Development and online operation of minimum bias triggers in ATLAS

Since the restart of the LHC in November 2009, ATLAS has recorded pp collisions to investigate new phenomena at the high energy frontier. One early measurement is of charged particle multiplicities in inelastic interactions, which was performed on data sets of about 360,000 and 10,000,000 minimum bias event at (s)1/2 = 900 GeV and 7 TeV respectively. This measurements will help to constrain various phenomenological models describing inelastic interactions. Understanding trigger efficiencies for different event types is crucial to minimize any possible bias in the event selection. ATLAS has a three level trigger system and utilises two main minimum bias triggers. A primary scintillator based trigger situated in the forward regions of 2.1 < |η| < 3.8 was proven to select pp collisions with high efficiency. A complementary control trigger runs on unbiased filled proton bunches and utilizes space points reconstructed in the silicon tracking volume to trigger on inelastic interactions. Both triggers were essential for the analysis of charged particle multiplicities. Their performance and trigger efficiency measurements as well as studies on possible bias sources are presented. The triggers are shown to form an effective and inclusive triggering solution for early LHC physics.