Diffractive Jet Production in Deep-Inelastic e+p Collisions 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 two or more high transverse momentum jets. 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 --> q qbar). The dijet data are thus highly sensitive to the gluon structure of the colourless exchange. Compared with previous H1 measurements of this process, the luminosity is increased by an order of magnitude (L=18 pb-1) and the kinematic coverage is extended (Q2>4 GeV2; pT,Jet>4 GeV). This makes it possible to extract double-differential cross sections for the first time and to study 3-jet as well as dijet production.
The data are compatible with Regge- and QCD-Factorisation hypotheses for diffractive DIS and are used to extract a value for the "pomeron intercept" alphaIP(0)=1.17, which controls the energy dependence of the cross section. Assuming these factorisation hypotheses, the dijet data tightly constrain the gluon distribution of the colourless exchange.
The figure shows the measured dijet cross section as a function of zIP, the momentum fraction of the colourless exchange carried by the quark or gluon involved in the hard scattering. Models based a partonic colourless exchange are compared with the data, using different parameterisations for the gluon distribution (shown in separate figures above the data). The data are clearly able to distinguish between the different gluon distribution parameterisations. The "H1 fit 2" parameterisation, derived from the H1 F2D(3) measurements, is in remarkably good agreement with the dijet data.
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. 2-gluon exchange models have been found to give a good description of elastic vector meson production and deeply virtual Compton scattering (DVCS). There are good reasons to believe that this approach may also be applicable to diffractive dijet production at low xIP.
The figure shows a comparison between the dijet data with xIP < 0.01 and two recent models based on 2-gluon exchange. The "saturation" model is able to reproduce the shapes of the distributions, but not the overall normalisation. The QCD based "BJLW" calculation is, depending on the choice of the cut-off parameter pT,gcut, able to describe the data.
In summary, the data yield complementary but consistent information - together with the inclusive F2D(3) data - on the partonic structure of the diffractive exchange. Progress in calculations based on 2-gluon exchange has led to improved agreement with the data.
Last Update on 08/12/2000 by Frank-Peter Schilling (fpschill@mail.desy.de)