1. In order to acquire the full physics potential of the LHC, the ATLAS electromagnetic calorimeter must be able to efficiently identify photons and electrons at energies in the wide range of 5 GeV to 5 TeV. The reconstruction of an electromagnetic object begins in the calorimeter, and the inner detector information determines whether the object is a photon - either converted or unconverted - or an electron. The ATLAS detector is currently in a fully operational state and has been collecting cosmic ray data during fall 2008 and summer 2009. Cosmic muons can produce electrons by ionization in the detector material. Since the probability of a muon inducing a delta-ray with high enough momentum to produce a standard e/ ɣ cluster in the electromagnetic calorimeter is low, a method of matching tracks to an EMTopoCluster is used. This allows for a study of electrons of momentum down to 0.5 GeV to be carried out, which is more suitable for the low energy electrons from the cosmic rays. This analysis is using the 3.5 M events from the ID cosmic trigger stream from the fall 2008 data. The ATLAS officially produced Monte Carlo cosmic samples are also included in the study. The 11 M events from the summer 2009 cosmic run is currently investigated. Elina Berglund - University of Geneva Electrons in Cosmic Ray Data with the ATLAS Detector CONCLUSION The search for the few electrons among the millions of muons in cosmic data is not an easy task. Due to the small acceptance of the ATLAS inner detector and the cosmic rays not originating from the primary vertex, the tracks are only required to have hits in the TRT (Transition Radiation Tracker) and not in the silicon detectors. A requirement of more than 25 hits in the TRT strips is applied to the tracks together with a fraction of high threshold TRT hits above 0.12. The Inner Detector geometryThe ATLAS Detector The track is extrapolated to an EMTopoCluster with additional requirements of Δφ between the track and the cluster if a match is initially found to be true. Selections are made on the moments of the cluster and E/p is required to be above 0.5, with the energy measured in the cluster and using the momentum of the track. Events with more than one track constitute a signal sample, assuming that one of the tracks originates from the muon, while events with only one track compose a background sample RESULTING ELECTRON CANDIDATES After applying all selection requirements on the signal and background samples from the 3.5 M fall 2008 data events, the number of remaining electron candidates can be seen in the table below: Track momentum > Signal sample 150k events before cuts Background sample 1.6M events before cuts 0.5 GeV40574 1 GeV35471 3 GeV17252 Atlantis event display of event with three tracks whereas one passes the cuts THE ANALYSIS PROCEDURE BACKGROUND ESTIMATION The fraction of high threshold TRT hits is plotted vs E/p for the signal and background samples after all other cuts have been applied, since these quantities are the most effective in distinguishing the electrons from the muons. This allows for a background and signal region to be defined, which is illustrated below. Fitting the background region of the background sample of the real data and the true events in the signal region of the MC data to the events passing the cuts gives a background estimate of 18.6% ± 7.2%, using TFractionFitter in Root. For the real data signal sample, this implies 75 ± 29 background events. TR ratio vs E/p in signal (left) and background sample for 2008 cosmic data The search for electrons in cosmic ray data together with the comparison to cosmic Monte Carlo is still in progress. However, by primarily considering the real cosmic data, the results indeed show a signal of low energy electrons. Using the above described analysis and background estimation indicates 405 electrons, whereas 75 ± 29 are background events. These electrons are the first to be identified by the ATLAS detector. INTRODUCTION TO ELECTRONS IN ATLAS