Identification of positrons and electrons in the cosmic radiation with the electromagnetic calorimeter ECAL for the AMS-02 experiment

In May 2011 AMS-02 detector has been successfully installed on the International Space Station (ISS), where it will take data on cosmic radiation from 1 to 1000 GeV for at least 10 years. Among all scientific objectives of the experiment, one of the most important is the search for Dark Matter (DM), which constitutes 80% of the Universe matter, but its nature is still unknown. A DM signal can be identified by studying the combined fluxes of positrons, photons, antiprotons and antideuterium. Thanks to its high acceptance and its performances, AMS-02 detector can extend primary cosmic ray physics search to a new energy range with high accuracy. A key role for these measurements, in particular for the electromagnetic channels, is played by ECAL calorimeter. This subdetector has been developed to measure e− and e+ energy with an accuracy of few %. Thanks to its 3D shower reconstruction imaging capabilities, it also has a high separation power between electromagnetic and hadronic showers (e/p rejection), essential to eliminate the proton background in the positron channel. Finally, it provides the trigger on photons which do not interact in the upper part of the detector (about 72% of the ones in ECAL geometrical acceptance). In Chapter 1 of this thesis, cosmic ray physics is introduced with details on Big Bang cosmology and on the DM problem. A summary of direct and in particular indirect searches for DM signature is presented. In Chapter 2 and 3 AMS-02 detector is presented with an overview of each subdetector features and performances. ECAL electromagnetic calorimeter is described in detail. Chapter 4 describes a flight equalization method, which has been developed and tested on August 2010 Test Beam data, with its application performances on ground and on flight data. In Chapter 5, the calorimeter capabilities have been used to develop e± identification algorithms, using both ECAL standalone and also tracker momentum measurements. The definition of algorithms, training and testing processes, data-MC comparisons and proton rejection spectrum are described.