Background

The main source of background originates from photoproduction events. Figure 3 shows the distribution of DIS and photoproduction events in a given x and Q² bin after all cuts, except for the cut on , have been applied. To calculate the number of photoproduction events with  GeV a fit is made to the data, shown as black dots, in each bin of the x-Q² plane. From the total number of events in each bin, the number of photoproduction events as calculated from the fit, is subtracted. The photoproduction background can rise to 12% at high y, but is a few percent for most of the bins. Other sources of background, like cosmic ray events and halo muons, contribute much less and are efficiently removed.

  
Figure 3: Distribution of DIS and photoproduction (php) events in a given x-Q² bin. The black dots show the data and the solid line represents the fit that is applied to the data.