Ingelman and
Schlein [3] proposed that the observation of jets in diffractive
events would probe the partonic nature of the exchanged object, whether
it is the pomeron or something else.
Their paper introduced the field of hard diffractive scattering, which refers
to the subset of traditional diffractive interactions characterized by
high
transverse momentum (
) scattering.
They assumed that the pomeron can be
treated as an object that exists within a proton, and that it is thus
sensible to define a flux of pomerons in the proton as well as a pomeron
structure function. They proposed a gluonic pomeron with either
a hard structure, as would be derived from two gluons sharing the
pomeron momentum 
,
or a soft structure like the gluonic structure of the proton 
,
where 
is the momentum fraction of the parton with respect to the
pomeron. With these assumptions they were able to make predictions
for diffractive jet production cross sections and properties.
The first experimental results on this subject were published by the UA8 Collaboration at CERN, and showed the existence of jets in single diffractive events [4] and that these jets had rapidity and longitudinal momentum distributions consistent with a hard pomeron structure [5]. There was also evidence for a ``super-hard'' or ``coherent'' pomeron, where the entire momentum of the pomeron participates in the hard scattering [5].
The UA8 Collaboration tagged diffractive events using a small angle spectrometer to detect and reconstruct the leading proton [6]. A proton spectrometer typically consists of machine magnets surrounded by a series of Roman pots, which are vessels that house position detectors. These pots can be positioned close to the beam and used to measure protons that are scattered through small angles, by measuring the bend of the track in the known magnetic field. Diffractive events can also be identified using rapidity gaps [7, 8], which are experimentally defined as the absence of particles or energy above threshold in some region of rapidity. Since the pomeron is a color singlet, radiation is suppressed in events with pomeron exchange typically resulting in large rapidity gaps in these events [9].
Figure 2 shows the diagram for hard single diffraction producing
two jets, a scattered 
, and a rapidity gap.
This figure is identical to
Fig. 1(b) for traditional diffraction except for the
production of jets. We use the
convenient language of Ingelman and Schlein to describe the process as occurring
in two steps.
First the pomeron is emitted from the , with an emission
probability described by the pomeron flux factor. The is scattered
but remains intact, while
the pomeron interacts with the proton in a hard
scattering producing jets and a rapidity gap in the region near the
. The charge conjugate diagram where the proton
remains intact and the is fragmented is equally likely.
The detailed study
of these interactions will yield
insight into the nature of the pomeron and reveal the validity of
this phenomenological picture.
Figure 2: The diagram for a hard single diffractive interaction
resulting in a final state with a scattered
and two jets.
The 
-
plot shows the distribution of particles in this event including
a rapidity gap near the scattered and the circles
which represent the two jets.