Monday, May 07, 2012

A confirmation of the 130 GeV dark matter-like bump

Three weeks ago, I discussed Christoph Weniger's finding that the Fermi satellite seems to show an excess of photons whose energy is close to 130 GeV in several regions of the sky.

A photograph of our galaxy using 120-140 GeV photons is on the left (with the location of the bubbles that have nothing to do with the excess, as is shown today). Smoothed excess signal is on the right. Circles indicate disk-shaped regions with the maximum significance. Click to zoom in.

Today, the second paper about this emerging discovery – the first independent confirmation – was published:
Fermi 130 GeV gamma-ray excess and dark matter annihilation in sub-haloes and in the Galactic centre (arXiv)
Three Estonian authors, Elmo Tempel, Andi Hektor, and Martti Raidal (with some Swiss affiliation as well) have found an even stronger signal than Weniger.

Their approach differs from Weniger's in some basic respects. Weniger tried to look for the strongest signals in places with the strongest gamma-ray radiation. One may essentially say that this assumed that the strength of the 130 GeV signal is correlated with the overall gamma-ray flux. This led him to identify the "optimum regions for a strong signal". And these regions inevitably overlapped with the shape of various structures that the Fermi satellite is seeing, in particular the Fermi bubbles.

This strategy has still worked for Weniger but it was easy to misinterpret the results. Some people were led to believe that he has shown that the 130 GeV line had something to do with the Fermi bubbles. However, he actually had no evidence of this sort, as I tried to explain to some commenters. He just chose the Fermi-bubble-like regions by his very methodology – the bubbles got imprinted to his the shape of his regions because of his assumptions.

The Estonian authors avoid this trap altogether. They just look for disk-shaped regions (parameterized by their radius and their location) in which they may find the strongest signal (using a method to reconstruct a probability distribution by smoothing the discrete photon data). And they find a couple of disks – now it's just ordinary disks and not some contrived shapes – where the signal reaches up to 4.5 sigma. The best region is a 3° vicinity of the galactic center and there are a few other, weaker disks, too.

Off-topic: Liberation Festival 2012 in Pilsen, some scenes. The number of troops who actually liberated our town is going down every year but I hope that they're happy that someone still knows what they stood for.

I think that Weniger should still be credited with the discovery but I do agree with the Estonians that their method to locate the position of the signal is more meaningful. In particular, they may show very clearly that the highest-signal places have no significantly correlated overlap with the Fermi bubbles so you should forget about this non-dark-matter-based explanation of the excess.

They also propose their own "model" of the dark matter particle. It seems somewhat contrived to me. They assume just one new scalar but because its couplings to the photons would be too weak, they assume that the cross section is enhanced by a resonance so they need a new 290 GeV heavy particle, too. One might say that they're constructing their minimal particle content "from nothing" which could be a bad idea; the assumption that SUSY has nothing to do with the dark matter is one of the "pillars" (or, more precisely, one of the tons of sand) that underlie their phenomenology.

Their particular combinations of channels and intermediate particles requires a 145 GeV particle which is the same figure I got in my previous blog entry – and that probably reproduced a calculation that Gordon Kane did previously because he told me the same result – but the Estonian folks use somewhat different logic to accidentally end up with the same mass.

Of course, one should be able to separate their cold work with the experimental data from their speculative musings about the model building... The evidence for a new particle that much/most of the dark matter is composed of has gotten stronger once again.

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