Potential dark matter signals from the galactic center

I would like today to come back to the topic of dark matter.

Recently, a couple of puzzling excesses have been observed in cosmological data and physicist are trying to provide explanations.

The nice features of all those excesses is that they can (kind of) easily be interpreted as signals for dark matter.

Of course, although the dark matter explanation is very appealing, it is today too early for being able to conclude about this explanation being correct or wrong.

I will describe below the excesses I want to talk about and how dark matter could help in re-establishing the agreement between data and theory.

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DARK MATTER IN A NUTSHELL

I already wrote long ago about what is dark matter in general (please check here if interested). I will thus only recapitulate a little bit and provide the minimum requirement for the understanding of this post.

Dark matter consists of one of the building blocks of the standard model of cosmology.

This conclusion results from a large amount of phenomena that can be easily explained by the presence of dark matter. We have for instance the rotation curves of the galaxies, cosmic microwave background data or big bang nucleosynthesis.

We must however keep in mind that even if dark matter is one of the best option, alternatives also exist (like when gravity is modified). Those alternatives are very-well alive and not excluded at all.

In this article, I will however restrict myself to the dark matter hypothesis.

Thus assumes that a large fraction of the matter in the Universe consists of a form of matter that is dark.

This actually means that dark matter is matter-like in the sense that it interacts gravitationally, and it is dark for the reason that it does not interact electromagnetically.

The true nature of dark matter is however unknown. Moreover, as there is no candidate for dark matter in the Standard Model of particle physics, the nature of dark matter is one of the most important issue of cosmology and particle physics.

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WHERE TO SEARCH FOR DARK MATTER?

One of the most popular candidate for dark matter is the so-called WIMP candidate, an acronym (yeah again) standing for Weakly-Interacting Massive Particles.

Such particles are very common in many new physics scenarios and their phenomenological consequences in terms of potential signals make them easily searchable in the sky and at colliders.

In particular, WIMP are expected to indirectly show themselves in gamma rays. An example is shown on the picture of the left where gamma ray sources are depicted.

Although I have already mentioned the mechanism behind such an indirect detection of dark matter in my previous post on the HAWC observatory, let me again recapitulate it a little bit.

One targets regions of the universe that are known to be dense in dark matter. As a result, dark matter particles are very likely to annihilate into Standard Model particles, and in particular into photons (that are gamma rays). We then only need to wait and observe these gamma rays on Earth.

One possible source for these gamma rays consists of dwarf galaxies, and this is the starting points of the search at the HAWC observatory, for instance.

But another very good region of the Universe that is full of dark matter is the center of the Milky Way. There is a strong evidence for the presence of a supermassive back hole there, but the galactic center is also believed to contain a very large amount of dark matter.

This is indeed what comes out of many standard models for dark matter distributions in the galaxy.

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GAMMA RAY EXCESS IN THE GALACTIC CENTER

The Fermi-LAT satellite (or NASA's Fermi Gamma-ray Space Telescope) is one of such experiments investigating the properties of the gamma rays. Data from the galactic center is currently exhibiting a slight excess than cannot be explained by standard mechanisms.

Physicists have indeed tried many (standard) hypotheses to explain the excess, ranging from undiscovered pulsars to cosmic ray collisions on gas clouds, but without any success.

On the image to the right, we observe what is left once all contributions from these standard sources of gamma rays are removed. It is unambiguous. There is something (the green-red spot in the middle of the picture).

Even after considering that the astrophysical noise may have been underestimated, the excess stays. In other words, even when considering the large uncertainties on the results and predictions, there is something in there.

We are therefore left to try explanations based on new phenomena, like, for instance, dark matter.

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EXCESSES IN COSMIC RAYS FROM AMS-02

Cosmic rays are also good probes for new phenomena and they are probed within many experiment.

AMS, whose name stands for the Alpha Magnetic Spectrometer, is one of these. It consists a module mounted on the International Space Station that aims to study the properties of different types of cosmic rays from various energies.

The picture of this device is shown on the left

In 2013, the AMS collaboration released its results and announced an unknown excess in the flux of positrons (the antimatter particle associated with the electron). The scientific article can be found here .

The conclusions are that there is no (standard) way to explain the relative differences observed in the amount of electron and positron cosmic rays and one must rely on something new.

One possible option for this 'something new' could be, once again, dark matter and in particular WIMP annihilation in the galactic center (see here for more information).

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DARK MATTER SEMI-ANNIHILATIONS

The reason of this post is that I recently read this article where the authors propose a common explanation for both the above excesses.

The idea is to consider a dark matter candidate that is a WIMP, but that semi-annihilates instead of annihilating. This consists of a process where two dark matter particles annihilate into a new dark matter particle and a Higgs boson or a Z-boson.

They have shown that both mechanisms (the one with the Higgs and the one with the Z-boson) yield setups consistent with data. This means that those models first reproduce the excesses and then allow to escape all existing experimental constraints on dark matter.

Those models however offer the advantage of being less severely constrained by dark matter direct detection experiments, and may thus survive better in the future in the light of future data.

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TAKE-HOME MESSAGE AND REFERENCES

In this article, I discussed excesses in cosmology that could be explained by the presence of dark matter. Both the considered excesses originate from the galactic center.

I have depicted a new dark matter semi-annihilation mechanism that has been proposed by some colleagues to explain those excesses.

Some further pieces information can be found in the following publications:

• Information on dark matter and the galactic center can be found here
• Information on gamma ray excesses in the galactic center found by Fermi-LAT can be found here.
• The AMS homepage is here.
• A possible WIMP explanation for the AMS and Fermi-LAT excesses has been detailed in this article.
• A bunch of wikipedia pages are very nice, like this one on dark matter or this one on gamma rays