Bubbles support \(10\GeV\) or \(50\GeV\) dark matter

March 2013 is expected to be a great dark matter month, especially due to the eagerly expected results from AMS-02 that may emerge as early as the next week (ANTARES has seen nothing a few days ago). Joseph S. has brought my attention to an excellent astro-ph paper by Tracy Slatyer (IAS) and Dan Hooper (FNAL)

Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter

that eliminates all doubts that the authors belong among the very top of the world's astroparticle physics. They looked at the spectrum of the Fermi bubbles – that Tracy co-discovered – and decided to write down the most natural model(s) that explain(s) the observer spectrum. What the models depend upon – and what the observations should therefore clarify – is what is the spectrum of electrons, the radiation, and masses and dominant decay channels of hypothetical dark matter particles that team up to produce the spectrum.

I think that they show their ability to split the data into regions that seem to be dominated by different effects, explain the partial datasets, and design economic theories that are able to explain several features of some partial datasets simultaneously. What does it mean in practice?




In practice, it means that they divided the picture of the galaxy to two regions, according to the latitude (angular distance from the galactic center). For the "tropical" and "polar" regions, they found two different spectra and two different models that explain them.

What are they?




Far from the galactic plane, at least 30 degrees from the equator, they see that the spectrum ceases to depend on the latitude. The gamma rays over there may apparently be described by inverse Compton scattering. Note that the inverse Compton scattering is the process \[

e^-+\gamma\to e^-+\gamma

\] in which the photon's energy increases (the photon steals some energy from the initially fast electron): that's why it's inverse. This description works if the electrons obey a power law spectrum and if they collide with an interstellar radiation field. Cutely enough, this model may also explain the microwave "haze" assuming that the electrons are moving in a magnetic field whose intensity is a microgauss or so.

For the "tropical" region, they need a different description and a different mechanism, however. The reason is that the spectrum seems to have a peak near \(1-4\GeV\) if \(E^2\,\dd N/\dd E\) is graphed against \(E\). The inverse Compton scattering is no good to produce such peaks. Instead, they decide that this part of the dataset is similar to results previously reported from the Galactic Center. The luminosity per volume seems to decrease as the \(r^{-2.4}\) power law with the distance from the Galactic Center.



The chief NASA administrator and a rapper gives a not terribly comprehensible but sufficiently stringy introduction to dark matter.

An important part of the answer is that this radiation seems to be consistent with the annihilation of dark matter particles! It's either due to \(10\GeV\) dark matter particles pair-annihilating into lepton pairs or due to \(50\GeV\) dark matter particles annihilating into quark-antiquark pairs. They seem to propose two comparably likely scenarios for the possible mass and dominant interactions of the dark matter particles.

Note that none of these two scenarios should be new to the TRF readers. A possible \(10\GeV\) dark matter particle has been discussed many times in the context of the "Is Dark Matter Seen" war between the direct search experiments. The allies in the "Dark Matter Is Seen" coalition do generally claim that they see collisions with numerous \(10\GeV\) or sub-\(10\GeV\) dark matter particles. The "Dark Matter Is Not Seen" axis is vehemently rejecting all these assertions.

However, even dark matter particles around \(50\GeV\) have previously been spotted by careful TRF readers. In January 2012, I mentioned that Virgo favored a \(20-60\GeV\) dark matter particle. This was based on a fresh preprint by Han et al. who looked into Virgo, Fornax, and Coma clusters through the Fermi satellite and concluded something remarkably similar to Slatyer and Hooper: there is either a \(20-60\GeV\) dark matter particle annihilating into \(b\bar b\) quark pairs, or a \(2-10\GeV\) dark matter particles annihilating into \(\mu^+\mu^-\).

Because this Han et al. paper doesn't seem to be cited by Tracy and Dan, you could view the agreement as a strong piece of circumstantial evidence that the possible masses and dominant annihilation channels of the dark matter are pretty much what Slatyer and Hooper say now. This sociological argument has only one delicate problem, aside from its being sociological and therefore worthless: in July 2012, Han et al. II wrote a new paper in which they added several new point sources. It seems they thought that they had to add them. Their original signal got contaminated and all the peaks of "extended emission" went away.

Now, another half a year later, Slatyer and Hooper are reviving a statement very similar to the Han et al. statement from January 2012. Cosmology entered a high-precision era 15 years ago but even when it depends on solid fellow disciplines such as astroparticle physics, you may see that it sometimes takes a lot of time to pick the right answers in similar uncertain situations and to choose the winners in assorted dark matter wars.

In these wars, March 2013 could turn out to be an analogous month to June 1944. I hope that some readers know what the D-Day means ;-) – the Battle of Stalingrad wasn't good enough for the previous sentence because it was too long and its date was therefore too fuzzy. At any rate, stay tuned. Things could get very exciting and very convincing very soon.