D* Meson Production in Deep-Inelastic Diffractive Interactions at HERA
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.
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 .