Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5–10 improvement in sensitivity in the 100 GeV–10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

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 pc 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 $⟨\sigma v⟩$. 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 $⟨\sigma v⟩$ values of $6\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ in the $W^{+}{W}^{-}$ channel for a DM particle mass of 1.5 TeV, and $2\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ in the ${\tau }^{+}{\tau }^{-}$ channel for a 1 TeV mass. For the first time, ground-based $\gamma$-ray observations have reached sufficient sensitivity to probe $⟨\sigma v⟩$ values expected from the thermal relic density for TeV DM particles.

A search for a very-high-energy (VHE; $\ge 100\mathrm{GeV}$) $\gamma$-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance $r\sim 45–150\mathrm{pc}$ from the Galactic center. The background-subtracted $\gamma$-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) $\gamma$-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual $\gamma$-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section $⟨\sigma v⟩$ as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of $\sim 1\mathrm{TeV}$, values for $⟨\sigma v⟩$ above $3\times {10}^{-25}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ are excluded for the Einasto density profile.

Gamma-ray line signatures can be expected in the very-high-energy ($E_{\gamma }>100\mathrm{GeV}$) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical $\gamma$-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H.E.S.S. $\gamma$-ray instrument, upper limits on linelike emission are obtained in the energy range between $\sim 500\mathrm{GeV}$ and $\sim 25\mathrm{TeV}$ for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic $\gamma$-ray line emission, flux limits of $(2\times {10}^{-7}–2\times {10}^{-5}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ and $(1\times {10}^{-8}–2\times {10}^{-6}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section $⟨\sigma v{⟩}_{\chi \chi \to \gamma \gamma }$ reach $\sim {10}^{-27}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$, based on the Einasto parametrization of the Galactic DM halo density profile.

Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac object PKS $2155-304$ ($z=0.116$) are used to derive upper limits at the 95% C.L. on the strength of the ALP coupling to photons, $g_{\gamma a}<2.1\times {10}^{-11}\mathrm{Ge}{\mathrm{V}}^{-1}$ for an ALP mass between 15 and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic field.