PI Science: Double-checking Dark Matter

For instance, you can think of each of the stars circling the centre of our galaxy as a ball being spun at the end of a rope. If the rope – gravity – weren’t there, or were too weak, the stars would fly off into space. The trouble is that the gravity created by the matter we can observe is far too weak a rope to hold the spinning stars together. It’s now widely thought that the visible parts of the galaxy are surrounded by a “halo” of dark matter – which cannot be seen because it does not give off light. What is dark matter? That’s one of the most interesting open questions in modern physics. We simply do not know. A leading candidate is weakly interacting massive particles, or WIMPs: some kind of stable new particle that does not interact with the strong or electromagnetic forces. Dozens of experiments have looked for evidence of WIMPs – but only one claims to have found them. Researchers at the DAMA experiment, located in the Gran Sasso underground laboratory in Italy, have been claiming for a decade that they have detected WIMPs. The wider dark matter research community has met this claim with skepticism. Now, an independent analysis by Perimeter researchers could put one of the stronger arguments against DAMA’s claims to rest. One can think of the DAMA experiment as an effort to chart what Perimeter Associate Faculty member Itay Yavin calls “a wind of WIMPs.” Our sun’s orbit around the centre of the galaxy has it moving at about 220 kilometers per second through the dark matter halo. Like running through still air, this creates a steady “wind.” The earth’s orbit around the sun has us pushing against the wind of WIMPs in the summer and puts the wind at our backs in the winter. The geometry of the orbit allows scientists to make two clear-cut predictions: we should observe a small yearly rise and ebb in any signal from dark matter, and the signal’s peak should be on June 2. DAMA sees exactly that. To track the strength of the WIMP wind, DAMA uses a scintillating detector – that is, an array of crystals that emit light when particles interact inside them. With 13 years of data collected, DAMA’s claim to have detected an annual rise and fall – a modulation – in the number of scintillations is not in doubt. However, there is something else that could make the DAMA detector scintillate, which also has a period of one year and a peak in the summer: cosmic muons. Cosmic muons come from cosmic rays – that is, the charged particles (mainly protons) moving at high speeds that are constantly showering down on the earth from space. When these particles hit the earth’s atmosphere, they produce a cascade of secondary particles, many of which are unstable and decay into muons. Some of these muons have high enough energies to penetrate the 1,400 metres of rock and reach the underground chamber where the DAMA detector is located. The number of high-energy muons produced depends on the temperature. The colder the air, the denser it is, and the more likely the secondary particles are to collide before they decay, decreasing the energy available to their decay products, the muons. Therefore, more high-energy, rock-penetrating muons are produced in the summer than the winter, which should result in an annual rise and fall in the number detected by an underground scintillator. So is DAMA seeing these relatively mundane and well-understood muons or dark matter? Perimeter Associate Faculty member Itay Yavin and Postdoctoral Researcher Josef Pradler set out to find out. “DAMA has made a strong claim for seeing dark matter, maintaining that muons cannot be the cause of the signal,” says Pradler. “But there is good reason to be skeptical – especially since the collaboration is somewhat secretive about their data. What we did was offer an independent analysis of the muon hypothesis.” The two researchers compared DAMA’s published results with cosmic muon data taken with the Large Volume Detector experiment, an apparatus that measures the cosmic ray muon flux and that sits next to DAMA in the Gran Sasso laboratory. They looked at the amplitude, phase, and power spectrum of two annual modulations – and found that they were very unlikely to be from the same source. For instance, with regard to the phase, they found that while the DAMA signal peaks in June, the muon flux peaks in July. Together with Spencer Chang, an Assistant Professor at Oregon University, they performed a correlation analysis and other tests, but all seemed to indicate that the two data sets – the muon set and the DAMA set – were insufficiently correlated. In other words, whatever DAMA is seeing, it is almost certainly not muons. These results have been accepted for publication in Physical Review D and Pradler has presented them to interested audiences at major dark matter and particle physics conferences. “The reason so many people care about this result is that dark matter is an exploding frontier,” says Yavin. “There are many experiments looking for dark matter in the lab: some at the SNOLAB underground facility here in Canada, some at the Soudan mine in Minnesota, more at Gran Sasso – 10 or 20 worldwide experiments looking for dark matter in the lab. We need to get a better handle on what that signal might look like.” In the interest of getting a better understanding of dark matter, the researchers have suggested a next step. If DAMA’s signal is from dark matter, it should – as a matter of pure mathematics – have higher harmonics. “It’s like the vibrations of a guitar string,” explains Yavin. “The dark matter signal is a ‘note’ with a frequency of one year. It should have overtones.” The researchers pointed out that such an harmonic analysis can uncover more information about the signal and offer additional discrimination against different backgrounds.

Further Exploration

• S. Change, J. Pradler, and I. Yavin, “Statistical Tests of Noise and Harmony in Dark Matter Modulation Signals,” Phys. Rev. D 85, 063505 (2012), arXiv:1111.4222
• Perimeter’s educational video The Mystery of Dark Matter