AMS-02 positron excess: new bounds on dark matter models and hint for primary electron spectrum hardening

TL;DR

This paper investigates the cosmic-ray spectrum hardening observed in various experiments, suggests it affects dark matter model bounds, and finds that primary electron spectrum hardening is necessary for accurate modeling, especially impacting dark matter annihilation constraints.

Contribution

It introduces the importance of primary electron spectrum hardening in cosmic-ray data analysis and refines dark matter model bounds considering this effect.

Findings

Primary electron spectrum hardening is needed in most models.

Dark matter annihilation channel χχ→μ+μ− is tightly constrained.

Decay channel χ→μ+μ− remains viable.

Abstract

The data collected by ATIC, CREAM and PAMELA all display remarkable cosmic-ray-nuclei spectrum hardening above the magnetic rigidity $\sim$ 240 GV. One natural speculation is that the primary electron spectrum also gets hardened (possibly at $\sim 80$ GV) and the hardening partly accounts for the electron/positron total spectrum excess discovered by ATIC, HESS and Fermi-LAT. If it is the case, the increasing behavior of the subsequent positron-to-electron ratio will get flattened and the spectrum hardening should be taken into account in the joint fit of the electron/psoitron data otherwise the inferred parameters will be biased. Our joint fits to the latest AMS-02 positron fraction data together with the PAMELA/Fermi-LAT electron/positron spectrum data suggest that the primary electron spectrum hardening is needed in most though not all modelings. The bounds on dark matter models have also been investigated. In the presence of spectrum hardening of primary electrons, the amount of dark-matter-originated electron/positron pairs needed in the modeling is smaller. Even with such a modification, the annihilation channel $\chi\chi \rightarrow \mu^{+}\mu^{-}$ has been tightly constrained by the Fermi-LAT Galactic diffuse emission data. The decay channel $\chi\rightarrow \mu^{+}\mu^{-}$ is found to be viable.