Background from Photoproduction Events

The occurance of photoproduction background events in the sample can be shown with the luminosity monitor. In figure 5.21 the distribution of the events remaining in the sample are shown. Of these events are tagged by the luminosity monitor positron tagger as genuine photoproduction events. These are shown as a hatched histogram. We tag an event as photoproduction if $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$ of that event, together with the energy detected in the luminosity monitor positron tagger (see section 3.2.5) is close to :

and no accompanying $\gamma$ in the photon tagger is observed which excludes that these events stem from overlay of bremsstrahlung with another event. However, only about of the photoproduction events have a positron tagged by the positron tagger because of acceptance, so it is not possible to use the above condition to remove photoproduction background.

The background can be shown more clearly using the distribution in the variable . Figure 5.22(a) shows the distribution of for the data events which have passed the cuts described so far with the hatched histogram showing the same variable for tagged photoproduction events. Charged current events have as seen in figure 5.22 (b) which shows the distribution for the Monte Carlo events.

The events due to photoproduction are therefore to be found at low : We find events with . The tagged events would be equivalent to in total which is consistent with the measurement of events.

Through this cut events are rejected and events remain.

Figure: for the data events remaining in the sample. The hatched histogram shows of those events the ones for which , they correspond to photoproduction events.

Figure: The distribution of for events that have passed the selection cuts described in section 5 for data (a) and Monte Carlo events (b). The hatched histogram in figure (a) shows tagged photoproduction events still in the data sample. Those events have been selected by requiring .