The transition radiation detector of the alpha magnetic spectrometer(AMS-02) on the International Space Station and the precision measurement of the cosmic ray positron fraction

Abstract

On May 19 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> 2011, the AMS-02 experiment was successfully deployed on board the Earth-orbiting International Space Station (ISS) and is constantly operating since then without interruption. AMS is a complex particle detector in space and conducts research in astroparticle physics by precisely measuring charged cosmic particles to study acceleration, propagation and contributions from new particle physics and astrophysical sources. This is made possible by the main features of the AMS detector, namely its large acceptance, the long duration mission about 20 years limited by the ISS lifespan and the unique high proton rejection power by combining the electromagnetic calorimeter and the transition radiation detector (TRD). The TRD is designed to separate positrons from the huge proton background with a high efficiency. A precision measurement of the positron fraction in the energy range from 0.5 to 350 GeV is presented. The accurate data show that the positron fraction is steadily increasing from 10 to 250 GeV with a decreasing slope by an order of magnitude between 20 and 250 GeV. The positron fraction spectrum shows no fine structure and no observable anisotropy. These observations show the existence of new physical phenomena whether from a particle physics or an astrophysical origin.