Towards Understanding the Origin of Cosmic-Ray Positrons

Abstract

Precision measurements of cosmic-ray positron flux by the Alpha Magnetic Spectrometer on the International Space Station are presented based on 1.9 million positrons up to 1 TeV. The positron flux exhibits distinct and complex energy dependence. The positron flux exhibits a significant excess starting from $25.2 \pm 1.8$\,GeV followed by a sharp drop-off above $284^{+91}_{-64}$\,GeV. In the entire energy range, the positron flux is well described by the sum of a diffuse term associated with low energy secondary positrons produced in the collision of cosmic rays, and a new source term of high energy positrons with a finite energy cutoff. This cutoff energy of the source term is determined to be $810^{+310}_{-180}$\,GeV with a significance of more than $4 \sigma$. These experimental data show that, at high energies, positrons predominantly originate either from dark matter collisions or from new astrophysical sources.