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MC Generators

There are a number of different MC models on the market to describe two-gamma physics. Some earlier models could recently be rejected, as the quality of the data now is good enough to discard them. There are mainly four different MC generators used in the $\gamma$$\gamma$-physics analysis today [28]:

• HERWIG 5.9 [31] is a general-purpose QCD generator to simulate Hadron Emission Reactions With Interfering Gluons. The old version of HERWIG 5.9 has been replaced with HERWIG 5.9+k_t (dyn) for $\gamma$$\gamma$ studies. In the new version the quarks inside the photon have a modified transverse momentum distributions (k_t).

• PHOJET 1.05 [32] is a minimum bias event generator for pp, $\gamma$p and $\gamma$$\gamma$ interactions, described with the Dual Parton Model (DPM) in terms of Reggeon and Pomeron exchanges combined with the QCD-improved parton model. PHOJET has in particular a detailed modelling for diffractive and soft interactions.

• TWOGAM [34] is a pure $\gamma$$\gamma$ event generator where all $\gamma$$\gamma$-interactions are stripped down to three different processes: The Quark Parton Model (QPM), the Vector Dominance Model (VDM) and also the Resolved Photon Contribution (RPC).

• PYTHIA [33] is a general-purpose event generator for high energy physics, which recently also is used to simulate $\gamma$$\gamma$ physics. The model uses, similar to TWOGAM, three components to describe the photon. In $\gamma$$\gamma$ interactions there should then be 3x3 event classes, but the 'off-diagonal' combinations appear pairwise. The program therefore uses six event classes to describe two-photon collisions.