Tuesday, February 18, 2014

7 keV sterile dark matter?

It's a good day when you wake up and see the U.S. medal in your favorite Winter Olympic sport (SBX), and you see a blog post at Resonannces with a good discussion about a topic of interest: dark matter.
The Resonnances blog post discusses a manuscript by Bulbul et al. recently uploaded to Arxiv about X-ray emission lines ~3.5 keV that can't be attributed to known atomic spectra. The authors of the manuscript attribute the emission to sterile dark matter particles with a mass of ~7 keV. Though, it should be noted that there are other, less likely, explanations to the emission at 3.5 keV. The manuscript discusses some of the other possible explanations. As seen below in the graph at the Resonannces website, the emission line at 3.5 keV is consistent with other experiments, and in region of parameter space that has yet to be ruled out.

(Image from http://resonaances.blogspot.com/)

What I'd like to add to the discussion is that this value of dark matter mass is very close to the 95% confidence window from computer simulations by  Horiuchi et al., whose 95% confidence window as 6-10 keV in one set of data and 8-13 keV in a second set of data. (shown below)

While there's still a large amount of uncertainty about what is the cause of dark matter, it appears that there is starting to be some convergence between experiments and computational simulations. And I hope that the recently submitted manuscript by Bulbul et al. will convince NASA to fund more research into analyzing X-rays in the ~0.5 keV to ~5 keV range as possible signals of sterile neutrinos decaying into fertile neutrinos. Of course, the term sterile and fertile neutrino are misnomers because sterile neutrinos aren't completely sterile (w.r.t. to the weak nuclear force) or else they wouldn't be able to decay to normal neutrinos, and it should be pointed out that normal neutrinos, electrons and quarks are not always fertile (w.r.t. to the weak nuclear force) because as they zig-and-zag, they go between being fertile and sterile.

I also want to point out that it does seem intuitively strange that "mostly" sterile neutrinos are heavier than the "mostly" fertile, normal neutrinos. This seems to violate the trend that the fundamental particles with more mass also have more forces with which they can interact. Therefore, it's important to point out that there is still a lot fundamental physics that we don't understand, even if it turns out that dark matter is ~7 keV sterile neutrinos.

Update: Here's a link to a paper by a separate group that also found a 3.5 keV signal in the X-ray spectra from two galaxies.

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