The Two-Photon Background

Figure 5.4: The two-photon background processes.

The two-photon process is shown in figure 5.4. The final state of this process does not really resemble the signal process, but the cross-section is extremely high, over a 1000 times greater than the signal processes cross-section, this means that some events may be accepted as signal events. There are two photon events where both beam leptons are lost down the beam pipe, these are called untagged events and will not contribute to the background. However, there are events where one of the beam leptons is detected, these are called tagged events. These may contribute to the background as the lepton may resemble that from a ${\rm W}^{+}{\rm W}^{-}\rightarrow{\rm q}\bar{\rm q}{\rm e}\bar{\nu}_{\rm e}$ event. Even with these events, the chances of it being accepted in the selection is extremely low, so the two-photon process is not as large a contributor to the background as the ${\rm Z}^{0}/\gamma \rightarrow {\rm q}\bar{\rm q}$ process.