Performance of the electromagnetic calorimeter of the AMS-02 experiment on the International Space Station and measurement of the positron fraction in the 1.5 - 350 GeV energy range

The AMS-02 experiment is a particle detector installed on the International Space Station (ISS) since May 2011, which measures the characteristics of the cosmic rays to bring answers to the issues risen by the astroparticle physics since a few decades, in particular the study of dark matter and the search of antimatter. The phenomenological aspects of the physics of cosmic rays are reviewed in the first part. In a second part we describe the in-flight performances of the different sub-detectors of AMS-02, in particular the electromagnetic calorimeter. It is shown, using particles at the ionizing minimum (MIPs), accounting for the main part of cosmic rays, that the calorimeter works as expected, and we find the same performances as on ground. This study is used to follow in time the evolution of the detector performances. It also allows the development of a charge estimator for the nuclei using the calorimeter. A third and final part, deals with the determination of the positron fraction. The main difficulty of this measurement is to identify the positrons by rejecting the protons thanks to the characteristics of the showers in the calorimeter. After having defined variables relevant for this separation, we build an estimator using a multivariate analysis and Monte-Carlo simulations of electrons for higher energies. Above 100 GeV, we obtain a rejection factor of about 10.000 at a 90% efficiency. After having estimated the charge confusion, this estimator, finally, allows us to determine the positron ratio for the first 18 months of data and energies ranging from 1.5 to 350 GeV. (author)