Abstract
For many dark matter models, the annihilation cross section to two-body final states is difficult to probe with current experiments because the dominant annihilation channel is velocity or helicity suppressed. The inclusion of gauge boson radiation for three-body final states can lift the helicity suppression, allowing a velocity-independent cross section to dominate the annihilation process, and providing an avenue to constrain these models. Here we examine experimental constraints on dark matter that annihilates to two leptons plus a bremsstrahlung boson, $\bar{\ell}+\ell+\gamma/W/Z$. We consider experimental constraints on photon final states from Fermi-LAT using both diffuse photon data and data from dwarf spheroidal galaxies, and compare to the implied constraints from 21 cm measurements. Diffuse photon line searches are generally the strongest over the entire mass regime. We in particular highlight the model in which dark matter annihilates to two neutrinos and a photon, and show that these models are more strongly constrained through photon measurements than through existing neutrino bounds.
Highlights
The annihilation cross section, ðσvÞann, is one of the key quantities that describes the nature of dark matter interactions with the Standard Model
We use Fermi-LAT diffuse gamma-ray data and data from dwarf spheroidals (dSphs), as well as the 21 cm observation of EDGES, to constrain the primarily two to two p-wave component DM annihilation models via their s-wave components arising from radiating a photon or electroweak gauge boson
The dominant annihilation channel can vary over the course of cosmic history due to a velocity dependence that will dramatically suppress p-wave annihilations relative to s-wave for nonrelativistic dark matter
Summary
The annihilation cross section, ðσvÞann, is one of the key quantities that describes the nature of dark matter interactions with the Standard Model. There exist s-wave annihilation channels to two fermion final states, for example, but these are chirality suppressed by mass ratio factors of ðmf=mDMÞ2 [7] This can be contrasted with dark matter in the Galactic halo, which has a virial velocity v2 ∼ 10−6, and thereby reduces the observational importance of the p-wave process. The degeneracy does not need to be extreme in order to obtain a large effect from vector bremsstrahlung, which provides a natural region of parameter space that can be probed by experiment These considerations suggest that, in the case of dark matter annihilation to fermion pairs, final state radiation and internal vector bremsstrahlung of photons and the electroweak W=Z bosons are irreducible processes which present an interesting target for observational searches.
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