[Paper Review] The cosmic ray electron and positron spectra measured by AMS-02

C. Corti|arXiv (Cornell University)|Feb 3, 2014

Dark Matter and Cosmic Phenomena1 references3 citations

TL;DR

This paper presents the first high-precision measurement of cosmic ray electron and positron spectra using the AMS-02 detector on the International Space Station, analyzing over 30 billion events in two years. It reports a steadily increasing positron fraction up to ~250 GeV, a rising positron flux above 30 GeV, and a smooth, slowly falling electron spectrum, with systematic uncertainties still under evaluation.

ABSTRACT

The AMS-02 detector is operating on the International Space Station since May 2011. More than 30 billion events have been collected by the instrument in the first two years of data taking. A precision measurement of the positron fraction and of the positron flux in primary cosmic rays up to 350 GeV, of the electron flux up to 500 GeV and of the combined electron plus positron flux up to 700 GeV are presented. The separate and combined electron and positron fluxes are preliminary and represent work in progress. Systematic uncertainties must still be investigated further.

Motivation & Objective

• To measure the cosmic ray electron and positron spectra with unprecedented precision using the AMS-02 detector in low Earth orbit.
• To determine the positron fraction and flux in primary cosmic rays up to 350 GeV, and electron flux up to 500 GeV.
• To investigate the origin of the observed positron excess in cosmic rays, distinguishing between astrophysical sources and dark matter annihilation.
• To validate the performance and stability of the AMS-02 detector over two years of operation in space.
• To provide a high-statistics dataset (9 million electrons, 6.8 million positron/electron events) for future astrophysical modeling and comparison with other experiments.

Proposed method

• AMS-02 uses a multi-layered detector system including a silicon tracker, transition radiation detector (TRD), time-of-flight (TOF) counters, ring imaging Cherenkov (RICH), electromagnetic calorimeter (ECAL), and anti-coincidence counters (ACC).
• Electron and positron identification is achieved through combined use of energy-momentum matching (E/p > 0.75), 3D shower shape in the ECAL, and TRD-based transition radiation detection.
• A Boosted Decision Tree (BDT) algorithm is applied to the ECAL shower shape to optimize electron identification and minimize proton contamination.
• Systematic uncertainties are evaluated using data-driven methods, including trigger efficiency, track reconstruction, and BDT selection efficiency, with Monte Carlo simulations based on GEANT4 for calibration.
• The analysis applies loose preselection to retain downgoing relativistic particles (β > 0.8) with signals in TRD and ECAL, and enforces a 1.25× Stoermer cutoff requirement to reject atmospheric background.
• Efficiency corrections are applied using probe samples and template fitting, with stability tested across varying BDT cut thresholds.

Experimental results

Research questions

• RQ1 What is the energy dependence of the cosmic ray positron fraction up to 350 GeV, and does it show any structure indicative of dark matter?
• RQ2 How does the electron flux vary from 0.5 to 500 GeV, and does it follow a smooth, hardening or softening trend?
• RQ3 What is the behavior of the combined electron and positron flux when multiplied by E³, and how does it compare to theoretical expectations?
• RQ4 How stable and reliable are the electron and positron identification efficiencies across the full energy range, and what are the dominant systematic uncertainties?
• RQ5 To what extent do the observed spectra support or rule out astrophysical sources such as pulsars as the origin of the positron excess?

Key findings

• The positron fraction increases steadily from 10 GeV to ~250 GeV, with the slope decreasing by an order of magnitude between 20 and 250 GeV.
• No fine structure or sharp features are observed in the positron fraction spectrum, suggesting a smooth energy dependence.
• The electron flux, multiplied by E³, rises up to 10 GeV and then follows a smooth, slowly decreasing trend above that energy.
• The positron flux, when multiplied by E³, increases up to 10 GeV, remains flat from 10 to 30 GeV, and rises again above 30 GeV, indicating a distinct spectral index from the electron spectrum.
• The measurement of the electron spectrum is based on ~9 million electrons selected from over 30 billion triggered events, with a 10% sample of the expected total AMS data.
• Systematic uncertainties are still under finalization, but the trigger efficiency is 100% above a few GeV, and track reconstruction efficiency agrees with Monte Carlo within 1% across a wide energy range.