Monday, 28 May 2012

How to make a line

I mean a gamma-ray monochromatic line, like the one seen in the Fermi data. Recall the story so far. An independent analysis of publicly available data from the Fermi gamma-ray telescope found a peak in the photon spectrum near 130 GeV. It may be interpreted as a 130 GeV dark matter particle annihilating into a pair of photons with the cross section of about 2*10^-27 cm^3/sec (or a 145 GeV dark matter particle with the cross section twice as large annihilating  into a photon and a Z-boson). Since then the finding was confirmed by other independent groups and there's little doubt that the feature is present in the data. The question however is whether it is indeed a signal of dark matter, or whether it is due to a boring astrophysical process or to an instrumental effect.  Actually, 1 month after the finding, the Fermi collaboration put their own gamma-ray line study on arXiv in which they don't claim any signal but only set upper limits on the production cross section. Inspired by the ostrich tactics, Fermi does not mention Weniger's result at all, nevertheless one can see their limits are in some tension with the annihilation cross section quoted above. 

Anyway, in this post I will brush off the doubt and discuss what sort of dark matter models could explain the monochromatic gamma-ray line in the current Fermi data.  One definitely needs something non-trivial. Dark matter cannot carry an electric charge, therefore, in a generic model,  annihilation into photons is mediated by a loop diagram,  leading to the cross section naively suppressed by 3-4 orders of magnitude  compared to other annihilation channels. This would be problematic for two reasons. Firstly, given the strength of the Fermi line, that total annihilation cross section would by far exceed the thermal cross section 3*10^-26 cm^3/sec, so one would have to invoke some non-thermal mechanism of populating dark matter in the universe. Secondly, Fermi observations of the continuum gamma-ray flux from Milky Way satellite galaxies constrain the allowed annihilation cross sections to about 10^-25 cm^3/sec for a 130 GeV dark matter particle. the precise value depending on the annihilation channel.

For this reason, models fitting the Fermi line have to somehow shut off the tree-level annihilation channels and make the annihilation into photons dominant or nearly dominant. This is tricky and typically requires introducing a turtle standing on an elephant standing upon a giant turtle, as you can see on arXiv every day. Here I pick 3 models on the market that involve imo the smallest number of elephants.
  • Chern-Simon portal.In this model the mediator between the dark and visible matter is a Z' vector boson, who is coupled in a funny way to the Standard Model via the Chern-Simons term Z' Z γ (one can consider it an effective coupling after some exotic chiral fermions charged both under the Standard Model and Z' have been integrated out). One assumes dark matter is charged under Z' thus it can annihilate into a pair of Z' as well as (via the Chern-Simons coupling)  into Zγ. Since these two processes depend on two independent adjustable parameters, it is no surprise one can fit both the line (via annihilation into Zγ) and the thermal cross section (via annihilation into Z'Z'). 
  • Higgs is space!  Here Z' is also the mediator, however this time on the visible side it couples to the top quark (this can even be motivated in some composite Higgs or Randall-Sundrum constructions). This means dark matter annihilates dominantly into a pair of top quarks, and one can choose the couplings of Z' such that the cross section is the thermal one. But if the dark matter mass is lighter that the top mass the annihilation rate can be suppressed by phase space. On the other hand,  the subleading annihilation channels into Zγ or Hγ proceeding via a loop of the top quark do not suffer from that phase space suppression, therefore their rate can be relatively large, so as to explain the strength of the Fermi line.  
  • Box-shaped line. It is hard to annihilate dark matter into photons, but it is much easier to annihilate it into another exotic particle X who, at least part of the time, decays into photons. Of course, this in general does not produce a line, but rather a box-shaped photon spectrum,  as the observed photon energy gets smeared by the motion of the X particle. However, as the mass of X approaches that of dark matter (so that dark matter annihilates into X at rest) the box gets narrower and at some point becomes a line for all practical purpose. So the Fermi line could be explained e.g by a 265 GeV dark matter particle annihilating with the thermal cross section into a 260 GeV particle X, who in turn decays about 10% of the times into a pair of photons.
Sincerely, what do I think?  Well, Nature is a bitch, and that has been especially true with regard to dark matter. So far we've been denied any insight into the identity of the dark matter particle, in spite of tedious efforts in numerous direct and indirect detection or collider experiments. A monochromatic gamma-ray line -- an undeniable smoking gun of dark matter -- just sounds too good to be true. Thus, my best guess is that she's screwing with us again, and the line will be explained away by some instrumental effect.

Which means: hurry up with your gamma-ray line paper, before it's gone ;-)


Phil said...

There's at least one preprint claiming ( that a broken power-law fits the data as well as a line. Broken power-laws are two-a-penny in high energy astrophysics, so even if the origin of this one is unclear it sounds like a plausible suggestion.

Tony Smith said...

This is a naive question trying to get understanding of
"... a 130 GeV dark matter particle annihilating into a pair of photons with the cross section of about 2*10^-27 GeV/cm^3 ...".

What would be the corresponding cross section for
a 130 GeV Standard Model Higgs annihilating into a pair of photons in the same units of GeV/cm^3


Jester said...

2*10^-27 cm^3/sec (sorry, I screwed up the units in the previous version) is equal to about 0.1pb -- a typical value for a weak interaction process. Thus, the Higgs annihilation cross section would be of similar order, however, since Higgs is not stable, there's not much of it in the galactic center

Anonymous said...

The Fermi paper you quote analyses 2 years of data, Weniger almost 4. If i remember correctly, he says in his paper that taking the 2 year data only he agrees with Fermi's findings.

Anonymous said...

Assuming of course that there is no process in the galactic centre generating copious Higgs........

Robert L. Oldershaw said...

Nature passes the 487th consecutive negative verdict on "WIMP" dark matter.

Will theoretical physicists wake up to reality - the dark matter is not in any form of subatomic particle? It is comprised of primordial Kerr-Newman ultracompact objects that roam nomadically throughout galaxies and intergalactic space.

Discrete Scale Relativity

Sesh Nadathur said...

Hi Jester, I'm a little late on this, but I just wanted to point out that the reason the Fermi paper does not mention Weniger's result is apparently that the collaboration requires papers to be accepted by journals before they are put on the arXiv. Therefore the Fermi paper was actually written some time before the date it was uploaded, and probably before Weniger's paper was available.