The structure of the Pomeron: Study of diffractive hard processes

The study of  high energy hadron-hadron scattering which is dominated by 'Pomeron' exchange has become a new testing ground of QCD. The
interest has been revived by the HERA e-p experiments.  In deep inelastic scatteringa virtual photon with four momentum transfer Q is scattered
from partons in the proton.  Since W2 = (1-x)Q/x , where  W    is the center of mass energy of teh gamma*-p system,
high energy gamma*-p scattering is equivalent to the measurement of the inclusive structure function F2 (W2, Q2 )
at low x. The observed rise of F2 towards low x is therefore equivalent to the rise of the gaamp*-p cross section for high center of mass energies  W2.
The gamma-p scattering process can of course also be described in the proton rest system. Then we have the following situation (see fig.1):


The photon fluctuates into a q-qbar  pair  far outside of the proton. The q-qbar pair travels an average  distance  d~ 1/x of about 50 fm before it hits
the proton and the transverse size of the pair is    r = .2 GeVfm /Q [GeV] . Gamma-p scattering is therefore equivalent to the scattering of a
color dipole (the q-qbar pair) of variable size with the proton. Its size can be adjusted by choosing Q2 . At high Q2 this pair has a size much smaller
than 1 fm which can be treated perturbatively and which has a small cross section because the color charge is well shielded. For photoproduction
(Q2 = 0) the dipole size has the size of a typical hadron, the total cross section for photoproduction rises therefore with the same slope as p-p at high energy.
HERA physics offers hence the chance to study the high energy cross sections of color dipoles of adjustable size with the proton.
      Hard diffractive scattering processes, where the proton is left intact offer a second chance to study Pomeron structure in more detail. They are
identified in the detector by either observing an elastically scattered proton or by requiring a 'rapidity gap' between the scattered proton and the
observed hadronic system. A typical event is shown in figure 2.


                   Event with a rapidity gap in the H1 detector                                                          Diagram for diffractive scattering in the 'resolved pomeron" model

These processes are described by the exchange of the colorless 'Pomeron' and the subsequent scattering of the photon on a parton with momentum
fraction beta inside the pomeron.
           Studies of inclusive deep inelastic diffractive processes at HERA have shown, that the pomeron structure is dominated by gluons . Our group
has therefore launched a program to measure diffractive dijet production in DIS and photoproduction because  these processes give a direct handle
on the gluon content of the Pomeron. They are dominated by photon-gluon processes e.g. the gluon in the pomeron enters the scattering process directly.
            Diffractive dijet cross -sections in the variable zp, the momentum fraction of gluons in the pomeron, are shown below  for DIS  and phtoproduction.

a) cross sections of diffractive dijet production  compared tp NLO QCD predictions
based on QCD factorisation and diffractive parton densities derived from QCD fits
to the diffractive structure function F2D        


b) same as  a) but now for photoproduction. The NLO prodiction has to be scaled
down by a factor S=0.6 +- 0.1 to describe the memasurements.     

Fig.a) shows the dijet cross section for DIS compared to predicitons based on the NLO diffractive gluon distribution*
pomeron flux  which are compatible with the observed

scaling violations of F2D. The NLO QCD prediction is in good agereement with the measurements. This underlines that the diffractive cross sections factorise for DIS.  Diffractive dijet cross sections in photoproduction are also well described in shape by the same parton distributions (b)
they are however suppressed by a factor S=0.6+- 0.1  . This supression is the same for both resolved and direct  contibutions.
This shows that factorisation does not hold for all diffractive processes in line with earlier measurements at the Tevatron.

New results are published as conference contributions:     dijets in diffractive photoproduction and DIS