Commissioning of the ATLAS Electron, Photon and Tau Trigger Selection

Since the start-up of the LHC end of 2009, the trigger commissioning is in full swing. The ATLAS trigger system is divided into three levels: the hardware-based first level trigger, and the software-based second level trigger and Event Filter, collectively referred to as the High Level Trigger (HLT). At design luminosity the electron, photon and tau trigger face the challenge of efficiently selecting interesting events containing electrons, photons and hadronic decays of tau leptons and achieving a huge background rejection factor of O(10000-100000) for electrons and taus and several thousand for photons within the time constraints of the trigger system. In addition, these triggers have to be robust with respect to changing beam conditions and detector problems. In the initial LHC running, events have been selected online based on the Level-1 selections with the HLT algorithms run but not rejecting any events. This has been an important step in the commissioning of these triggers to ensure their correct functioning and subsequently to enable the HLT selections. Since then, electrons from J/Psi, bottom, charm, W and Z decays, prompt photons and a first sample of tau hadronic decays from W have been efficiently selected. This talk gives an overview of the implementation of the electron, photon and tau trigger trigger selection algorithms and the first experience running these triggers online. Selected examples from the various co mmissioning steps will be shown. The software performance of the system will be discussed in terms of stability, timing and physics sample selected.