$e^\pm$ Excesses in the Cosmic Ray Spectrum and Possible Interpretations

TL;DR

Multiple cosmic ray experiments observe an electron/positron excess above 100 GeV, prompting various astrophysical and dark matter models; current data favor astrophysical explanations due to less exotic assumptions.

Contribution

This review synthesizes observational data and compares different theoretical models, highlighting the challenges in distinguishing between astrophysical sources and dark matter explanations.

Findings

Electron/positron excess observed above 100 GeV

Dark matter models require more exotic assumptions

Future observations can differentiate models

Abstract

The data collected by ATIC, PPB-BETS, FERMI-LAT and HESS all indicate that there is an electron/positron excess in the cosmic ray energy spectrum above $\sim$ 100 GeV, although different instrumental teams do not agree on the detailed spectral shape. PAMELA also reported a clear excess feature of the positron fraction above several GeV, but no excess in anti-protons. Here we review the observational status and theoretical models of this interesting observational feature. We pay special attention to various physical interpretations proposed in the literature, including modified supernova remnant models for the $e^\pm$ background, new astrophysical sources, and new physics (the dark matter models). We suggest that although most models can make a case to interpret the data, with the current observational constraints the dark matter interpretations, especially those invoking annihilation, require much more exotic assumptions than some astrophysical interpretations. Future observations may present some ``smoking-gun'' observational tests to differentiate among different models and to identify the correct interpretation to the phenomenon.