Towards Understanding the Origin of Cosmic-Ray Positrons and Electrons

Abstract

Precision measurements of cosmic-ray positron flux and electron flux by the Alpha Magnetic Spectrometer on the International Space Station are presented based on 1.9 million positrons up to 1 TeV and 28.1 million electrons up to 1.4 TeV. The positron flux and electron flux have distinctly different magnitudes and energy dependences. 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. The finite cutoff energy of the source term, $E_s$, is determined to be $810^{+310}_{-180}$ GeV, with a significance of more than $4 \sigma$. The electron flux exhibits a significant excess starting from $42.1^{+5.4}_{-5.2}$ GeV compared to the lower energy trends, but the nature of this excess is different from the positron flux excess. Below 1.9 TeV, the electron flux does not have an exponential energy cutoff at more than $5 \sigma$ level. These experimental data show that, at high energies, positrons predominantly originate either from dark matter collisions or from new astrophysical sources, whereas most high energy electrons originate from different sources than high energy positrons.