The Sciences

Looking for Dark Matter in All the Wrong Places

Cosmic VarianceBy Sean CarrollFeb 4, 2009 5:40 PM


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David Harris at symmetry breaking points to a paper and accompanying commentary on the search for high-energy cosmic antiprotons by the PAMELA satellite experiment. (What one defines as "high-energy" depends on one's upbringing; we're talking about energies of up to 100 times the mass of the proton.) The impression is given that this is a brand-new result casting doubt on the earlier claims that PAMELA might have detected evidence for dark matter; that's not really a correct impression, so it's worth getting it all straight. The PAMELA satellite, an Italian/Russian/German/Swedish collaboration, looks at high-energy cosmic rays from orbit, and pays particular attention to the presence of antimatter -- basically, positrons (anti-electrons) and anti-protons. Part of the idea is that a high-energy matter particle can simply be a particle that had been lying around for a while and was accelerated to large velocities by magnetic fields or other astrophysical processes, whereas you need some pretty high energies to produce antiparticles in the first place. Say, for example, from the annihilation of dark matter particles with each other. There are certainly some high-energy collisions in the ordinary non-dark-matter world, so you expect to see a certain fraction of antimatter, but that fraction should noticeably diminish as you get to higher and higher energies. So in October the experiment released two papers back to back:

A new measurement of the antiproton-to-proton flux ratio up to 100 GeV in the cosmic radiation Authors: O. Adriani et al. arXiv:0810.4994Observation of an anomalous positron abundance in the cosmic radiation Authors: O. Adriani et al. arXiv:0810.4995

If you look closely, you'll notice the second paper has 10 trackbacks to its abstract on arxiv, while the first doesn't have any (until now!). The reason is clear: the second paper has the word "anomalous" in the title. The PAMELA measurements of positrons deviate significantly from the theoretical expectation, while the measurements of anti-protons reported in the first paper are exactly what you might have predicted. Who wants to write about observations that fit theories we already have? You might remember the PAMELA positron result as the one that created a stir when they gave a talk before submitting their paper, and theorists in the audience snapped pictures of the data with their cell phone cameras and proceeded to write papers about it. Those wacky theorists. Here is the relevant positron plot, from paper 2 above:

The vertical axis is the fraction of positrons in the total sample of electrons+positrons, plotted against energy. The red dots are the data, and the black curve is the theoretical prediction from ordinary astrophysical processes. Not the best fit, eh? At low energies that is not a surprise, as "weather" effects such as solar activity can get in the way of observing low-energy positrons. But at high energies the prediction should be more robust, and that's where it's the worst. Indeed, it's pretty clear that the fraction of positrons is increasing with energy, which is pretty baffling, but could conceivably come from dark matter annihilations. See Resonaances for more discussion. And here is the version for antiprotons, from paper 1 above:

Now that's what we call a fit to the data; again, fraction of antiprotons plotted versus energy, and the data go up and down just as predicted. What happened is that the PAMELA collaboration submitted their second paper (anomalous positrons) to Nature, and their first paper (well-behaved antiprotons) to Physical Review Letters. The latter paper has just now appeared in print, which is why Simon Swordy's commentary in Physics appeared, etc. Although the idea behind Physics (expert-level commentary on recently published articles) is a good one, it's sponsored by the American Physical Society, and therefore pretends that the only interesting articles are those that appear in journals published by the American Physical Society. Which Nature is most surely not. So one might get the impression that the antiproton result is a blow against the idea that we are seeing dark-matter annihilations. Which it is; if you didn't know any better, you would certainly expect to see an excess of antiprotons in dark-matter annihilations just as surely as you would expect to see an excess of positrons. But it's not a new blow; the papers appeared on arxiv (which is what really matters) at the same time! And it's not a blow that can't be recovered from. All you have to do is declare that your dark matter candidate is "hadrophobic," and likes to annihilate into electrons and positrons rather than protons and antiprotons. Not an easy task, but that's why theorists get paid the exorbitant salaries we do. (Without ready access to champagne and caviar, we can hardly be expected to justify unusual branching ratios in WIMP annihilations.) The favorite model out there right now belongs to Arkani-Hamed, Finkbeiner, Slatyer, and Weiner, featuring a new gauge force that is broken at relatively low energies. But there are various models on the market, and the number is only going to grow. Most likely the PAMELA positron excess is coming from something that can be fit quite nicely into the Standard Model of particle physics, like pulsars. That's my guess, anyway. Happily, there's all sorts of data coming down the pike that will help us sort it out.

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