D* Meson Production in Deep-Inelastic Diffractive Interactions at HERA 
Diffractive scattering processes have been the subject of intensive research in the H1 experiment at the ep collider HERA. In Deep-Inelastic Scattering (DIS), a virtual photon probes the quarks and gluons of the proton. Approximately 10% of the DIS events are diffractive. In this case, the proton remains intact, losing only a small fraction xIP of its momentum during the interaction. In addition, a complex hadronic system X is produced. Diffractive interactions have been interpreted as being due to the exchange of a net colour-singlet state between the colliding proton and the virtual photon. Historically, this so-called pomeron was assumed to be dominated by gluons. Its exact partonic structure however remained unclear.

It was a breakthrough for diffraction when it became possible at HERA to study the QCD structure of the colourless exchange. The total cross section for diffractive DIS, presented in a previous paper in the form of a structure function F2D(3), has led to the extraction of diffractive parton distributions, i.e. the composition of the colour singlet state in terms of quarks and gluons. Whilst the F2D(3) data yield tight constraints on the quark content of the colourless exchange, the gluon distribution is rather poorly known. 

In this analysis, we focus on the subset of diffractive events for which the system X contains at least one D* meson. If the diffractive exchange is indeed dominated by gluons, then these events are expected to arise dominantly from the Boson-Gluon-Fusion (BGF) process (gamma* g --> c cbar). The D* data are thus highly sensitive to the gluon structure of the colourless exchange. Due to the large mass of the charm quark, from which the D* meson is produced, the data can be compared with the predictions of perturbative QCD calculations.

In the analysis, the total D* production cross section is measured for the first time at HERA. In the studied region about 6 % of the total D* cross section is produced diffractively. The statistics of the measurement are sufficient to allow the data to be measured at two values. For example, the figure below shows the measured D* cross section as a function of xIP, the momentum fraction of the proton carried by the colourless exchange. The xIP distribution is plotted twice to allow comparison with the following two groups of models
Resolved Pomeron Model

Models based on a partonic colourless exchange are compared with the data, using different parameterisations for the gluon distribution (shown in the legend above the data). The models exceed the data in the low xIP (low mass of the system X) region. The "H1 fit 2" parameterisation, derived from the H1 F2D(3) measurements, comes closest to the data in this region. All three parameterisations are consistent with the data in the high mass region.
2 Gluon Models

Several groups have attempted to interpret diffractive DIS in terms of the exchange of a pair of gluons, taken from the proton parton distributions. This highly appealing approach gives a natural interpretation for the diffractive exchange at the parton level. The 2-gluon exchange models are directly applicable in the low xIP region (xIP <0.01). The figure shows a comparison between the D* data and two recent models based on 2-gluon exchange. The "saturation" model is able to reproduce the normalisation of the cross section in the low xIP range, in which it is expected to be applicable, but also provides a good description of the data in the remaining region of phase space. The QCD based "BJLW" calculation is, depending on the choice of the cut-off parameter kT,gcut, also able to describe the data. The scattering of the qqbar fluctuation, which is shown as a shaded zone, forms a sizeable component of the total two gluon exchange cross section . 

Last Update on 08/12/2000 by Frank-Peter Schilling (fpschill@mail.desy.de)