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

Abstract.

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type $ep \rightarrow eXY$, where Y is a hadronic system with mass $M_Y< 1.6$ GeV and where the squared momentum transfer at the pY vertex, t, is limited to $|t|<1\,{\rm GeV}^2$, is studied as a function of the invariant mass $M_X$ 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 $Q^2$. A model with soft colour interactions is also successful.