Prospects for detecting heavy WIMP dark matter with the Cherenkov Telescope Array: The Wino and Higgsino

TeV-scale particles that couple to the standard model through the weak force represent a compelling class of dark matter candidates. The search for such weakly interacting massive particles has already spanned multiple decades, and whilst it has yet to provide any definitive evidence for their existence, viable parameter space remains. In this paper, we show that the upcoming Cherenkov Telescope Array (CTA) has significant sensitivity to uncharted parameter space at the TeV mass scale. To do so, we focus on two prototypical dark matter candidates, the Wino and Higgsino. Sensitivity forecasts for both models are performed including the irreducible background from misidentified cosmic rays, as well as a range of estimates for the Galactic emissions at TeV energies. For each candidate, we find substantial expected improvements over existing bounds from current imaging atmospheric Cherenkov telescopes. In detail, for the Wino we find a sensitivity improvement of roughly an order of magnitude in $⟨\sigma v⟩$, whereas for the Higgsino we demonstrate that CTA has the potential to become the first experiment that has sensitivity to the thermal candidate. Taken together, these enhanced sensitivities demonstrate the discovery potential for dark matter at CTA in the 1–100 TeV mass range.

Figure 1

Left panel: VHE astrophysical emissions in the GC region. The energy differential fluxes are plotted for the ${\pi }^0$ (pink open triangles) and ICS (cyan open triangles) components of the galactic diffuse emission measured by Fermi-LAT extracted from Ref. [105], and for the ${\pi }^0$ (pink filled triangles) and ICS (cyan filled triangles) components derived from an alternative analysis [106] performed in the inner $15°\times 15°$ of the GC. The filled and open triangles are used for fitting a power law or a power law with exponential cutoff, and then we use the fitted curves to normalize the maps in the CTA energy range. The H.E.S.S. Pevatron spectrum (red dots) and the low-latitude spectrum of the Fermi bubbles (blue filled squares) [95] are also shown. Conservative (dashed blue line) and optimistic (log-dashed blue line) parametrizations of the Fermi bubbles component are plotted. Note that the region corresponding to the Galactic ridge emission is excluded from the region of interest (see Sec. 4b). Right panel: expected differential count rate as a function of energy for the expected signal and backgrounds in the ROI (5,11), a $0.5°\times 0.5°$ squared pixel centered at $l=2.75°$, $b=-0.25°$ (see Sec. 4b for more details on the ROI definition). The expected DM rates are plotted for a 3 TeV Wino with an annihilation cross section of $2.3\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ (orange solid line), and for a 1 TeV Higgsino with an annihilation cross section of $1.1\times {10}^{-28}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ ($9.2\times {10}^{-29}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$) (green solid lines) assuming $\delta m_N=200\mathrm{keV}$ ($\delta m_N=2\mathrm{GeV}$) and $\delta m_{+}=480\mathrm{MeV}$ ($\delta m_{+}=350\mathrm{MeV}$). For the background we show the residual background, the backgrounds for GDE scenarios 1 and 2, and the maximum Fermi bubbles contribution. The DM signal curves correspond to the full spectra, including the line component, and have been convolved with the CTA energy resolution.

Figure 2

Galactic diffuse emission maps in Galactic coordinates. The maps are expressed in terms of the flux $\mathrm{\Phi }$ integrated in the energy bin centered at 1 TeV with width $\mathrm{\Delta }{\log }_{10}(E/1\mathrm{TeV})=0.2$, and in a pixel of size $0.5°\times 0.5°$. The maps are given for the GDE scenario 1 (top panels) and scenario 2 (bottom panels). The separate ${\pi }^0+\mathrm{Brem}$ (left panels) and ICS (right panels) components are shown.

Figure 3

Fermi bubbles emission maps in Galactic coordinates for the conservative (left panel) and the optimistic (right panel) models. The maps are expressed in terms of flux integrated in a pixel of size $0.5°\times 0.5°$ and in the energy bin centered at 1 TeV with bin width $\mathrm{\Delta }{\log }_{10}(E/1\mathrm{TeV})=0.2$.

