Physics Analysis
Diffraction
Diffractive scattering was first observed 40 years ago in hadron - hadron collisions. It has become a hot topic again at HERA where it was unexpectedly rediscovered in deep inelastic collisions. In diffractive collisions the proton somehow manages to remain intact even though the high energetic electron deeply disturbes the internal structure.
Diffraction is usually described as a wave phenomena, in analogy to optics. This explanation is, however, in contradiction with the infinite momentum parton picture in which, in leading order, a virtual photon just counts quarks inside the proton. The contradiction can be resolved in different ways. Either one can assume the existence in the proton of a new species, called the pomeron, with its own substructure. Or one can invoke the so called dipole picture. In that picture the incoming virtual photon couples first to a quark-antiquark pair which subsequently interacts with the proton via the exchange of a gluon cloud called pomeron.
The dipole picture provides a direct connection between diffractive scattering and inelastic reactions via the optical theorem. In that way one can study the properties of the pomeron, i. e. the gluon (and sometimes quark) emission process simultaneously in inlusive DIS processes (F2) and in diffractve processes.
The emission properties in the transverse plane are of particular interest. They are accessible through the determination of the t-distributions. Here the variable t denotes the square of the four momentum transferred to the diffractively scattered proton. t can be measured well either using forward proton detectors (in ZEUS called Leading proton spectrometer, LPS), or by selecting various diffractive vector meson production processes.
Kerstin Borras 
Henri Kowalski 
Bernd Löhr 
Uta Stösslein
Günter Wolf
Dorota Szuba 
Graeme Watt
Isabell Melzer 
Raquel Santamarta