The user interface for the ATLAS trigger and its use in physics studies

The ATLAS detector at the Large Hadron Collider (LHC) will be exposed to proton-proton collisions at a rate of 40 MHz. This rate needs to be reduced to an output rate of 100-200 Hz, compatible with the foreseen storage and analysis capability. To achieve this while retaining the most interesting physics, the trigger uses novel techniques, such as seeded, step-wise reconstruction and early rejection. As the luminosity increases more potentially interesting events will be produced than can be kept for analysis. To maximise the physics reach within the available bandwidth, the trigger menu will need to adapt to the increasing luminosity according to the ATLAS physics program. To study the bias introduced by the trigger selection, detailed online information needs to be used in physics analyses. A user interface was developed that, for each recorded event, allows easy access to information produced by the trigger as well as to its configuration. A first version of this interface was made available earlier this year and is now routinely used in physics studies. We describe the ATLAS trigger operation and present in particular the trigger-user interface, focusing on the accessibility it provides to both online quantities and the trigger configuration. We also discuss the impact of the trigger selection on ATLAS physics studies relevant for the initial phase of LHC running.