Development and Online Operation of Minimum Bias Triggers in ATLAS

Since the restart of the LHC in November 2009, ATLAS has recorded inelastic $p - p$ collisions to perform first measurements on charged particle multiplicities. These measurements will help to constrain various phenomenological models describing soft parton interactions. Understanding trigger efficiencies for different event types is therefore crucial to minimize any possible bias in the event selection. ATLAS uses two main minimum bias triggers, utilizing complementary detector components and trigger levels. A hardware based first trigger level situated in the forward regions of $2.1 < |eta| < 3.8$ has been proven to select $p - p$ collisions with high efficiency. A complementary Inner Detector based minimum bias trigger uses a random seed on filled bunches and central tracking detectors for event selection. Both triggers were essential for the analysis of kinematic spectra of charged particles. Their performance and trigger efficiency measurements as well as studies on possible bias sources will be presented.