Speaker
Description
Dark matter need not be electromagnetically dark. Through the Higgs-to-diphoton loop mechanism and graviton exchange in perturbative quantum gravity, non-zero $\gamma\chi \to \gamma\chi$ cross sections arise without - or with only minimal extensions to - the Standard Model. Both UV-complete channels, however, produce optical depths far below observational reach: the gravitational cross section falls short of CTA sensitivity by 46-54 orders of magnitude, while the Higgs-mediated channel constrains only WIMPZilla-scale dark matter using Fermi-LAT photon flux data.
To identify which dark matter-photon interactions could ever be observable, we scan all gauge-invariant effective operators for scalar, Majorana, and Dirac dark matter. Our observable is distinct from prior work: rather than annihilation spectra or direct-detection recoils, we constrain operators through the spectral attenuation of background photons - the frequency-dependent suppression imprinted on the diffuse gamma-ray sky by $\gamma\chi$ scattering. This channel is orthogonal to collider and direct-detection constraints. We extract the first exclusion curves derived strictly from Galactic diffuse emission morphology. Utilising NFW-weighted optical depth calculations benchmarked against Via Lactea II subhalo parameters, we embed our spatial attenuation model directly into a full multi-component MCMC template fit of the Fermi-LAT sky, following Totani (2025). Embedding the attenuation model directly into this sky-map fit modifies the inferred DM annihilation cross section required to explain the observed 20 GeV halo excess, offering a physically motivated path to resolving existing tensions with dwarf galaxy bounds.
We extend the $\gamma\chi$ cross section to curved spacetime, deriving for the first time the polarisation-resolved scattering amplitudes on a Schwarzschild background for both weak and gravitational interaction channels. A tower of photon-sphere resonances is identified in the $\perp$-polarisation channel. While these resonances do not amplify astrophysical signals sufficiently to offset Planck-scale suppression locally, they hold profound cosmological implications: for Primordial Black Holes (PBHs) near the Hawking evaporation threshold, these resonant frequencies intersect the Cosmic Microwave Background (CMB) band. This mechanism provides a novel, non-evaporative channel to constrain cosmological PBH abundances via localised, secondary modifications to the CMB lensing power spectrum. Finally, the polarisation ratio between the two interaction channels - a direct, model-independent signature of spin-2 graviton exchange absent in the Higgs-portal case - constitutes a clean target for upcoming gamma-ray polarimeters. We present full polarisation maps around Sagittarius A* and compare against projected sensitivities of COSI and AMEGO-X.
