Lab report

The search for dark matter continues with mixed results. China unveils its new particle accelerator, but it's not for particle physics
June 3, 2009
Dark matter continues to puzzle

No one seems sure what to make of the latest results from a space telescope searching for dark matter. Some reports have claimed that the Fermi Gamma-Ray telescope may have detected a faint trace of the elusive stuff. Other accounts imply that the Fermi data actually squelches earlier claims to that effect.

Dark matter is so named because it can't be seen directly and isn't composed from any known subatomic particles. But we know it exists from the gravitational tug it exerts on visible matter and it makes up around 85 per cent of the matter in the universe.



There are experiments that aim to catch dark-matter particles in underground detectors, where most of the distracting "background" particles never reach. And the Large Hadron Collider, when running again, might even be able to make dark matter.

As well as Fermi, two other experiments are looking for dark matter by studying high-energy cosmic rays: a detector called Pamela on a satellite run by the European Space Administration, and a project called Atic which uses balloon-borne instruments over the Antarctic. The idea behind these searches is that some cosmic rays—high-energy particles streaming through space—might have been created as dark-matter particles annihilate one another in rare collisions, or as they decay. Those processes should generate electrons and their antimatter counterparts positrons, which Pamela and Atic can pick up, as well as gamma rays that Fermi can detect. (But as we don't know what dark matter particles actually are, this is only an informed hunch.)

Pamela has detected more positrons at high energies than expected from the usual "background" of cosmic rays. And last November the Atic team reported something even more enticing: a spike in the number of cosmic-ray electrons with a certain range of energies, which might indicate how massive dark-matter particles are. But the spike might have an entirely different origin, including (most boringly) a quirk of the detection method.

That's what the Fermi results, reported in early May, now seem to suggest. They show a slight excess of gamma rays within the energy range studied by Atic, but not a spike. This fits with Pamela's data, and implies that, if it is a signal of dark-matter decay, the particles could be too massive to make at the LHC.

Alternatively, the excess could be produced by known astrophysical objects such as superdense stars called pulsars. We don't even know for sure that dark matter will signal its presence at all. There's an awful lot of guesswork at this frontier of physics.


China's shiny new toy

Not all particle accelerators are built for particle physics. Many synchrotrons—in which particles such as electrons are boosted to enormous speeds around circular tracks—exist to harvest the light that streams from the superfast particle beam as it spins. This "synchrotron radiation" supplies bright beams of X-rays for conducting more prosaic (some would say more useful) research, such as deducing the structures of materials like protein crystals. Users range from archaeologists to molecular biologists.

They're expensive to make, so the big synchrotrons, such as the European facility in Grenoble, tend to be oversubscribed. Now China has unveiled a new synchrotron in Shanghai. It cost $176m, more than any other single scientific facility in China. It is not China's first synchrotron, but its previous two were small fare and limited in the X-ray energies they could produce, forcing Chinese scientists to look abroad. The new synchrotron is already struggling to meet the demand from prospective users.

Africa's genetic mix

It seems remarkable that a new study of genetic relationships between African populations makes no reference to the long history of ethnic tension in the continent. Perhaps that topic is too volatile to be broached yet. But the new survey, probably the most extensive so far and conducted by a large team including researchers from Kenya, Nigeria, Sudan, Tanzania and other African nations, certainly has implications for the issue. It is no great surprise that the analysis of genetic diversity in 121 African populations finds "high levels of mixed ancestry" in most cases. But this surely poses a challenge to the rigidity with which ethnic divisions are asserted, especially in areas of conflict.

Admittedly, the study had other priorities. Studying genetic ancestry in Africa is interesting not only because of the immense number of ethnic groups (more than 2,000) and because Africans have the greatest in-group genetic diversity in the world, but also because of course modern humans originated in Africa 200,000 years ago. The study allows the place of human migration out of Africa to be pinpointed with almost unnerving precision, close to the coastal border of Namibia and Angola in southwest Africa. Africa's cultural diversity also means that links between genes and language can be studied: languages mostly follow ethnicity, but have been supplanted in some ethnic groups.

A study this big, involving genetic analysis of over 2,400 people, would have been unthinkable just a few years ago, and shows how new genome technologies will reveal the historical and demographic narratives of our species in extraordinary detail.