Figure 4

Expected upper limits at 95% C.L. on the Wino annihilation cross section as a function of its mass for 500 h of CTA observations towards the GC. The predicted NLL cross section is shown (solid gray line) and the thermal Wino DM mass is marked (cyan solid line and bands). The only background considered here is the residual background. The full Wino spectrum is included in the expected signal. Left panel: mean expected upper limits at $2\sigma$ (red solid line) for an Einasto profile are shown together with the $1\sigma$ (green band) and $2\sigma$ (yellow band) containment bands. Mean expected upper limits at $5\sigma$ (red dashed line) are also shown. The H.E.S.S.-like $2\sigma$ sensitivity extracted from Ref. [68] is shown as a blue solid line. Right panel: the expected limits are shown for cored DM profiles of size from 300 pc to 5 kpc.

Figure 5

Impact on the 95% C.L. mean expected upper limits, on the Wino annihilation cross section, of the continuum and end point contributions to the Wino spectrum. The limits for the full Wino Spectrum (red solid line) are compared to those that do not include the continuum contribution (red dashed line). The major improvement on the sensitivity to the Wino with respect to a simple line comes from the end point gamma rays. The effect of the continuum on the limits is much smaller than the end point contribution, to a greater degree than observed with H.E.S.S., due to the improved CTA energy resolution.

Figure 6

95% C.L. expected upper limits on the line Higgsino annihilation cross section as a function of its mass for the Einasto profile (red solid line) and cores of size from 300 pc to 5 kpc. The theoretical cross section is printed in gray. Top left panel: limits computed assuming mass splittings $\delta m_N=200\mathrm{keV}$ and $\delta m_{+}=350\mathrm{MeV}$. The mean expected limits are shown at $2\sigma$ (red solid line) and $5\sigma$ (red dashed line), respectively. Top right panel: limits computed assuming mass splittings $\delta m_N=2\mathrm{GeV}$ and $\delta m_{+}=480\mathrm{MeV}$. Bottom panels: 95% C.L. expected mean upper limits for CTA on the Higgsino annihilation cross section as a function of its mass, for an Einasto DM profile and 500 hour homogeneous exposure in a 10°-side squared region centered at the GC region. The expected limits (red solid line) are shown together with the $1\sigma$ (green band) and $2\sigma$ (yellow band) containment band obtained from the Asimov dataset. Only the residual background is considered here. The predicted leading order cross section is shown (solid gray line) and the thermal Higgsino DM mass is marked (cyan solid line and bands). The sensitivity is computed for the mass splittings $\delta m_N=200\mathrm{keV}$ and $\delta m_{+}=350\mathrm{MeV}$ (bottom left panel) and $\delta m_N=2\mathrm{GeV}$ and $\delta m_{+}=480\mathrm{MeV}$ (bottom right panel). The line-only constraints are shown as red dotted lines.

Figure 7

Impact on the 95% C.L. expected mean upper limits, for the Wino and Higgsino annihilation cross sections, of the astrophysical sources of gamma-ray emission that contribute to the overall background. The limits are shown for residual background only (red solid line) and with additional Fermi bubbles (FB) emission (blue solid line), GDE gamma rays (cyan solid line), and VHE Fermi-LAT pointlike (PL) sources (black solid line). The most impactful background component is the Fermi bubbles, assuming the “FB max” spectrum. The PL sources have a negligible effect. The impact on the constraints for the Wino DM is given in the left panel, and for Higgsino DM in the right panel assuming splittings $\delta m_N=200\mathrm{keV}$ and $\delta m_{+}=350\mathrm{MeV}$.

Figure 8

95% C.L. expected mean upper limits on the Wino annihilation line cross section as a function of the DM core radius for a 3 TeV Wino. The limits are shown for the case of residual background only (red dots), for the case of additional astrophysical background from the GDE emission (green dots) in scenarios 1 (green filled dots) and 2 (green empty dots), and for all backgrounds (black dots).