This poster shows approved plots from https://twiki.cern.ch/twiki/bin/view/Atlas/ElectronGammaApprovedCosmicPlots and will be presented at the Hadron Collider Physics Symposium 16th -20th November 2009 in Evian

Kraus, J

Schmitt, C

Von Törne, E

Wermes, N

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ATL-PHYS-SLIDE-2009-36423November2009First observation of electronsin the ATLAS detectorJana Kraus, on behalf of the ATLAS collaborationCosmic rays and the ATLAS detector    •   The ATLAS detector as one of four         experiments at the LHC in Geneva        providing high luminosity proton-proton         collisions at 14 TeV promises precise         measurements of Standard Model           parameters and searches for new             physics phenomena (Higgs, SUSY,...)         with very small cross sections•   Therefore it is necessary to perform      accurate and efficient measurements of    the different particles occuring in the      detector•    No data from collisions is available yet,        but cosmic rays (muons) penetrate the     whole volume of the detector from above         at random times (1 Hz/m2)•   They can be used to test and improve       the performance of ATLAS (calibration,      alignment, trigger...) and to investigate            the efficiency in detecting and identifying        particles, e.g. electrons Electrons from cosmic rays in   ATLAS    •  Reconstructed electrons in      ATLAS are objects, which leave        a cluster in the electromagnetic        calorimeter and a track in the        inner detector matching the        position of the cluster•  In the cosmic ray data        electrons originate primarily           from ionisation of atoms in the         inner detector by the muons (at      least 2 tracks)•  Fake electrons are produced       by high energetic muons,             which leave a cluster in the      calorimeter by emitting        bremsstrahlung (only 1 track)    Isolation of an electron sample in cosmic data     •  The definition of events with one track as a background-like and with at least two tracks as a signal-like          sample and the application of the cuts enables an identification of real electrons from ionisation (36 events) •   Background estimation: A twodim. binned maximum likelihood fit is applied to the E/p vs. HT TRT ratio         distribution of the ionisation electron candidates excluding all events, which overcome the cuts. The integral         of the fitfunction over this excluded signal region yields the number of remaining background events (~20%)     •  This first observation of electrons demonstrates both the excellent commissioning of the Inner         Detector with efficient Transition Radiation and of the calorimeter and it makes confident that early         electrons will be reconstructed and identified in ATLAS. Electron identification    •  Electrons have typical attributes        concerning the shape of the shower       in the calorimeter, the track quality     and the matching of cluster and       track, which are used as cut criteria       to identify real electrons and reject      background events •  For cosmic ray data some standard    electron cuts have to be modified,     because the events don't originate      from the center of the detector•  Important variables are:      E/p, Energy measured in the        calorimeter over momentum        measured by the track, this variable      is around 1 for electrons        Ratio of high to low threshold            hits in Transition Radiation Tracker,         electronic signal caused by transition         radiation overcomes high threshold,         electrons should produce a higher         fraction (probability ~ 1/mass)     Sketch of electron production by   muon ionisation Sketch of fake electron production   by muon bremsstrahlung  Eoverp vs. fraction of high threshold TRT hits for events with at least two tracks, lines show   cuts applied to bulk of events      electron candidates after application of several identification cuts (85)      final electron candidates after application of additional cuts shown here (36)  Eoverp vs. fraction of high threshold TRT hits for events with only one track, lines show cuts   applied to bulk of events      electron candidates after application of several identification cuts (1229)      final electron candidates after application of additional cuts shown here (19)Event display of a typical electron produced by muon ionisation(muon track in yellow, electron track and electromagnetic cluster/cells in green, TRT hits in blue, high threshold TRT hits in red)  Typical shower shape variable for electrons - the fraction of total energy left in first layer of   electromagnetic calorimeter. Distribution of final ionisation electrons compared to Monte   Carlo generated single electron events  Projection of twodimensional background fit to E/p distribution for electron candidates from   ionisation after application of all cuts except on E/p

First observation of electrons in the ATLAS detector

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