**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 W ^{2 }= (1-x)Q^{2 }/x ,
where
W is the center of mass energy of teh gamma*-p system,**

**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 Q ^{2 }.
At
high Q^{2 }this pair has a size much smaller**

**
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**

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

** **