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$\gamma$$\gamma$ Interactions

A hadronic $\gamma$$\gamma$ collision can be described by any of the hadronic terms in equation 6.1, which gives rise to a wide range of possible interactions. Previously these processes could be reduced into two main classifications, a vector dominance process for soft hadronic interaction (Fig 6.1) and a direct coupling to pointlike quarks (Fig 6.2) [25].

In the late 80's experiments started to produce results that could not be explained by these two processes [26]. As the photon can fluctuate into a q$\overline{q}$ state, which can fluctuate into more complex partonic states, it is convenient to define Parton Density Functions (PDF's) for the photon in a similar way that is done for the hadron. This means that the photon is described as an object with smaller partonic constituencies of quarks, antiquarks and gluons. One of the photons may react directly with one of the partons of the other photon (Fig. 6.3), or partons from both of the photons can react with each other (Fig. 6.4) [25].

Figure 6.1: The VDM process
Figure 6.2: The QPM process

Figure 6.3: The single resolved process
Figure 6.4: The double resolved process