Constraining the dark matter origin of the halo-like 20 GeV $γ$-ray excess with the AMS-02 antiproton data

TL;DR

This work tests whether the recently reported ~20 GeV gamma-ray excess in the Milky Way halo could be explained by dark matter annihilation with $m_\chi \sim 0.5-0.8$ TeV and $\langle \sigma v \rangle \sim (5-8)\times10^{-25}\ \mathrm{cm^3\,s^{-1}}$ by examining the associated antiproton flux observed by AMS-02. Using GALPROP to model cosmic-ray propagation (diffusion with re-acceleration, solar modulation via the force-field approximation) and incorporating both secondary antiprotons and a DM-induced component for $b\bar b$ and $W^+W^-$ channels, the authors propagate DM scenarios through a constrained transport parameter ensemble obtained from AMS-02 and related data. They find that the DM-induced antiproton flux, especially around $E\sim100$ GeV, would overshoot the AMS-02 measurements by substantial margins (up to $\sim15-18\sigma$ for the two channels), and the total flux including secondaries would exceed data across the energy range. When compared with the gamma-ray–inferred signal region, the antiproton constraints imply a cross section about $\sim$40 times ($<2\times10^{-26}\ \mathrm{cm^3\,s^{-1}}$) smaller than what would be required to explain the gamma-ray excess, leading to the conclusion that the 20 GeV halo gamma-ray excess is not a viable DM signal. The result strengthens the case for alternative astrophysical explanations and demonstrates the constraining power of AMS-02 antiprotons on DM interpretations of gamma-ray features.

Abstract

Very recently, a significant $\sim 20$ GeV gamma-ray excess in the Milky Way halo has been reported and a dark matter origin has been suggested. The inferred dark matter parameters are $ m_χ\sim 0.5-0.8 $ TeV and $ \langle σv \rangle \sim (5-8) \times 10^{-25}~{\rm cm^3~s^{-1}}$ for the $ b\bar{b} $ channel. If correct, prominent antiproton emission is produced and can be directly tested by the AMS-02 data. In this work we calculate the corresponding antiproton emission and show that the expected flux at $\sim 100$ GeV is already above the AMS-02 observation. A proper treatment on the antiproton background resulting from the high energy cosmic ray propagation would suggest an annihilation cross section of $< 2\times 10^{-26}~{\rm cm^3~s^{-1}}$, which is a few$\times 10$ times lower than that needed to interpret the potential signal. We therefore conclude that the $\sim 20$ GeV gamma-ray excess in the Milky Way halo is not a viable dark matter signal.

Figures (2)

• Figure 1: Antiproton spectra produced by dark matter annihilation through different channels. The left panel shows annihilation into $b\bar{b}$, while the right panel shows annihilation into $W^+W^-$. The green band represents the background antiproton flux (secondary production), the red band denotes the contribution from dark matter annihilation, and the blue band indicates the total (background + DM) flux. The error bars correspond to AMS-02 antiproton data published in 2021 (black) and 2025 (red). The total flux at 100 GeV exceeds the AMS-02 measurements by $15\sigma$ for the $b\bar{b}$ channel and by $18\sigma$ for the $W^+W^-$ channel. Here $\sigma$ refers to the experimental uncertainty of AMS-02 data.
• Figure 2: Comparison of dark matter annihilation signals derived from different astrophysical messengers. The red region in the upper-right corner represents the $2\sigma$ signal range inferred from Totani's work. The central green region corresponds to the $2\sigma$ annihilation signal obtained using AMS-02 antiproton data, while the black curve denotes the corresponding 95% confidence-level upper limit.