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Gamma-ray production in the cosmic-ray -- dark matter scattering as a probe of the axion-like particle -- proton interaction

Victor P. Goncalves, Emmanuel Moulin, Igor Reis, Aion Viana

Abstract

The production of very-high-energy (VHE, $E_γ \gtrsim 100$ GeV) gamma rays resulting from the scattering of high-energy cosmic-ray protons off axion-like particles (ALPs) populating the dark matter halo of the Milky Way is investigated. By employing the latest instrument response functions for current and future facilities, we demonstrate that ground-based VHE gamma-ray observatories, such as H.E.S.S., CTAO, and SWGO, provide a promising and complementary avenue to probe the yet uncharted ALP-proton coupling $g_{ap}$. Our results show that these experiments can reach sensitivity to couplings above $10^{-2}$ in the $1 - 10^{8}$ eV ALP mass range, a region that remains largely unexplored by supernova and neutron star cooling observations. Interestingly, we demonstrate that this search channel is capable of probing QCD axion dark matter models, assuming two benchmark models for it: the Kim-Shifman-Vainshtein-Zakharov (KSVZ) Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) models, specifically within the MeV mass range. These findings highlight the potential of VHE gamma-ray astronomy to provide unique constraints on the interaction between ALPs and the baryonic sector.

Gamma-ray production in the cosmic-ray -- dark matter scattering as a probe of the axion-like particle -- proton interaction

Abstract

The production of very-high-energy (VHE, GeV) gamma rays resulting from the scattering of high-energy cosmic-ray protons off axion-like particles (ALPs) populating the dark matter halo of the Milky Way is investigated. By employing the latest instrument response functions for current and future facilities, we demonstrate that ground-based VHE gamma-ray observatories, such as H.E.S.S., CTAO, and SWGO, provide a promising and complementary avenue to probe the yet uncharted ALP-proton coupling . Our results show that these experiments can reach sensitivity to couplings above in the eV ALP mass range, a region that remains largely unexplored by supernova and neutron star cooling observations. Interestingly, we demonstrate that this search channel is capable of probing QCD axion dark matter models, assuming two benchmark models for it: the Kim-Shifman-Vainshtein-Zakharov (KSVZ) Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) models, specifically within the MeV mass range. These findings highlight the potential of VHE gamma-ray astronomy to provide unique constraints on the interaction between ALPs and the baryonic sector.
Paper Structure (6 sections, 6 equations, 3 figures)

This paper contains 6 sections, 6 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Gamma - ray production via proton CR - ALP interactions
  3. Methodology
  4. Observational strategy
  5. Statistical analysis
  6. Results and Discussion

Figures (3)

  • Figure 1: Gamma - ray production via ALP - proton interaction. The blue blob represents the ALP - proton coupling.
  • Figure 2: Energy differential gamma-ray flux for the expected ALP-CRp IC scattering signal, as well as the expected residual background from a $5^\circ$ region around the GC region. Here we assume three different observatories: a H.E.S.S.-like array, the CTAO and the SWGO strawman design SWGOirf. The expected fluxes assume an ALP of mass $m_a = 1$ keV.
  • Figure 3: Sensitivity on the ALP-proton coupling $g_{ap}$ versus ALP mass $m_a$. The sensitivity is expressed as 95% C.L. mean expected upper limits. The sensitivity in the ($m_a$, $g_{ap}$) plane are derived assuming the IC scattering of ALPs and CRp in the GC region, as observed by a H.E.S.S.-like observatory, CTAO and SWGO. We also show exclusion regions from SN1987A observations, SNO, Torsion Balance and Casimir, all extracted from Ref. AxionLimits. The golden curve and shaded region represents the portion of the parameter space corresponding to QCD Axions.