Event Selection

The event selection is dominated by two selection steps, the trigger and the offline selection. The ZEUS trigger is a three step algorithm and has an efficiency greater than 95% for DIS events in the x and Q² region discussed here. A total of events passed the DIS trigger. To obtain an event sample with little background and high purity, several cuts were applied. The most important are listed below:

•  GeV, , where E_i denotes the energy deposit in calorimeter cell i and the polar angle of the centre of the cell with respect to the event vertex; (For fully contained DIS events  GeV, whereas photoproduction events peak at low values of , as the scattered electron remains within the rear beampipe.)
•  GeV; the energy of the scattered electron should be greater than 5 GeV for a reliable electron finding and a low background from photoproduction;
• y_JB > 0.04 to ensure a good measurement of the hadronic angle and of x.

A total of 46k DIS NC events corresponding to an integrated luminosity of 0.54 pb^-1 survived all cuts.

  
Figure 2: Distribution of the primary measured quantities, a) angle of the scattered electron , b) angle of the hadronic final state , c) Q², and d) x. The dots are the data and the shaded areas represent the MC.

The distributions for the measured and the reconstructed variables for the DA method for data and MC are shown in figure 2. The good agreement between the data and MC distributions demonstrates our good understanding of the relation between MC and data. The structure function used as an input for the MC distribution has been reweighted to the measured structure function.