Sunday, July 5, 2015

Updates on 3.55 keV line

Update 8/15/2017
I just wrote a new post. I still think that dark matter is sterile neutrinos. However, I'm leaning now towards their masses being on the order of 10^10 GeV  (ten to the ten GeV), i.e.
For more information on the predictions of Type I Seesaw models, see this Jan 2017 arxiv manuscript by Stephen F. King. This model appears to be quite predictive as far as being able to predict the angles for neutrino mixing as well as the CP violation term, delta. The CP violation term is very nearly equal to -90 degrees, which means near maximal CP violation.

Original Post
Well, it looks like the emission peak around 3.55 keV may not actually be from a decaying sterile neutrino. In a previous post, I wrote about the news of a possible sterile neutrino with a rest mass of 7.1 keV. However, recent experimental data does not appear to back up this claim.

For example, see the following papers:

Where do the 3.5 keV photons come from? A morphological study of the Galactic Center and of Perseus
Carlson et al. Jan 2015

Constraints on 3.55 keV line emission from stacked observations of dwarf spheroidal
Malyshev et al. Aug 2014

Discovery of a 3.5 keV line in the Galactic Centre and a critical look at the origin of the line across astronomical targets
by Jeltema and Profumo Aug 2014

The main evidence (as it seems to me) against the claim for the 7.1 keV particle with a sin squared 2 theta of ~10^-10 is that there is essentially no signal from dwarf spheroidal galaxies.

But all is not lost for sterile neutrinos as warm dark matter. It is still possible to have a sterile neutrino with a smaller value of sin squared 2 theta. A smaller value of theta is possible if there is a larger lepton asymmetry in the universe. The constraints on lepton asymmetry in the universe is extremely weak because N_eff is very loosely constrained. Lepton asymemtry of +/- 0.1 is still entirely possible, whereas only a value of 0.001 would be needed to stay below the constraints set by X-rays emission from dwarf spheroidal galaxies.

As such, it's possible that the sterile neutrino could have a smaller value of theta and still be consistent with cosmological constrains on N_eff.

Finally, I'd like to point out that most of the papers that analyze X-ray emission in order to put constraints on dark matter are pretty ad hoc (the same applies to papers using Gamma-ray emission to detect Cold Dark Matter.) There is dark matter all over the place. Everywhere you look, there's dark matter. It's fairly evenly distributed through the universe. True, it's slightly lumpy here and here, but there would be X-ray signal from every direction (with some redshift depending on how far away the source is.)
As such, researchers in this area needs to be a much systematic about searching for X-ray (or gamma-ray) emissions from possible dark matter. This means doing a correlation analysis against dark matter lensing maps (after subtracting off known sources of X-rays or gamma-rays.)
(Such as detailed by Fornengo and Regis or Zandanel et al. on how to do these correlations, though not actually done by them.)
This is not easy to do because you need to know the density of dark matter as function of the distance from us across the whole sky. But we know have maps of dark matter density vs. z and we have X-ray emissions as a function of energy. It should be possible to do a full-sky analysis of decaying dark matter (rather than just this silly looking at few galaxies with known, unknown sources of X-rays.)

This means that, in order to claim a 'detection' of a decaying dark matter particle, researchers need to match their signal with actual data on the 3D density of dark matter (i.e. two spatial dimension plus distance, z.)

If anybody is aware of such a detailed, full-sky correlation analysis between dark matter and X-ray emission, please provide me a link in the comment section below.

1 comment:

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