The Trigger for Early Running

The ATLAS trigger and data acquisition system is based on three levels of event selection designed to capture the physics of interest with high efficiency from an initial bunch crossing rate of 40 MHz. The selections in the three trigger levels must provide sufficient rejection to reduce the rate to 200 Hz, compatible with offline computing power and storage capacity. The LHC is expected to begin its operation with a peak luminosity of 10^31 with a relatively small number of bunches, but quickly ramp up to higher luminosities by increasing the number of bunches, and thus the overall interaction rate. Decisions must be taken every 25 ns during normal LHC operations at the design luminosity of 10^34, where the average bunch crossing will contain more than 20 interactions. Hence, trigger selections must be deployed that can adapt to the changing beam conditions while preserving the interesting physics and satisfying varying detector requirements. In this paper, we provide a menu of trigger selections that can be deployed during the startup phase at each trigger level and show its evolution to higher luminosities. The studies presented in this paper are based on simulated data.