DAPNIA, Gif-sur-Yvette, France
¹⁰ DESY, Hamburg, Germany
¹¹ Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany
¹² Max-Planck-Institut für Kernphysik, Heidelberg, Germany
¹³ Physikalisches Institut, Universität Heidelberg, Philosophenweg 12, 69120, Heidelberg, Germany
¹⁴ Kirchhoff-Institut für Physik, Universität Heidelberg, Heidelberg, Germany
¹⁵ Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic
¹⁶ Department of Physics, University of Lancaster, Lancaster, UK
¹⁷ Department of Physics, University of Liverpool, Liverpool, UK
¹⁸ Queen Mary and Westfield College, London, UK
¹⁹ Physics Department, University of Lund, Lund, Sweden
²⁰ CPPM, CNRS/IN2P3 - Univ. Mediterranee, Marseille, France
²¹ Departamento de Fisica Aplicada, CINVESTAV, Mérida, Yucatán, Mexico
²² Departamento de Fisica, CINVESTAV, Mérida, Mexico
²³ Institute for Theoretical and Experimental Physics, Moscow, Russia
²⁴ Lebedev Physical Institute, Moscow, Russia
²⁵ Max-Planck-Institut für Physik, München, Germany
²⁶ LAL, Université de Paris-Sud 11, IN2P3-CNRS, Orsay, France
²⁷ LLR, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France
²⁸ LPNHE, Universités Paris VI and VII, IN2P3-CNRS, Paris, France
²⁹ Faculty of Science, University of Montenegro, Podgorica, Montenegro
³⁰ Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republic
³¹ Faculty of Mathematics and Physics, Charles University, Praha, Czech Republic
³² Dipartimento di Fisica Università di Roma Tre and INFN Roma 3, Roma, Italy
³³ Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria
³⁴ Institute of Physics and Technology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
³⁵ Paul Scherrer Institut, Villigen, Switzerland
³⁶ Fachbereich C, Universität Wuppertal, Wuppertal, Germany
³⁷ Yerevan Physics Institute, Yerevan, Armenia
³⁸ DESY, Zeuthen, Germany
³⁹ Institut für Teilchenphysik, ETH, Zürich, Switzerland
⁴⁰ Physik-Institut der Universität Zürich, Zürich, Switzerland
⁴¹ Physics Department, National Technical University, Zografou Campus, 15773, Athens, Greece
⁴² Rechenzentrum, Universität Wuppertal, Wuppertal, Germany
⁴³ University of P.J. Šafárik, Košice, Slovak Republic
⁴⁴ CERN, Geneva, Switzerland
⁴⁵ Max-Planck-Institut für Physik, München, Germany
⁴⁶ Comenius University, Bratislava, Slovak Republic
⁴⁷ Helmholtz Humboldt Research Award, DESY and University Hamburg, Hamburg, Germany

^* e-mail: obehnke@mail.desy.de

Received: 14 March 2007
Published online: 19 June 2007

Abstract

Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q²<0.01 GeV²) and deep-inelastic scattering processes (DIS, 4<Q²<80 GeV²). The event topology is given by ep→eXY, in which the system X, containing at least two jets, is separated from a leading low-mass baryonic system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5±0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.