Multiplicity structure of the hadronic final state in diffractive deep-inelastic scattering at HERA

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type ep → eXY, where Y is a hadronic system with mass MY < 1.6 GeV and where the squared momentum transfer at the pY vertex, t, is limited to \t\ < 1 GeV2, is studied as a function of the invariant mass Mx of the system X. Results are presented on multiplicity distributions and multiplicity moments, rapidity spectra and forward-backward correlations in the centre-of-mass system of X. The data are compared to results in e+ e- annihilation, fixed-target lepton-nucleon collisions, hadro-produced diffractive final states and to non-diffractive hadron-hadron collisions. The comparison suggests a production mechanism of virtual photon dissociation which involves a mixture of partonic states and a significant gluon content. The data are well described by a model, based on a QCD-Regge analysis of the diffractive structure function, which assumes a large hard gluonic component of the colourless exchange at low Q2. A model with soft colour interactions is also successful. a Supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, FRG, under contract numbers 7AC17P, 7AC47P, 7DO55P, 7HH17I, 7HH27P, 7HD17P, 7HD27P, 7KI17I, 6MP17I and 7WT87P b Supported by the UK Particle Physics and Astronomy Research Council, and formerly by the UK Science and Engineering Research Council c Supported by PNRS-NFWO, IISN-IIKW.