Marco Cirelli: Indirect Searches of Dark Matter, and the Hooperon
As a way of introduction, Marco said that DM exists. It is a feebly interacting corpuscle. It dilutes with the volume of the universe as the universe expands. The rest is wishful thinking. Is it a weakly-interacting particle ? Many of us believe in the WIMP miracle, of a particle with weak-scale mass and weak interactions, which gives automatically the correct abundance that you need in cosmology.
The range of possible candidates spans 90 orders of magnitude in mass! Several possible hints have appeared all around the board. The Hooperon, the Bulbulon, the Wenigon, the Pamelon (credit of the naming goes to Adam Falkowsky from the resonaances blog).
One can search for DM annihilation in the center of the galaxy. This produces fluxes of charged particles. We look for antimatter because this reduces astrophysical backgrounds.
The problem is that the diffuse flux this generates must reach you before you collect a signal. It can spiral around, lose energy, be blown away by galactic wind, suffer spallations on the gas in the disk in-between the stars. Finally, they have to go through the heliosphere, the influence of the sun’s magnetic field. All of this is complicated astrophysics.
The way we deal with it is to solve a complicated diffusion equation, within a sort of tuna-can box which encapsulates the galaxy which traps the particles due to magnetic fields. The size of the box is unknown, so if the box is 1kParsec thick you get little, but if it is less thick you expect to collect larger signals.
We can determine plausible values of the unknown parameters that determine the propagation. In a specific model of propagation you can go from production to detection, but if you move to another model things of course change.
Gamma rays are easier as they propagate linearly. One can have also secondary emission of gamma rays, from outside the center of the galaxy. Inverse compton scattering also provides a signal: positrons from DM annihilation can produce TeV-scale gamma rays coming to us. A second type of secondary radiation is the one from bremsstrahlung of electrons and positrons. This is very relevant at low energy.
Dark matter can produce photons in a number of ways, and one has to consider all of them for a complete analysis. Prompt and secondary emission can each take on different forms, producing lines and sharp features or more diffuse spectra. Secondary emission depends on the environment. It is important, and different kinds of this are important in different regimes.
Let us talk about the Hooperon as a typical example of claim. It is that there is a diffuse excess from around the Galaxy center. Published in 2010. But millisecond pulsars, which are distributed spherically as the Hooperon around the galaxy center. The excess can thus be due to the pulsars. But the Hooperon is not totally ruled out. They redid the analysis in 2014 and found that the residuals can be fit by DM annihilating into b-quark pairs, with a reasonable annihilation cross section. They even claimed a 40 sigma effect. [Editor note: 40 sigma means a probability of the null hypothesis of 10^-350 or so, so this is beyond ridiculous as a claim, as nobody will ever be able to know systematic effects in the tails to such an astounding accuracy.]
If a DM annihilation goes into b-quark pairs, one should look for the associated constraints coming from antiproton fluxes. These are relevant but not robust. Propagation models here are crucial, as different choices of propagation parameters can give an exclusion of the signal region or allowance for it. In conclusion, there are choices that may allow the Hooperon to survive.
In conclusion, we have hints (claims of dark matter) in gamma rays, positrons, x-rays; and we have seen constraints coming from gammas, antiprotons, neutrinos, and cosmology. The sad thing is that the constraints do not kill all these hints. There are hopes, to rest on future experiments. The hope is also that all these ideas will benefit from new theory directions. As a final result, any convincing result must be multi-messenger.