Search for dark matter annihilations towards the inner Galactic halo from 10 years of observations with H.E.S.S (1607.08142v1)

Abstract: The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using $\gamma$-ray observations towards the inner 300 parsecs of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant $\gamma$-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section $\langle \sigma v\rangle$. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach $\langle \sigma v\rangle$ values of $\rm 6\times10^{-26} cm^3s^{-1}$ in the $W^+W^-$ channel for a DM particle mass of 1.5 TeV, and $\rm 2\times10^{-26} cm^3s^{-1}$ in the $\tau^+\tau^-$ channel for 1 TeV mass. For the first time, ground-based $\gamma$-ray observations have reached sufficient sensitivity to probe $\langle \sigma v\rangle$ values expected from the thermal relic density for TeV DM particles.

• The paper presents a comprehensive search for dark matter annihilation in the inner Galactic halo using a decade of H.E.S.S. observations, deriving stringent upper limits on the annihilation cross-section.
• It employs a 2D maximum likelihood method that analyzes both spatial and spectral gamma-ray data to robustly measure background events.
• The results set unprecedented constraints on dark matter models by limiting the annihilation cross-section for TeV-scale WIMP interactions.

Analysis of Dark Matter Annihilation in the Inner Galactic Halo Using H.E.S.S.

This paper presents a comprehensive paper by the H.E.S.S. Collaboration focused on detecting dark matter (DM) annihilation events within the inner Galactic halo. The data comprises ten years of $\gamma$-ray observations conducted by the High Energy Stereoscopic System (H.E.S.S.) array, targeting the central 300 parsecs of the Milky Way.

Observational Motivation and Methodology

The density of dark matter in the inner regions of the Milky Way makes it an ideal site to search for DM self-annihilation. The investigation utilizes the rich data set obtained from H.E.S.S., which provides sensitivity to very-high-energy (VHE) $\gamma$-rays. The absence of a significant $\gamma$-ray signal above the background enables the derivation of stringent upper limits on the annihilation cross-section of DM particles covering a mass spectrum from 160 GeV to 70 TeV.

The analysis employs a 2D maximum likelihood method that accommodates both spatial and spectral properties of the DM $\gamma$-ray signal. The background events are robustly measured using regions symmetric to the signal regions, thereby enhancing the method's reliability.

Statistical Analysis and Results

The paper derives upper limits on the DM velocity-weighted annihilation cross-section $\langle \sigma v \rangle$, assuming both Einasto and Navarro-Frenk-White DM density profiles. The results are significant, attaining a cross-section value of $$\langle \sigma v \rangle = 6 \times 10^{-26} \, \text{cm}^3\text{s}^{-1}$$ for DM interactions via the $W^+W^-$ channel at a DM mass of 1.5 TeV, under the Einasto profile assumption. Similarly, analyses in the $\tau^+\tau^-$ annihilation channel produce limits of $2 \times 10^{-26} \, \text{cm}^3\text{s}^{-1}$ for a 1 TeV mass.

These constraints are unparalleled in the TeV mass range, offering the most stringent limits known from H.E.S.S. observations and surpassing prior measurements from Fermi-LAT observations of dwarf galaxies for masses above 400 GeV.

Implications and Future Work

The non-detection result implies the derived constraints on DM models and cosmological DM density assumptions. The limits are crucial for refining DM particle physics models, specifically for weakly interacting massive particles (WIMPs) that predict $\langle \sigma v \rangle$ consistent with the thermal relic scenario.

This paper sets the stage for subsequent works, notably utilizing the ongoing H.E.S.S. II observations with lowered energy thresholds and improved detector sensitivities. Future investigations will focus on wider Galactic regions and optimized observation strategies to further reduce uncertainties in the DM density profiles and provide improved chances of detecting DM signatures.

In conclusion, the meticulous work conducted by the H.E.S.S. Collaboration exemplifies the intersection of advanced observational techniques and rigorous statistical methods to enhance the quest for understanding dark matter, pushing the boundaries of current experimental physics.