Using data from the ZEUS detector the interaction between positrons and protons has been studied by our group; specifically we investigated the so-called ‘photon structure’, that is, the hadronisation of the virtual photon exchanged between the positron and proton, in low positron deflection angle situations. The important quantity to be measured was xgobs, the proportion of the photons momentum that went into the interaction. This was reconstructed from the knowledge of the energies, momenta and pseudorapidity of the hadron jets which emerge from the interaction.

The interaction – an overview

The interactions come about when positrons (which are considered equivalent to electrons) and protons are fired at each-other at high energies.

The events that were studied all involved low positron deflection angles. When the positron and proton interact, they do so electromagnetically by emitting a virtual photon. This photon was studied to determine the photon structure (the two particles could also interact weakly; the process is similar but is not what is studied here).

A schematic representation of the most basic interaction can be seen in figure 1b. In this case the photon is emitted by the positron and absorbed by the proton, which then breaks up into a number of indistinguishable jets of hadrons. It is these jets that are detected by ZEUS. This event is called a direct photon interaction.

This is not the only interaction that can occur however. The photon may split into a particle/antiparticle pair (permitted to do so by the uncertainty principle), a meson, and these particles may exist for long enough to collide with the proton instead of the photon. The meson must have the same quantum numbers as those of the photon, and it is usual for the particle concerned to be a quark/antiquark pair. This interaction is known as a resolved photon interaction, and can be seen in figure 1a.

These two types of interaction were distinguished by looking at the properties of the detected jets in each event. Depending on whether a direct or resolved photon interaction occurred different things are seen to emerge from the event. From the relative numbers of events of each type an estimate of the fraction of a photons life spent in a resolved state can be made.