Diffraction  

The analyses which will be discussed in this report both make use of the operational definition of diffraction [4]:

  A process is diffractive if and only if there is a large rapidity gap in the produced-particle phase space which is not exponentially suppressed.

They are, in addition, studies of hard diffraction in the sense that the events all possess a large (negative) squared momentum transfer, Q², or a high energy scale, Q. The hard diffraction events are further subdivided into two classes both of which have gone by a number of different names.

The first class of events may be called hard diffractive scattering, hard double-dissociation diffraction or high-t diffraction. They proceed as shown in Figure 3(a), via the exchange of a colour singlet object of large negative squared invariant mass, t. (t, in both event classes, refers to the square of the momentum transfer across the exchanged colour singlet object. This object is called a pomeron and denoted $I\!\!P$.)

 

Figure:   Hard diffractive scattering at HERA. The diagram for this process is shown in (a). The exchanged colour singlet object is denoted $I\!\!P$ and the negative of its squared invariant mass, -t, sets the energy scale of the interaction, ($Q = \protect\sqrt{-t}$). In the final state, shown in (b), there are two high transverse energy jets and two remnant jets with a gap in particle production in the central rapidity region.

Owing to the absence of colour flow across the middle of the event a gap in the production of particles is expected to be observable. These events thus contain a central rapidity gap as illustrated in Figure 3(b). This may be contrasted with the situation, for example, where a gluon is exchanged in place of the pomeron in Figure 3(a). Central rapidity gap events will be examined in Sect. 2.

The second class of events has been called diffractive hard scattering, hard single-dissociation diffraction and low-t diffraction. These events are understood to occur when a colour singlet object, travelling collinearly with the proton, is probed by the hard subprocess. An example is shown in Figure 4(a).

 

Figure:   The diffractive hard photoproduction process at HERA is shown in (a). The pomeron, denoted $I\!\!P$, is shown being emitted from the proton with a squared momentum transfer t. A quark from the pomeron subsequently enters the hard subprocess which is mediated by the exchange of a gluon, denoted g, and characterized by the energy scale Q. The topology of the final state is shown in (b). There are two high transverse energy jets associated with the hard subprocess. There may be a photon remnant. However the proton is not broken up and disappears down the forward beam pipe leaving a gap in particle production at high rapidities.

Because the object emitted by the proton does not carry colour, particle production into the forward, or high-$\eta$,region of phase space is suppressed. This process thus leads to the formation of a forward rapidity gap as illustrated in Figure 4(b). This process is studied in Sect. 3.