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Closing in on Dark Matter: Another "Tentative" Step


Dark matter around Abell 1689 galaxy cluster

A galaxy cluster with the distribution of dark matter marked by purple overlay. Credit: NASA, ESA, E. Julio (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM).

When physicists and mathematicians want to get an idea into circulation before going through all the hoo-hah of peer-reviewed publication, they often post a paper on the arXiv server, where anyone who is curious can go and read it. Some arXiv papers turn out to be important, but much evaporates on closer inspection. Judging whether a new arXiv paper is one or the other can be extremely difficult. That is certainly the case with physicist Christoph Weniger’s paper, “A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope,” posted on April 12, on dark matter.

Dark matter, invisible and undetectable, makes up more than a quarter of the universe and has been an enigma to physicists and astronomers for more than a century. While physicists can’t look at dark matter directly, they can try to tell-tale trails that dark matter was present. Weniger has produced an analysis of data that—if it holds up—is a major step forward in explaining dark matter, and might provide the first unambiguous evidence of what this mysterious and elusive substance is.

Of course, we’ve heard dramatic claims like this before that didn’t pan out—and it’s certainly possible this one won’t either. We won’t know which way it goes until other scientists digest the analysis and weigh in, which could take months. And even so, it may take years before the findings are confirmed. In the meantime, it’s worth having a look at this latest experimental claim, if only to see how an outsider —a theorist unaffiliated with an experimental collaboration— occasionally tries to make a splash in the big collaboration world of physics.

The outsider, of course, is Weniger. A post-doc at the Max-Planck Institute of Physics, he is not a member of the collaboration that works on the Fermi Large Area Telescope (the collaboration goes by the acronym Fermi-LAT). However, Fermi-LAT makes its data publicly available, which allowed Weniger to use it for his investigation. In fact, his analysis goes over ground that researchers collaborating on the Fermi-LAT project have already trod. When they analyzed their data in previous years, the Fermi-LAT researchers found no strong evidence for dark matter. Weniger, however, wasn’t convinced. He and a few colleagues opted to re-crunch the Fermi-LAT data and in March, posted hints of dark matter that they had spotted. Weniger’s April 12 paper goes a step further, suggesting he’s spotted an even stronger signal at a specific energy.

Weniger’s analysis relies on a theory that predicts that when particles of dark matter meet, they will annihilate one another and create photons. In principal, you should be able to spot these photons in the form of high-energy gamma rays. Since the Large Area Telescope was built to study gamma rays, it’s an ideal instrument for this kind of search.

Weniger analyzed 43 months of data, which yielded strong evidence for a gamma ray source in the outskirts of the galaxy—a region called the galactic halo—which is exactly where theorists would predict you could find dark energy annihilations. Specifically, he’s claimed to spot the candidate gamma rays at 130 billion electron volts. For those of you keen on the statistical details, he’s claiming it with as much as 4.6 sigma certainty—which is to say, a high degree of certainty. For context: In current particle physics, evidence for the Higgs boson would be accepted as a discovery at 5 sigma certainty, so 4.6 is pretty good. That said, when he incorporates the necessary statistics for his targeted search and sample size, his results drop to a 3.5 sigma certainty, barely strong enough for publication.

What makes Weniger think that he got it right while the insiders at Fermi-LAT got it wrong? His is the first to include a full 43 months of data. Previous Fermi-LAT collaboration publications, such as results published in 2010, are limited to just 11 months. In addition, to updating the dataset, Weniger has developed his own algorithms for the dark matter search, which he believes do a better job understanding the region of the galaxy where dark matter is alleged to be. This improves his chances of distinguishing the sought out gamma rays from other galactic events.

But before we pop open the champagne, there are several important caveats. As Weniger himself acknowledges, several more years of data will be needed before it’s clear whether what he thinks he’s seen is real. In addition, because Weniger isn’t a member of the team that gathers data at Fermi-LAT, it’s possible he doesn’t entirely understand how the technology involved in detecting and collecting the data may affect the data. This is something that only collaborators are likely to have studied with enough care to correct for in their analysis. The paper could amount to nothing more than another dark matter dead end.

Things might get interesting if the Journal of Cosmology and Astroparticle Physics, to which Weniger is submitting this paper, opts to publish. That stamp of approval would set Weniger’s work above a great many other arXived efforts. Another development to watch for is a response from the folks on the Fermi collaboration. They know this data better than anyone, and if there’s something to be learned from Weniger’s approach, they’ll want to take it seriously. If nothing else, this is one more in a string of recent examples that shows how we are closing in on dark matter. For now, we watch and wait.

The views expressed are those of the author and are not necessarily those of Scientific American.

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