Introduction

Charged current scattering events are characterized by a substantial missing transverse momentum which is carried off by the undetected neutrino. This also causes the $({{E_{\mbox{\em tot}}}\, - \, {P_z}})$ of the event to be lower than the which should be measured for fully contained events. Furthermore the events should have characteristics consistent with the event having originated from the nominal interaction point.

The selection criteria are thus designed to extract events with the above characteristics. Some additional cuts are necessary to remove events which are caused by detector malfunctions. Moreover, cosmic ray muons, either alone or overlaying a bona fide e-p-interaction need to be rejected. Finally cuts are needed to remove events which are due to resolution effects in e-p-interactions with large cross sections such as neutral current events with a poorly measured scattered positron and photoproduction events with imbalance caused by an incomplete measurement of the produced final state.

The event selection consists of an online trigger and an offline selection process. The main task of the online trigger system is to select events which are true high- positron-proton scattering events while the offline selection procedure selects from those events the charged current events.

In this chapter we will first describe the online trigger and then the offline selection procedure for charged current events.