Electron Trigger Performance Studies for the Event Filter

In this note a comprehensive study of the high-Pt single electron trigger at the event filter of ATLAS is presented. An overall trigger efficiency of 70%(66%) for single electrons with Pt=30(20)GeV at design (low) luminosity within the stringent time constraints of the online system can be achieved. The single electron trigger rate is about 80(25)Hz of which 65%(50%) are due to irreducible processes like W -> e nu. The high trigger rate for single electrons compared to the total target rate of 100Hz is of big concern. Therefore it might be necessary to raise the threshold for single object triggers and to pre-scale the lower threshold trigger items. A case study using the physics channels W -> e nu, Z -> e e, and H -> 4e will give an indication how double object trigger items or the missing transverse energy can be used to save the trigger efficiency while reducing the trigger rate.