The ATLAS Trigger System

Johannes Haller
Uni Hamburg

With the high bunch-crossing and interaction rates and potentially 
large event sizes the experiments at the LHC challenge data 
acquisition and trigger systems. The LHC bunch-crossing rate of 40 MHz 
has to be reduced to the few 100 Hz allowed by the storage systems. In 
ATLAS, a three-level trigger system has been designed to achieve this 
goal. The first level trigger utilises custom built hardware to derive 
a trigger decision within 2.5 microsec and delivers Regions-of-Interest 
(RoI) as seeds for reconstruction to the High Level Trigger (HLT) 
system. The HLT consists of two consecutive software triggers: Level-2 
and Event Filter, which run on commodity PC farms. In this presentation 
I will give a overview of the system, concentrating on its technical 
implementation, expected performance and its status.