Probing Dark Photon Dark Matter with CTAO

Abstract

The dark photon is a new hypothetical gauge boson arising in extensions of the Standard Model, and constitutes a compelling dark matter candidate. As dark photon dark matter (DPDM), it can interact with electromagnetic fields via kinetic mixing, and the inelastic scattering process $γγ' \to e^+ e^-$ becomes kinematically allowed for gamma rays above a characteristic energy threshold. This interaction imprints unique spectral attenuation features at very-high-energies (VHE), offering an observational probe of DPDM models. Using the Cherenkov Telescope Array Observatory (CTAO) Instrument Response Functions (IRFs), we simulate observations of VHE sources and forecast novel sensitivities to the kinetic mixing parameter for the photon-dark photon scattering process. Our study focuses on three key astrophysical targets: the Crab Nebula and the blazars Markarian 421 and Markarian 501. Additionally, we investigate the impact of dark matter spikes around black holes on the upper limits. Our results demonstrate that CTAO can probe the DPDM parameter space down to a mixing parameter of $\varepsilon \sim 10^{-8}$ for masses around $m_{A^{\prime}} \sim 10^{-1}~\textrm{eV}$ through high-energy spectral attenuation, at a $95\%$ confidence level.

Figures (12)

• Figure 1: Photon–dark-photon scattering cross section $\sigma_{A'}$ as a function of the photon energy for representative dark-photon masses. Solid (dashed) lines correspond to $\varepsilon = 10^{-3}$ ($10^{-6}$).
• Figure 2: Radial dark matter density profiles for Mrk 421 (blue) and Mrk 501 (red) host galaxies, modeled with a dark matter spike (parameters in Table \ref{['tab:profiles']}). The dotted black line shows the Milky Way's NFW profile for comparison, normalized to the local density at $r_\odot$ (black dotted line).
• Figure 3: Line-of-sight (LOS) integrals of the dark matter mass for the host galaxies of Mrk 421 (solid lines) and Mrk 501 (dash-dotted lines), computed for different source emission regions $R_{\mathrm{em}}$, where $R_S$ denotes the Schwarzschild radius of the central SMBH.
• Figure 4: The black points show one simulated CTAO flux realization for the Crab Nebula as $E^2 dN/dE$ across 25 logarithmically-spaced energy bins. The blue line represents the best-fit log-parabola ($H_0$), while the red line illustrates the dark photon scattering attenuation effect for $m_{A^\prime} = 0.1~\mathrm{eV}$ and $\varepsilon = 5\times10^{-4}$. The best-fit parameters for the null hypothesis are shown in Table \ref{['tab:bestfit']}.
• Figure 5: The black points show one simulated CTAO flux realization for Mrk 421 as $E^2 dN/dE$ across 25 logarithmically-spaced energy bins. The blue line represents the best-fit ECPL function ($H_0$), while the red line illustrates the dark photon scattering attenuation effect for $m_{A^\prime} = 1~\mathrm{eV}$ and $\varepsilon = 5\times10^{-8}$. The best-fit parameters for the null hypothesis are shown in Table \ref{['tab:bestfit']}.