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Data and MC

It has been shown that the QCD-RPC component of the Monte Carlo is needed to describe the data. Further more the p_T^min cut-off limit for the parton density functions in this model was adjusted to 2.05. From double tag data it is also clear that the radiative corrections are needed to better describe data. Excellent agreement of all important observables in data and Monte Carlo could then be obtained for both VSAT single tag and double tag data.

The VSAT+STIC double tag data has also been probed for the first time with very promising results. In all about 220 VSAT+STIC events were found, which shall be compared with the 260 VSAT double tag events that could be obtained after background cuts. The remaining background is slightly higher in the VSAT double tag sample, so the expected $\gamma$$\gamma$-signal is nearly the same for both samples (around 200).

As data and Monte Carlo agree very well, the TWOGAM generator could be used to study physics within the statistical limits of the data. The TWOGAM generator has the advantage of producing separate MC samples for each model. The Q² and invariant mass dependence of the QPM, VDM and QCD-RPC contributions can be seen in Fig. 8.3 and 8.4.

Figure 8.3: The Q² dependence of the three model contributions in the TWOGAM generator, taken in the W interval 15-60 GeV.
Figure 8.4: The W dependence of the VDM,QPM and QCD-RPC models, taken in the Q² interval 40-50 GeV.

The curves has been obtained by running the TWOGAM generator for different Q² and W intervals. The value of p_T^min will change the size of the QCD-RPC distribution, and as seen from Fig. 8.4 this is especially important at large invariant masses. From the analysis is it clear that the value of p_T^min should be between 2.0 and 2.15, resulting in an uncertainty of the QCD-RPC cross section in Fig. 8.3 and 8.4 of about -15% to +8%.