A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500 GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above ∼200 GeV the positron fraction no longer exhibits an increase with energy.
3Over the last two decades, there has been a strong interest in the cosmic ray positron fraction in both particle physics and astrophysics [1]. The positron fraction is defined as the ratio of the positron flux to the combined flux of positrons and electrons. The first results from the Alpha Magnetic Spectrometer (AMS) on the positron fraction were reported in [2]. They generated widespread interest [3]. In this Letter we report new results based on all the data collected during 30 months of AMS operations on the International Space Station (ISS), from 19 May 2011 to 26 November 2013. Due to the excellent and steady performance of the detector, and an increase of the data sample by a factor of 1.7, the measurement of the positron fraction is extended up to 500 GeV with improved precision.AMS detector.-The layout of the AMS-02 detector [4] is shown in Fig. 1. It consists of 9 planes of precision silicon tracker with two outer planes, 1 and 9, and the inner tracker, planes 2-8 [5]; a transition radiation detector, TRD [6]; four planes of time of flight counters, TOF [7]; a permanent magnet [8]; an array of anti-coincidence counters, ACC [9], inside the magnet bore; a ring imagingČerenkov detector, RICH [10]; and an electromagnetic calorimeter, ECAL [11]. The figure also shows a high energy positron of 369 GeV recorded by AMS. AMS operates without interruption on the ISS and is monitored continuously from the ground.The timing, location and attitude of AMS are determined by a combination of GPS units affixed to AMS and to the ISS. The AMS coordinate system is concentric with the center of the magnet. The x axis is parallel to the main component of the magnetic field and the z axis points vertically. The (y-z ) plane is the bending plane. The maximum detectable rigidity over tracker planes 1-9, a lever arm of 3 m, is ∼2 TV. Detector performance, described in detail in [2,4], is steady over time.Three main detectors provide clean and redundant identification of positrons and electrons with independent suppression of the proton background. These are the TRD (above the magnet), the ECAL (below the magnet) and the tracker. The TRD and the ECAL are separated by the magnet and the tracker. This ensures that most of the secondary particles produced in the TRD and in the upper TOF planes are swept away and do not enter into the ECAL. Events with large angle scattering are also rejected by a quality cut on the measurement of the trajectory using the tracker. The matching of the ECAL energy, E, and the momentum measured with the tracker, p, greatly improves the proton rejection.To differentiate between e ± and prot...
