First observation of electrons in the ATLAS detector

The special topology of cosmic events traversing all subdetectors offers the unique opportunity to investigate the combined performance of ATLAS in identifying and reconstructing particles before first collisions. High-energy delta electrons in cosmic data are studied which are produced by cosmic muons through ionisation of the inner detector material. A method of separating knock-on electrons from the large background of muon bremsstrahlung is presented accounting for the special nature of cosmic events and utilizing the ATLAS tools to identify electrons with their characteristic properties. The resulting isolation of a sample of 32 delta electrons out of 3.5 million cosmic ray events with a high-level trigger track candidate in the inner detector barrel has for the first time enabled an observation and investigation of real electrons in ATLAS.