Friday, 24 July 2009

FERMI is seeing something?

It has become a tradition that release of new astrophysical data proceeds in the atmosphere of scandal, sex, and intrigues. Less than two weeks ago in this blog I was whining that the FERMI collaboration is guarding their secrets too effectively. Not any more. Not even guns and barbed wire fences could keep theorists off, once they have smelled real data.

Once again the story is related to the searches of indirect signals of dark matter in cosmic rays. In the previous episodes, the PAMELA satellite reported an excess of cosmic ray positrons between 10 and 100 GeV, and FERMI announced that the spectrum of electrons and positrons is harder (falls off more slowly with energy) than predicted by conventional cosmic ray propagation models. Although there exist plausible explanations in terms of mundane astrophysics, the excess positrons and electrons can also be understood as products of annihilation or decay of dark matter in our galaxy. If that is the case, there is one robust consequence. The electrons and positrons produced by dark matter throughout the galaxy should interact with the photons of the cosmic microwave background and starlight in the process known as the inverse Compton scattering, ICS in short. A high energy electron scattering off a photon transfers most of its energy to the photon. Thus, dark matter models explaining the PAMELA and FERMI results also predict an excess of gamma ray photons from the galactic center at energies 100 GeV and more. That's why the astroparticle community has been eagerly awaiting the release of FERMI measurements of gamma rays from the galactic center.

A week ago FERMI announced some new results at the TeV Particle Astrophysics conference held at SLAC. The new data included measurements of the gamma ray spectrum from the galactic center. The results from the one-by-one degree square around the galactic center, while providing new constraints on dark matter models, do not show any exciting features, see the upper plot. However, the data from a larger portion of the sky referred to as the inner galaxy do show an excess, or a hardening of the spectrum, starting at 100 GeV, see the plot on the left. The hardening occurs exactly where the dark matter models predict it! The FERMI collaboration did not want to post the latter result because it is still contaminated with poorly understood backgrounds. But somehow, mysteriously, the plot made it into the summary talk given by Persis Drell and the slides were posted at the conference page. These slides have now been removed; too late alas too late. Today there is a new paper on arXiv that interprets the new FERMI data in terms of the PAMELA/FERMI motivated models dark matter. The plot below reproduced from that paper shows the FERMI data together with expected backgrounds and predictions from dark matter models.

So is FERMI seeing dark matter? Most likely not. Members of the FERMI collaboration suspect that the feature in the gamma ray spectrum around 100 GeV is due to an unexpected background from other cosmic ray particles. Further analysis should clarify the situation. What is definitely true is that we're living in interesting times...

Sunday, 19 July 2009

Bullets Fly

I'm sure that everybody has heard of the Bullet cluster aka 1E 0657-56:

This picture from August 2006 made the headlines because it offers a new way to see the presence of dark matter in the universe (the key of course is to paint it blue). The depth of the gravitational potential deduced from gravitational lensing is marked blue, while the matter that shines ordinary photons is marked red. The picture is interpreted as showing two clusters of galaxies that have recently undergone a head-on collision. The dark matter components (and also most of the ordinary stars in the galaxies) just passed through each other with little or no interaction, while the interstellar gas made of familiar protons and electrons collided and got left behind. The observation of the Bullet cluster gave another blow to dark matter alternatives like MOND-type modified gravity theories: in the latter context it is hard to explain why the gravitational potential is not spatially correlated with the ordinary matter distribution.

What might be a little less known is that the Bullet cluster is not the only one. In August 2007 Abell 520 aka Train Wreck was revealed:

This one is much more messy. In fact, in this case the dark matter interpretation is less straightforward. The reason is that the galaxies seems to have been removed from the densest core of dark matter, and it is not clear what mechanism could have caused it. Then in August 2008 we had a pleasure to meet MACS J0025.4-1222 aka Baby Bullet:
which is another pretty clear evidence in favor of dark matter. Apparently, galactic collisions happen every year in August, so next month we should be presented with another picture of this kind :-)

The most important thing about these observations is that they look cool in pictures. But they also carry some practical consequence for particle theorists who sweat to construct models of dark matter. From the fact that the dark matter components pass through each other so easily one can derive a constraint on the self-interaction cross section of dark matter. The paper of Randall et al (not that Randall) based on the analysis of the Bullet cluster quotes the limit

$\sigma/M \leq 3 \cdot 10^3/GeV^3$.

That's an order of magnitude better than the so-called Spergel-Steinhard bound that can be deduced from the dynamics of our galaxy. While this bound is irrelevant for a standard 100 GeV WIMP, it might be a useful constraint for recently popular theories of dark matter where the dark sector consists of strongly interacting particles bound by some new unknown GeV scale forces.

Friday, 10 July 2009

That's Another One for the Fire

It's a lazy summer season: everybody's on the beach and nothing's much going on. To stay in business I have to feed you with some microwaved news. Last week the FERMI collaboration uploaded a pile of papers on arXiv, one of which caught my attention. FERMI is a space gamma-ray observatory, but first of all he is a ruthless terminator with a mission to eliminate other astrophysical experiments. A while ago in May the widely publicized measurement of the cosmic-ray electron+positron spectrum pierced the ATIC balloon that had been pumped for several months. Earlier this year FERMI made another kill: it shot down EGRET, its direct predecessor in cosmic gamma-ray observations. That result has been presented at conferences for a few months, but only last week it made it to arXiv (there's also a longer PRL paper announced).

EGRET was a sattelite gamma-ray observatory that in particular studied the diffuse gamma ray emission in the 30 MeV-100 GeV range. Diffuse means spread over the sky rather than originating from point sources. The main source of diffuse radiation is scattering of the cosmic rays on the milk of the Milky Way. Dark matter annihilation into standard model particles can also contribute to the diffuse flux. The EGRET results showed an excess of gamma rays above 1 GeV, which was quickly hailed as the harbinger of dark matter and supersymmetry. But FERMI's measurement now demonstrates that there's nothing exciting going on below 10GeV: the experimental curve nicely follows the theoretical prediction of the standard propagation model. No exotic physics in sight. ATIC, EGRET...who's next?

FERMI's goal is to measure the diffuse gamma-ray spectrum up to some 300 GeV. The high energy data around are even more interesting for theorists as many popular models of dark matter - especially those that explain the PAMELA positron excess - predict a large signal peaking around a few hundred GeV. More results are expected in August since on August 12th the collaboration is supposed to make all their photon data public. If you hear cries and squealing later this summer that's the dark matter models being slaughtered. Or maybe FERMI sees an excess in which case all hell will break loose? The rumor is...the weird thing is that there are no rumors. Last time, quite accurate descriptions of the electron spectrum circulated in the community months before publication of the FERMI data, and theory papers from outside of the collaboration were out days after FERMI's publication (or even violating causality in one case). Probably because of that experience the collaboration has now entrenched in its camp with barbed wire fences, dogs, booby traps to keep off the theorists. Oh come on, dont be so serious, we also wanna know ;-)