Status of the ATLAS trigger system for LHC startup

The ATLAS experiment at Large Hadron Collider (LHC) has a goal to explore the mechanism of Electro Weak symmetry breaking and search for new physics. The design of the detector fulfills extreme requirements for high precision muon momentum measurement, efficient tracking, very good electromagnetic calorimetry for photon and electron identification. With LHC bunch crossing rate of 40 MHz and about 23 interactions per bunch crossing, highly selective trigger system is required to reduce the rate of 1 GHz interactions to about 100 Hz for data storage. For better understanding of trigger efficiencies, a number of software tools have been used, from trigger event selection to offline physics analysis. The ATLAS trigger system is based on three levels, Level 1, hardware based, Level 2 and Event Filter, together called High Level Trigger (HLT), software based. A number of tests have been performed to emulate data taking conditions and test the full experiment setup in combined operation and cosmic runs. The system performed successfully in the first beam events. Different trigger menus are prepared for varying conditions, to demonstrate the readiness of ATLAS for data taking with collisions.