Probing thermal-relic dark matter with the Inner Galaxy Survey

September 2022

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Something is wrong when we look at galaxies in space, because the formation of the entire structure and the way billions of stars run through space cannot be explained by visible matter alone. A possible solution to this problem is to postulate the existence of a lot of dark matter in addition to the luminous one. Many experimental efforts have been carried out in the last decades and repeatedly confirm that 85% of the total matter content in the Universe is made of the so-called dark matter (DM). The investigation of the Coma galaxy cluster in the early 1930s brought the astronomer Fritz Zwicky to conclude that the combined mass of all stars in the individual galaxies in the galaxy cluster could not explain the total mass necessary to explain the movements of the galaxies in the cluster [1]. Among other evidence for DM are the formation of large-scale structures in the universe, the kinematics of stars in galaxies and the way light bends near massive clusters of galaxies. Nevertheless, the nature of DM is still a mystery.

A compelling solution to the DM problem is to assume it is made of a new kind of particles. Some of the properties of these particles can be inferred from observations. They are not Standard Model particles, which are usually detected or produced in experiments on Earth. DM interacts almost only gravitationally, therefore it is not easily detected with direct observatories. These particles should be massive, electrically neutral, and should not interact via the strong force; they should move with non-relativistic velocities, be almost collisionless with ordinary matter and itself, and should be stable or have a lifetime comparable to the age of the Universe. Weakly Interactive Massive Particles (WIMPs), which arise in many extensions of the Standard Model of particle physics, are among the most promising candidates to satisfy these criteria. Sufficiently massive WIMPs could annihilate in regions of the sky with high mass density. Following a complex transition of hadronization, radiation and decay of the Standard-Model particles produced in the annihilation process, gamma rays will eventually emerge from the annihilation and can be detected with H.E.S.S.

The most promising targets from which to detect such an indirect DM signal with H.E.S.S. are the Galactic Center (GC) region [2,3], nearby dwarf galaxies satellites of the Milky Way [4,5,6,7, SOM August 2019 and SOM January 2021], and DM subhalos populating the Galactic halo [8, SOM July 2021]. The center of the Milky Way is predicted as the brightest source of DM annihilation in the Universe because of its proximity and the high DM density in this region. The DM density distribution in the GC region can be conveniently described by the Einasto profile parametrization [9].

The H.E.S.S. collaboration is currently surveying the GC region to cover the inner several hundred parsecs and achieves the best possible sensitivity for DM annihilation signals and other astrophysical phenomena. This Inner Galaxy Survey (IGS) is the first ever conducted deep very-high-energy (VHE, E> 100 GeV) gamma-ray survey of the Galactic Center region [11]. To cover the so far unexplored regions in VHE gamma rays, the IGS observations used here are performed with a grid of telescope pointing positions. The complete dataset, collected between 2014 and 2020, includes a total of 546 hours of high-quality observations with at least ten hours of time exposure up to b ≈ +6°. The exposure obtained for the H.E.S.S. IGS dataset is shown in Fig. 1 in Galactic coordinates. This dataset is used to search for a DM-induced gamma-ray signal that is distinguishable against background gamma rays given the expected spectral and spatial properties of the anticipated signal.

Fig. 1: H.E.S.S. Inner Galaxy survey (IGS) exposure map in Galactic Coordinates. The IGS pointing positions and the position of the supermassive black hole Sagittarius A* are shown as black crosses and black triangle, respectively. (Image credit: H.E.S.S. Collaboration [11]).

WIMP annihilations would produce a flux of gamma rays depending on the amount of DM in the source and its distance to the observer on Earth, usually expressed by the so-called J-factor, as well as the annihilation cross section between the two DM particles. No significant signal expected from DM annihilations is detected by H.E.S.S. in the present dataset. Therefore, upper limits on the DM density can be computed. For an assumed distribution of DM that follows a so-called Einasto cuspy profile, the annihilation cross section <σv> of the DM particles can be constrained.

Fig. 2 shows the observed 95% upper limits on the cross-section as a function of the (unknown) DM particle mass for the annihilation into W+W- particles, one of the possible end products of DM annihilation, derived from the full dataset (solid black line). All <σv> values above the solid black line are excluded. Our limits challenge in the TeV mass range the thermal-relic <σv> values (shown as the horizontal dashed grey line), which is fixed to the level of the annihilation cross section that is expected if all of the DM was thermally created in the Early Universe [11].

Fig. 2: H.E.S.S. IGS constraints from 546 hours of high-quality data on the annihilation cross section for DM particles annihilating into the W+W- annihilation channel for an Einasto DM density profile. These observed limits can be compared to the expected H.E.S.S. limits and the value of the natural scale for <σv> expected for thermal-relic WIMPs. (Image credit: H.E.S.S. Collaboration [11]).
Fig. 3 shows a summary of the main upper limits on the DM annihilation cross section in the TeV mass range. Limits obtained with observations of the GC region are shown for the previous H.E.S.S. analysis of 254 hours of observations [2] as well as for HAWC and Fermi-LAT observations. Limits obtained with measurements from the cosmic microwave background with Planck and with observations of dwarf spheroidal galaxies with Fermi-LAT are shown too.
Fig. 3: The H.E.S.S. IGS constraints are compared to previous H.E.S.S. constraints and recent results obtained with other experiments. (Image credit: H.E.S.S. Collaboration [11]).

The new limits obtained by H.E.S.S. improve the previous ones significantly and are the most constraining limits so far in the TeV mass range. They challenge natural annihilation cross section values expected for the thermal-relic WIMPs in the TeV DM mass range. The improved sensitivity results from the observation strategy carried out within the IGS program and the use of the full five-telescope array. H.E.S.S. observations of the central region of the Milky Way are unique for studying in detail WIMP models [11]. These observations constitute an unprecedented dataset to explore the yet-uncharted parameter space of multi-TeV DM models. The IGS observation program is an important H.E.S.S. legacy and paves the way for future observations [10].


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[11] Abdalla, H. et al. "Search for dark matter annihilation signals in the H.E.S.S. Inner Galaxy Survey", accepted for publication in Physical Review Letters, 2022, arXiv:2207.10471