Measure of the positrons fraction in cosmic rays with AMS-02

AMS-02 (Alpha Magnetic Spectrometer) is a cosmic rays detector installed in May 2011 on the International Space Station (ISS); data taking started few days after the installation and is expected to go on for at least 10 years. Cosmic rays are particles arriving on Earth from the Space and are mainly composed of protons, nuclei and electrons; a small component is due to anti-protons and positrons which are thought to be created by interactions of nuclei with the interstellar gas and therefore their amount is expected to decrease with energy. Recent results from the PAMELA experiment show an increase with the energy of the e+/(e− + e+) ratio (positrons fraction): this unexpected behaviour could be a mark of dark matter (DM) annihilation. AMS-02 will verify the PAMELA results and will extend the positrons fraction measurement up to ≈ 1 TeV with high accuracy. As protons are the main background for this measure, since p/e+ ∼ 10^4, AMS must provide a proton rejection factor at 10^5 ÷ 10^6 level. This thesis will focus on the use of AMS electromagnetic calorimeter ECAL to attain the required rejection power. ECAL is a lead segmented calorimeter capable of measuring e− and e+ energy with a few percent resolution over a wide energy range (1 GeV up to 1 TeV). ECAL longitudinal and lateral segmentation allows a 3D reconstruction of the shower, so to enable the requested discrimination between electromagnetic and hadronic showers to reduce proton background. A multivariate analysis approach has been adopted to separate leptons from hadrons: the algorithm chosen was a Boosted Decision Tree (BDT), which uses many of the variables constructed from ECAL raw data to classify events as signal or background. Results on the positrons fraction as preliminarily measured by AMS-02 will be presented with an estimation of the main systematic errors