The User Interface for the ATLAS Trigger and its Use in Physics Studies

T. Eifert, N. Berger, T. Bold, S. George, R. Goncalo, J. Haller, 
J. Stelzer, M. Wielers

   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 maximize 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 early 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.